GC157 Paediatric Chemical Pathology
Paediatric chemical pathology is the subspecialty of clinical biochemistry focused on the diagnosis and monitoring of metabolic, endocrine, and biochemical disorders in neonates, infants, and children.
Paediatric Chemical Pathology
This lecture (GC 157) sits at the intersection of paediatric medicine and chemical pathology. The overarching message is deceptively simple but profoundly important: "Children are not small adults." In chemical pathology, this means reference ranges, differential diagnoses, investigations, sample handling, and management all differ from adult practice. The lecture centres on four core paediatric biochemical problems—neonatal cholestasis, hyperlactatemia, hyperammonemia, and hypoglycemia—and then pivots to the broader topic of inborn errors of metabolism (IEM), including their classification, clinical presentation, laboratory approach, epidemiology, and the expanding role of newborn screening. Two illustrative cases round out the lecture: an OTC deficiency case and a primary CoQ10 deficiency case.
Learning Objectives (per slide 2) [1]:
- Enhance understanding of "children are not small adults" from a chemical pathology perspective.
- Discuss chemical pathology approaches to common paediatric biochemical problems: neonatal cholestasis, hyperlactatemia, hyperammonemia, and hypoglycemia.
- Understand general principles and laboratory investigations for suspected IEM.
- Understand the importance and latest updates on newborn screening for IEM and genetic diseases.
High Yield: Every learning objective maps directly to past exam stems. The examiners love giving a neonatal/infant case and asking you to interpret biochemistry, narrow the DDx, and order the right confirmatory test.
"In Chemical Pathology perspective: Reference intervals are different; DDx are different; Tests are different; Sample requirement and considerations are different; Treatment and monitoring are different." [1]
| Aspect | Adult Practice | Paediatric Difference | Why It Matters |
|---|---|---|---|
| Reference intervals | Standard adult ranges | Age-, sex-, and sometimes Tanner-stage-specific | e.g. ALP is physiologically high in growing children (bone isoenzyme); bilirubin reference differs in neonates |
| DDx | Acquired diseases dominate | IEM, congenital anomalies, genetic syndromes far more common | A "rule-out sepsis" picture in a neonate may actually be an IEM |
| Tests | Standard panels | Specialised metabolic screens (plasma amino acids, urine organic acids, acylcarnitines) | These tests are rarely ordered in adult medicine but are first-line in paediatrics |
| Sample requirements | 5–10 mL tubes | Micro-samples; capillary/heel-prick; smaller tubes | Over-sampling can cause iatrogenic anaemia in neonates |
| Treatment | Standard dosing | Weight-based dosing; different drug metabolism; some drugs contraindicated | e.g. aspirin → Reye syndrome in children |
2. Neonatal Cholestasis
Neonatal cholestasis = conjugated (direct) hyperbilirubinaemia in the first 3 months of life. The conjugated bilirubin is > 20% of total bilirubin, or absolute conjugated bilirubin > 17 µmol/L.
Why this matters: Unconjugated neonatal jaundice (physiologic, breast-milk, haemolytic) is extremely common and usually benign. Conjugated jaundice is ALWAYS pathological and demands urgent workup.
"Surgical: Bile duct abnormalities — then need to exclude biliary atresia since infants need to undergo surgery ASAP before the age of 60 days. Medical causes: Infections, Toxins/drugs, Endocrine diseases, IEM, Chromosomal diseases." [1]
| Category | Examples | Key Points |
|---|---|---|
| Surgical | Biliary atresia, choledochal cyst | Biliary atresia — Kasai portoenterostomy must be done before 60 days of life for best outcomes. After 90 days, success rate drops dramatically. |
| Infections | TORCH (Toxoplasma, Others, Rubella, CMV, HSV), UTI (especially E. coli), hepatitis B/C, sepsis | CMV is the most common congenital infection causing cholestasis |
| Toxins/Drugs | TPN-associated cholestasis, drugs | Prolonged parenteral nutrition in preterm neonates is a classic cause |
| Endocrine | Hypothyroidism, hypopituitarism, cortisol deficiency | Congenital hypothyroidism can cause prolonged conjugated jaundice |
| IEM | Galactosemia, tyrosinemia type I, alpha-1 antitrypsin deficiency, Niemann-Pick type C, CF | Galactosemia = jaundice + E. coli sepsis + cataracts in a milk-fed neonate |
| Chromosomal | Alagille syndrome (JAG1 mutation), Down syndrome | Alagille = paucity of intrahepatic bile ducts + butterfly vertebrae + posterior embryotoxon + characteristic facies |
Critical Time Window
Biliary atresia is a surgical emergency. The Kasai procedure must be performed before day 60 for the best chance of bile drainage and delaying the need for liver transplant. Any neonate with conjugated jaundice persisting beyond 2 weeks must have biliary atresia excluded urgently. Do NOT attribute prolonged jaundice to "breast-milk jaundice" without checking direct bilirubin.
"First-line Investigations of Neonatal Cholestasis" [1] (slide 6)
The lecture lists a systematic panel (inferred from the standard approach):
| Investigation | Purpose |
|---|---|
| Total and direct bilirubin | Confirm conjugated hyperbilirubinaemia |
| LFT (ALT, AST, ALP, GGT, albumin) | Hepatocellular damage vs cholestatic pattern; synthetic function |
| Coagulation (PT/INR) | Assess hepatic synthetic function; vitamin K deficiency in cholestasis |
| Full blood count | Infection, haemolysis |
| Blood culture, urine culture | Sepsis, UTI |
| TORCH screen | Congenital infections |
| Thyroid function (TSH, fT4) | Congenital hypothyroidism |
| Metabolic screen — galactose, urine reducing substances, alpha-1 antitrypsin level/phenotype, sweat chloride | IEM causes |
| Abdominal ultrasound | Biliary atresia (absent/small gallbladder, triangular cord sign), choledochal cyst |
| HIDA scan (hepatobiliary iminodiacetic acid scan) | Non-excretion into bowel suggests biliary atresia |
| Liver biopsy | If diagnosis unclear; bile duct proliferation in biliary atresia |
3. Hyperlactatemia
Lactate is the end-product of anaerobic glycolysis. Pyruvate is normally converted to acetyl-CoA (via pyruvate dehydrogenase) and enters the TCA cycle. When oxygen is insufficient OR when specific metabolic pathways are blocked, pyruvate is shunted to lactate by lactate dehydrogenase.
Normal plasma lactate: < 2 mmol/L. Hyperlactatemia is generally defined as > 2 mmol/L; lactic acidosis when > 5 mmol/L with pH < 7.35.
"Type A lactic acidosis: With hypoxia. Type B lactic acidosis: Without hypoxia (Acquired / Genetic)." [1]
| Type | Mechanism | Examples |
|---|---|---|
| Type A | Tissue hypoxia → anaerobic glycolysis → excess lactate | Shock, severe anaemia, cardiac arrest, severe hypoxia, CO poisoning |
| Type B — Acquired | Hypoxia, exercise, seizure, severe dehydration, infections, severe catabolic state, poisoning | Drug-related (metformin, NRTIs), thiamine deficiency, liver failure, malignancy |
| Type B — Genetic | Enzyme defects in lactate-pyruvate metabolism | See below |
"Disorders of lactate-pyruvate oxidation; TCA cycle deficiencies; Respiratory chain defects; Disorders of gluconeogenesis; Disorders of glycogen metabolism" [1]
| Genetic Cause | Example | Mechanism |
|---|---|---|
| Pyruvate metabolism defects | Pyruvate dehydrogenase deficiency, pyruvate carboxylase deficiency | Cannot convert pyruvate → acetyl-CoA or oxaloacetate → lactate accumulates |
| TCA cycle deficiencies | Fumarase deficiency | Block in Krebs cycle → upstream metabolite build-up including lactate |
| Respiratory chain (mitochondrial) defects | MELAS, Complex I/II/III/IV deficiency | Cannot oxidise NADH → NAD+ ratio falls → pyruvate shunted to lactate |
| Gluconeogenesis defects | Fructose-1,6-bisphosphatase deficiency | Cannot convert lactate back to glucose via gluconeogenesis |
| Glycogen metabolism defects | GSD type I (von Gierke) | Glucose-6-phosphatase deficiency → excess glycolysis → lactate |
Clinical tip: The lactate-to-pyruvate (L:P) ratio helps differentiate:
- L:P ratio > 25 → suggests mitochondrial/respiratory chain defect (NADH/NAD+ ratio is high, so pyruvate is converted to lactate)
- L:P ratio normal (10–20) → suggests pyruvate dehydrogenase deficiency or gluconeogenesis defect
4. Hyperammonemia
Ammonia (NH₃/NH₄⁺) is produced by amino acid catabolism, gut bacterial urease activity, and renal glutaminase. It is detoxified in the liver via the urea cycle, which converts ammonia to urea for renal excretion. In neonates and infants, the urea cycle can be immature or defective.
Normal plasma ammonia: < 60 µmol/L in older children; neonates may have slightly higher levels but should still be < 100 µmol/L.
Why ammonia is toxic: Ammonia freely crosses the blood-brain barrier. In astrocytes, glutamine synthetase converts glutamate + NH₃ → glutamine. Excess glutamine causes osmotic swelling of astrocytes → cerebral oedema → encephalopathy → coma → death.
| Category | Examples |
|---|---|
| Urea cycle defects (UCD) | OTC deficiency (X-linked), CPS1 deficiency, ASS deficiency (citrullinemia), ASL deficiency (argininosuccinic aciduria), arginase deficiency |
| Organic acidemias | Propionic acidemia, methylmalonic acidemia, isovaleric acidemia |
| Fatty acid oxidation defects | MCAD, VLCAD, LCHAD |
| Liver failure | Any cause — viral, drug, metabolic |
| Transient hyperammonemia of the newborn (THAN) | Preterm infants; resolves spontaneously |
| Infections | Severe sepsis, herpes |
| Others | Valproate therapy, urinary tract infection with urease-producing organisms |
"3-yo Hong Kong Chinese boy; full term; non-consanguineous parents; unremarkable history; good past health and normal development; 3 days of repeated vomiting and diarrhea; bizarre behavior on day 2; comatose on day 3." [1]
Key investigation findings [1]:
- CBP, LFT, RFT, ABG: normal
- Plasma ammonia: 54 µmol/L (< 60 — i.e. normal at presentation!)
- Urine toxicology: negative
- EEG: generalised slowing with transient sharp slow wave
- CT brain: normal
- CSF and blood cultures for viruses: negative
- Plasma and urine amino acids: normal
- Stool culture: positive for rotavirus
- Urinary organic acids: marked elevation of orotic acid
Exam Trap — Normal Ammonia Does NOT Rule Out UCD
In this case, ammonia was normal at the time of sampling. UCD patients can have intermittent hyperammonemia triggered by catabolic stress (infection, fasting). Between crises, ammonia may normalise. The KEY clue was the elevated urinary orotic acid.
"Plasma urea: LOW. Ammonia: HIGH. Plasma amino acids: low arginine, elevated glutamine. All are normal in this patient, except the urine orotic acid." [1]
Why these patterns?
- Low urea: The urea cycle is blocked, so less urea is produced
- High ammonia: Cannot convert ammonia to urea
- Low arginine: Arginine is the immediate precursor of urea; in most UCDs it is consumed/not produced
- High glutamine: Ammonia is detoxified to glutamine as a compensatory mechanism
- Orotic acid elevation: Specific to OTC deficiency — carbamoyl phosphate (which cannot enter the blocked urea cycle) spills into the pyrimidine synthesis pathway → orotic acid
"DDx of Orotic Aciduria" [1] (slide 18)
| Condition | Mechanism |
|---|---|
| OTC deficiency | Carbamoyl phosphate diverts to pyrimidine pathway |
| Hereditary orotic aciduria | UMP synthase deficiency (very rare) — megaloblastic anaemia unresponsive to B12/folate |
| Lysinuric protein intolerance | Defective dibasic amino acid transport → secondary urea cycle dysfunction |
"Genetic analysis of OTC gene unraveled a hemizygous missense mutation of arginine-to-glutamine substitution on amino acid 277 (p.R277Q)" [1]
Why hemizygous? OTC deficiency is X-linked. Males have only one X chromosome, so a single pathogenic variant = disease. This is the only common X-linked UCD; all others are autosomal recessive.
Why so atypical? The p.R277Q mutation is a known "late-onset" or partial OTC deficiency variant. The enzyme has residual activity (unlike null mutations that present in the neonatal period with severe hyperammonemia). These patients may be well for years and only decompensate during catabolic stress (e.g. the rotavirus infection in this case).
"The child remained unconscious for three days under intensive care. He regained consciousness spontaneously without any neurological deficit. He was discharged on the third hospital day with a diagnosis of transient encephalopathy of undetermined cause." [1]
Clinical lesson: Without the orotic acid finding and subsequent genetic testing, this child would have been discharged without a diagnosis and would be at risk of fatal hyperammonemic crisis in the future.
"Liver biopsy: Refused by parents; Invasive; Sending overseas; Patients have to pay. Genetic analysis: Non-invasive; Definitive; Local expertise available." [1]
In modern practice, genetic testing (Sanger sequencing, NGS panels, WES) has largely replaced enzyme assays on liver biopsy for diagnosing UCDs and many IEMs. The workflow involves DNA extraction → PCR → sequencing → mutation identification [1].
5. Hypoglycemia
"Definition of plasma glucose level" [1]
Neonatal hypoglycemia: plasma glucose < 2.6 mmol/L (operational threshold; varies by guideline). In older children/adults: < 3.3 mmol/L with symptoms (Whipple's triad).
Why it matters: The developing brain is critically dependent on glucose. Prolonged or recurrent hypoglycemia causes irreversible neurological damage.
"Ketone +/−? Urine stix vs quantitative measurement of ketone. Acetoacetate vs beta-hydroxybutyrate. Insulin – check any hemolysis in the sample. Glucose transfusion rate." [1]
| Consideration | Explanation |
|---|---|
| Ketone status | The MOST important discriminator. Normal fasting → ketones should be present (ketotic hypoglycemia). Absent ketones = non-ketotic = hyperinsulinism or fatty acid oxidation defect |
| Urine dipstick ketones | Detects acetoacetate only; misses beta-hydroxybutyrate. In early fasting, the predominant ketone body is beta-hydroxybutyrate |
| Quantitative blood beta-hydroxybutyrate | More accurate; preferred in paediatric workup |
| Insulin measurement | Must check for sample haemolysis — RBC contain insulin-degrading enzyme; haemolysed sample gives falsely LOW insulin, masking hyperinsulinism |
| Glucose transfusion rate (GTR) | The amount of IV glucose (mg/kg/min) needed to maintain normoglycemia. GTR > 8 mg/kg/min strongly suggests hyperinsulinism (normal neonates need ~4–6 mg/kg/min) |
The lecture references a diagnostic algorithm (slide 9, from crashingpatient.com) [1]. The key branch point is ketotic vs non-ketotic hypoglycemia:
| Feature | Ketotic Hypoglycemia | Non-Ketotic (Hypoketotic) Hypoglycemia |
|---|---|---|
| Ketones | Appropriately elevated | Inappropriately low/absent |
| Free fatty acids | Appropriately elevated | Low (hyperinsulinism) or High (FAO defect) |
| Insulin | Appropriately suppressed | Detectable/elevated |
| Common causes | Idiopathic ketotic hypoglycemia (most common cause of hypoglycemia in children aged 1–5), GH deficiency, cortisol deficiency, GSD III/VI/IX | Congenital hyperinsulinism, insulinoma, FAO defects (MCAD, VLCAD), GSD type I |
| Hepatomegaly | May be present in GSD | Present in GSD type I; absent in hyperinsulinism |
| Acidosis | Mild ketoacidosis | May have lactic acidosis (GSD I) or no acidosis (hyperinsulinism) |
Critical Sample
When a child presents with hypoglycemia, draw a "critical sample" BEFORE correcting the glucose. This includes: glucose, insulin, C-peptide, cortisol, GH, free fatty acids, beta-hydroxybutyrate, lactate, ammonia, acylcarnitine profile, and urine organic acids. These results are interpretable only in the hypoglycemic state. Once glucose is corrected, you lose the diagnostic window.
6. Inborn Errors of Metabolism (IEM) — General Principles
"Phenotypically and genetically heterogeneous group of disorders. Defective enzyme or transporter in metabolic pathways — leading to disease due to metabolic malfunction and/or accumulation of toxic intermediate metabolites." [1]
Think of a metabolic pathway as an assembly line. If one worker (enzyme) is missing:
- The product downstream is deficient (e.g. low cortisol in CAH)
- The substrate upstream accumulates and may be toxic (e.g. phenylalanine in PKU)
- Alternative pathways may be activated, producing abnormal metabolites (e.g. orotic acid in OTC deficiency)
The lecture lists all 15 SSIEM groups [1]:
| # | Group | Classic Example |
|---|---|---|
| 1 | Amino acid & peptide metabolism | PKU, MSUD, homocystinuria |
| 2 | Carbohydrate metabolism | Galactosemia, GSD |
| 3 | Fatty acid & ketone body metabolism | MCAD deficiency |
| 4 | Energy metabolism | Mitochondrial disorders, PDH deficiency |
| 5 | Purine, pyrimidine & nucleotide metabolism | Lesch-Nyhan syndrome |
| 6 | Sterol metabolism | Smith-Lemli-Opitz syndrome |
| 7 | Porphyrin & haem metabolism | Acute intermittent porphyria |
| 8 | Lipid & lipoprotein metabolism | Familial hypercholesterolaemia |
| 9 | Congenital disorders of glycosylation | CDG syndromes |
| 10 | Lysosomal disorders | Gaucher, Fabry, MPS |
| 11 | Peroxisomal disorders | Zellweger syndrome, X-linked ALD |
| 12 | Neurotransmitter metabolism | AADC deficiency |
| 13 | Vitamin & cofactor metabolism | Biotinidase deficiency |
| 14 | Trace element & metal metabolism | Wilson disease |
| 15 | Xenobiotic metabolism | Variants in drug metabolism |
"Mostly autosomal recessive" [1]
- Both parents are carriers (Nd × Nd → 1/4 NN, 2/4 Nd, 1/4 dd)
- 25% chance each pregnancy
- Notable exceptions: OTC deficiency (X-linked), some mitochondrial diseases (maternal inheritance), Hunter syndrome (X-linked)
"Usually presented at birth, early infancy and childhood (but some can present in adulthood) — with nonspecific clinical symptoms (esp. in neonatal period) such as appetite, vomiting, acute or chronic encephalopathy, myopathy, hypoglycaemia or hepatic syndromes, rule out sepsis pattern." [1]
High Yield — The 'Rule-Out Sepsis' Pattern
A neonate who presents with poor feeding, lethargy, vomiting, acidosis, and a "septic" picture but whose cultures are negative should be investigated for IEM. This is one of the most important clinical pearls in paediatric chemical pathology.
"Group 1: leading to intoxication. Group 2: with energy metabolism defects. Group 3: involving complex molecules." [1]
| Group | Mechanism | Examples | Key Features |
|---|---|---|---|
| Group 1 — Intoxication | Accumulation of toxic metabolites proximal to the enzyme block | UCDs, organic acidemias, MSUD, galactosemia | Symptom-free interval after birth (toxin needs to accumulate); acute decompensation triggered by feeds/catabolism; potentially treatable by removing the toxic substrate (dietary restriction, dialysis) |
| Group 2 — Energy deficiency | Defects in energy production (mitochondria, glycolysis, gluconeogenesis) | Mitochondrial respiratory chain defects, FAO defects, GSD, PDH deficiency | Chronic symptoms (hypotonia, myopathy, cardiomyopathy); lactic acidosis; hypoglycemia; less responsive to acute dietary manipulation |
| Group 3 — Complex molecules | Defects in synthesis or degradation of complex molecules | Lysosomal storage disorders, peroxisomal disorders, CDG | Progressive, often with organomegaly, coarse facies, skeletal dysplasia; storage diseases — symptoms develop gradually |
The lecture provides an extensive list [1] (slide 35). Key indications:
- Failure to thrive
- Neurological abnormalities: developmental delay, lethargy, hypotonia, seizures, encephalopathy, stroke, psychiatric problems
- Unexplained episodic illness with "rule-out sepsis" picture
- Abnormal biochemical findings: high anion gap metabolic acidosis, ketosis, hypoglycemia, Type B lactic acidosis, hyperammonemia
- Organomegaly, cardiomyopathy, myopathy, rhabdomyolysis
- Family history of unexplained sudden death or life-threatening event
- Maternal history of AFLP or HELLP syndrome (suggests FAO defect in fetus — fetal FAO defect → maternal liver injury)
- Jaundice, Reye syndrome, unexplained liver/renal derangement
- Abnormal odour (e.g. "sweaty feet" in isovaleric acidemia, "maple syrup" in MSUD, "musty" in PKU)
"Individually rare; collectively common. Collective incidence overseas amounted to about 1:4,000 newborns screened. Local collective incidence of IEM ranges from 1 in 4,122 to 1 in 7,580 — comparable to worldwide figures." [1]
This is the fundamental justification for newborn screening — even though each IEM is rare, together they affect a significant number of newborns.
"Plasma amino acids (PAA), Plasma acylcarnitines (PAC), Urine organic acids (UOA)" [1]
| Test | Method | What It Detects | Classic Findings |
|---|---|---|---|
| Plasma amino acids (PAA) | Ion-exchange chromatography or MS/MS | Aminoacidopathies, UCDs | ↑Phe in PKU; ↑Leucine/Isoleucine/Valine in MSUD; ↓Arginine + ↑Glutamine in UCD |
| Urine organic acids (UOA) | GC-MS (gas chromatography–mass spectrometry) | Organic acidemias, FAO defects, other IEMs | ↑Methylmalonic acid in MMA; ↑3-OH isovaleric acid in IVA; ↑Orotic acid in OTC deficiency |
| Plasma acylcarnitines (PAC) | Tandem mass spectrometry (MS/MS) | FAO defects, organic acidemias | ↑C8 (octanoylcarnitine) in MCAD; ↑C3 in propionic acidemia; ↑C5 in isovaleric acidemia |
Additional tests (context-dependent):
- Plasma lactate and L:P ratio
- Plasma ammonia (free-flowing sample, on ice, processed rapidly — delay → falsely elevated)
- Very long chain fatty acids (peroxisomal disorders)
- Urine purines and pyrimidines
- CSF amino acids and neurotransmitters (for neurotransmitter defects)
- Enzyme assays (on fibroblasts, lymphocytes, or liver tissue)
- Genetic testing (increasingly the definitive step)
8. IEM Diagnosis and Management
"Confirmatory test: mainly through biochemical testing, supplemented with genetic testing." [1]
The modern approach is often to go directly to gene panels or whole exome sequencing (WES) after initial biochemical screening, particularly when:
- Biochemical results are ambiguous
- Enzyme assay requires invasive tissue (liver biopsy)
- Local genetic expertise is available (as in the OTC case) [1]
"Dietary control; Enzyme replacement therapy; Steroid treatment; Surgical: liver & kidney transplantation; Genetic counselling." [1]
| Strategy | Examples |
|---|---|
| Dietary restriction | Low-phenylalanine diet in PKU; low-protein diet in UCD/organic acidemias; galactose-free diet in galactosemia |
| Cofactor supplementation | Biotin in biotinidase deficiency; CoQ10 in CoQ10 deficiency; B12 in B12-responsive MMA |
| Enzyme replacement therapy (ERT) | Gaucher (imiglucerase), Fabry (agalsidase), Pompe (alglucosidase alfa), MPS I (laronidase) |
| Substrate reduction therapy | Miglustat in Niemann-Pick C; Nitisinone in tyrosinemia type I |
| Transplantation | Liver transplant in UCD (provides the missing enzyme); kidney transplant in some; bone marrow transplant in some lysosomal diseases |
| Steroid treatment | e.g. in infantile spasms associated with IEM (as in the CoQ10 case) |
| Emergency management | IV glucose + stop protein intake in acute hyperammonemia; dialysis; nitrogen scavengers (sodium benzoate, sodium phenylbutyrate) |
| Genetic counselling | Recurrence risk, carrier testing, prenatal diagnosis |
9. Newborn Screening (NBS)
"Newborn screening becoming the gold standard worldwide." [1]
Wilson & Jungner criteria (adapted for NBS):
- Important health problem
- Accepted treatment available
- Facilities for diagnosis and treatment available
- Recognisable latent or early symptomatic stage
- Suitable test available
- Test acceptable to the population
- Natural history of the disease understood
- Agreed policy on whom to treat
- Cost of case-finding economically balanced with cost of treatment
- Continuous process, not a "once and for all" project
"Conventional screening test: one analysis of one metabolite (e.g. PKU). Nowadays screening test: many metabolites for different diseases in one analysis — by tandem mass spectrometry (MS/MS). MS/MS might be more cost-effective." [1]
| Method | How It Works | Advantage |
|---|---|---|
| Guthrie test (conventional) | Bacterial inhibition assay for Phe on dried blood spot | Simple, cheap; one test = one disease |
| Tandem mass spectrometry (MS/MS) | Analyses amino acids and acylcarnitines simultaneously from a single dried blood spot | One blood spot → screening for 20–50+ diseases; fast; small sample volume |
"Expanded Newborn Screening" [1] (slide 38)
The HA/HK NBS program [1] (slide 39) currently screens for:
- Congenital hypothyroidism (TSH)
- G6PD deficiency (enzyme activity)
- Congenital adrenal hyperplasia (17-OHP) — in some centres
- Expanded metabolic screen by MS/MS: amino acid disorders (PKU, MSUD, etc.), organic acidemias (MMA, PA, IVA, etc.), fatty acid oxidation defects (MCAD, VLCAD, etc.)
"Preventable Morbidities & Mortalities: Biochemical and Molecular Diagnosis of Tyrosinemia Type I — Recommendation for Expanded Newborn Screening in Hong Kong — Preventable liver transplantation." [1]
This slide emphasises that tyrosinemia type I is a treatable IEM (with nitisinone = NTBC, which blocks the pathway upstream of toxic metabolite accumulation). Without screening, patients present with liver failure requiring transplant. With screening → early treatment → normal life.
10. Case 2 — Primary CoQ10 Deficiency (Infantile Spasm)
"FTNSD, AN/PN unremarkable; Chinese, non-consanguineous; first child in family, FHx unremarkable; noted developmental delay then infantile spasm at 6 months; spasm resolved after short course of steroid; P/E: microcephaly, hypotonia; no positive findings from extensive blood/urine tests." [1]
"WES (Classified as pathogenic): Compound heterozygous for COQ4 c.370G>A, p.Gly124Ser and c.371G>T, p.Gly124Val" [1]
- Whole exome sequencing (WES) was used because standard metabolic workup was negative
- COQ4 gene encodes a protein involved in CoQ10 biosynthesis
- Compound heterozygous = two different pathogenic variants, one on each allele (consistent with autosomal recessive inheritance)
"Also known as coenzyme Q10, CoQ10, ubiquinone, ubidecarone. Non-protein-bound lipid-soluble prosthetic group. Ubiquitous in animals and most bacteria. Component of electron transport chain — Carrier of both proton and electrons — Accepts electrons from complex I or II — Donates electrons to complex III." [1]
Why CoQ10 deficiency causes disease: CoQ10 is essential for shuttling electrons between complexes I/II and complex III in the mitochondrial respiratory chain. Without it → impaired oxidative phosphorylation → energy failure → affects high-energy-demand tissues (brain, muscle, heart, kidney).
"Incidence in Italy < 1:100,000. Autosomal recessive." [1]
| System | Manifestations |
|---|---|
| Neurological | Fatal neonatal encephalopathy with hypotonia; Dystonia/Spasticity; Parkinsonism/Ataxia; Seizure; Intellectual disability |
| Multisystem | Severe infantile multisystem disease |
| Renal | Steroid-resistant nephrotic syndrome |
| Cardiac | Hypertrophic cardiomyopathy |
| Muscular | Isolated myopathy |
| Ocular | Retinopathy / optic atrophy |
| Auditory | Sensorineural hearing loss |
"Potentially treatable disease. Response variable, even within same family. High dose oral CoQ10 supplements as early as possible (No serious side effects). Adult: 2400 mg/day in 3 doses. Paedi: 30 mg/kg/day in 3 doses. No human data on CNS bioavailability. Limit disease progression + Reverse some manifestations. CANNOT reverse established severe neurological/renal damage." [1]
High Yield — CoQ10 Deficiency Is TREATABLE
Primary CoQ10 deficiency is one of the few mitochondrial disorders that is potentially treatable. The key is EARLY diagnosis and EARLY supplementation. Once severe neurological or renal damage is established, CoQ10 cannot reverse it. This underscores the importance of newborn/early screening and WES in unexplained neurological presentations.
Case outcome [1]:
- Started CoQ10 at 9 months old
- Parents reported increased alertness
- Developmentally making slight progress; head control improved
- Still cannot sit alone by age 3
- CoQ10 dose escalated to 41 mg/kg/day (liposomal CoQ10 for better absorption)
11. Integration with Related Lectures
GC 154 [2] covers the general principles of screening (sensitivity, specificity, PPV, NPV, Wilson-Jungner criteria). The newborn screening section of GC 157 directly applies these principles to neonatal metabolic screening via MS/MS.
This seminar [3] provides deeper dives into specific IEMs and their biochemical diagnosis, complementing the overview in GC 157.
The data interpretation sessions [4] practice interpreting panels of biochemical results in clinical scenarios, including paediatric metabolic emergencies.
GC 078 [5] covers glucose metabolism, diabetes mellitus, and DKA. Hypoglycemia in children can present similarly to adult DKA in reverse (altered consciousness, metabolic derangement), and the approach to blood glucose abnormalities overlaps.
The electrolytes lecture [6] covers acid-base disorders and electrolyte disturbances that often accompany IEM presentations (e.g. high anion gap metabolic acidosis in organic acidemias).
12. Exam Intelligence
- Neonatal cholestasis: Distinguish conjugated from unconjugated; urgency of excluding biliary atresia
- Hypoglycemia workup: Critical sample; ketotic vs non-ketotic; GTR significance
- Hyperammonemia: UCD vs organic acidemia; orotic acid significance; OTC as X-linked
- IEM general principles: Autosomal recessive inheritance; "not small adults"; newborn screening
- Lactic acidosis: Type A vs Type B; L:P ratio interpretation
- Newborn screening: MS/MS; expanded screening; why it's done
| Trap | Why Students Fall For It | Correct Approach |
|---|---|---|
| Normal ammonia at one time point excludes UCD | Ammonia fluctuates; may be normal between crises | Look for other clues (orotic acid, amino acid patterns); repeat ammonia during illness |
| Urine dipstick negative for ketones = no ketosis | Dipstick detects acetoacetate, not beta-hydroxybutyrate | Order quantitative blood beta-hydroxybutyrate |
| Haemolysed sample insulin = true insulin level | Haemolysis degrades insulin → falsely low | Reject haemolysed sample; re-draw |
| Breast-milk jaundice explains conjugated jaundice | Breast-milk jaundice causes UNCONJUGATED hyperbilirubinaemia | Always fractionate bilirubin; conjugated = pathological |
| All IEM present in neonatal period | Late-onset forms exist (e.g. partial OTC deficiency) | Metabolic stress can unmask late-onset IEM at any age |
| All mitochondrial disorders are untreatable | CoQ10 deficiency is treatable | CoQ10 supplementation can halt disease progression |
| Scenario | OTC Deficiency | CPS1 Deficiency | Organic Acidemia |
|---|---|---|---|
| Ammonia | ↑↑ | ↑↑ | ↑ (moderate) |
| Orotic acid | ↑↑↑ | Normal/Low | Normal |
| Plasma amino acids | ↓Arginine, ↑Glutamine, ↓Citrulline | ↓Arginine, ↑Glutamine, ↓Citrulline | Variable |
| Urine organic acids | Orotic acid only | Normal | Specific organic acids (MMA, PA, etc.) |
| Anion gap | Normal | Normal | High |
| Inheritance | X-linked | AR | AR |
13. Past Paper Questions
After searching all indexed past paper files, the following questions are relevant to the topics covered in this lecture:
"A 55-year-old male suffering from acute cholangitis has a body temperature of 39 degrees Celsius, heart rate of 110 beats per minute and blood pressure of 85/40 mmHg. Which of the following blood test is MOST USEFUL to evaluate systemic tissue perfusion? A. Albumin B. Creatinine C. Lactate D. Troponin" [7]
Answer: C. Lactate
Rationale: Lactate is the best marker of tissue hypoperfusion in sepsis/shock. This is Type A lactic acidosis — with tissue hypoxia. While creatinine reflects kidney function and troponin reflects myocardial injury, lactate directly reflects the adequacy of oxygen delivery to tissues and is used for monitoring response to resuscitation. This connects to the lecture's discussion of Type A vs Type B lactic acidosis [1].
"A 60-year-old man presented with malaise and weight loss. Chest X-ray showed a mass lesion in the left upper zone. Biochemistry: ... Urea 18.6, Creatinine 172, Calcium 3.45, ALP 550... (a) What are the major biochemical abnormalities? (b) What are the underlying causes? (c) Most common ECG abnormality? (d) Briefly discuss management." [8]
Answer:
- (a) Raised urea and creatinine (renal impairment); hypercalcaemia; raised ALP
- (b) Lung cancer with bone metastases → osteolytic lesions release calcium (hypercalcaemia) and stimulate osteoblastic activity (raised ALP); hypercalcaemia causes nephrogenic DI → dehydration → pre-renal AKI
- (c) Shortened QT interval (hypercalcaemia)
- (d) IV saline rehydration; bisphosphonates (e.g. zoledronic acid); treat underlying malignancy; monitor calcium/renal function
Relevance to lecture: While this is an adult case, the biochemical interpretation of calcium, ALP, and renal function parallels the approach to paediatric biochemical problem-solving emphasised in GC 157 [1].
"A 65-year-old woman presented to A&E after a witnessed convulsion at home. Serum calcium 1.60 mmol/L, albumin 34 g/L. (a) Name two physical signs. (b) What ECG abnormality? (c) What is the adjusted calcium? (d) Name three causes." [9]
Answer:
- (a) Chvostek sign, Trousseau sign
- (b) Prolonged QT interval
- (c) Adjusted Ca = 1.60 + 0.02 × (40 − 34) = 1.72 mmol/L (still low)
- (d) Post-thyroidectomy hypoparathyroidism, vitamin D deficiency, CKD, magnesium deficiency
Relevance: Calcium homeostasis principles apply across ages. In paediatrics, hypocalcaemia can present with seizures in neonates and must be distinguished from IEM-related seizures [1].
"A 5-year-old girl was referred to a paediatrician. She had recurrent fever, cough, runny nose. Incidentally, she was noticed to be not growing well. Her height was 90 cm (< 3rd centile), head circumference 41 cm (25th centile), and weight 6.5 kg (25th centile). On physical examination, there was webbed neck and low-set ears. What is the MOST LIKELY diagnosis for her growth problems? A. Achondroplasia B. Beckwith-Wiedemann syndrome C. Marfan syndrome D. Turner syndrome" [10]
Answer: D. Turner syndrome
Rationale: Short stature + webbed neck + low-set ears in a girl = Turner syndrome (45,X). This is a chromosomal disorder — relevant to the lecture's emphasis that chromosomal diseases are an important differential in paediatric chemical pathology, including as a cause of neonatal cholestasis [1].
Note: No additional past paper questions directly testing IEM/neonatal metabolic screening were found in the indexed past papers. The above questions test related biochemical interpretation principles.
| Problem | Key Definition | Critical DDx | First Clue | Urgent Action |
|---|---|---|---|---|
| Neonatal cholestasis | Conjugated bilirubin > 20% of total | Biliary atresia vs medical causes | Pale stool + dark urine + conjugated jaundice > 2 weeks | Exclude biliary atresia; Kasai before day 60 |
| Hyperlactatemia | Lactate > 2 mmol/L | Type A (hypoxia) vs Type B (no hypoxia) | Acidosis + raised lactate | Treat underlying cause; check L:P ratio if genetic cause suspected |
| Hyperammonemia | NH₃ > 60 µmol/L | UCD vs organic acidemia vs liver failure | Encephalopathy + ↑NH₃ + ↓urea | Stop protein; IV glucose; nitrogen scavengers; dialysis if severe |
| Hypoglycemia | Glucose < 2.6 mmol/L (neonate) | Ketotic vs non-ketotic | Low glucose + altered consciousness | Critical sample THEN correct with IV dextrose; GTR > 8 = hyperinsulinism |
High Yield Summary
- Children are not small adults — reference intervals, DDx, tests, sample handling, and treatment all differ.
- Neonatal cholestasis — always pathological; exclude biliary atresia urgently (Kasai before day 60).
- Hyperlactatemia — Type A (with hypoxia) vs Type B (without hypoxia, acquired or genetic); L:P ratio helps distinguish mitochondrial defects.
- Hyperammonemia — think UCD first; check orotic acid (elevated = OTC deficiency, which is X-linked); ammonia may be normal between crises.
- Hypoglycemia — draw critical sample BEFORE correcting glucose; ketotic vs non-ketotic is the key discriminator; GTR > 8 mg/kg/min = hyperinsulinism.
- IEM — individually rare, collectively common (~1:4,000–7,500); mostly AR; presented with nonspecific symptoms mimicking sepsis.
- Three IEM groups: (1) intoxication, (2) energy deficiency, (3) complex molecules.
- Major metabolic investigations: PAA, UOA, PAC — supplemented by genetic testing.
- Newborn screening by MS/MS is cost-effective and the gold standard worldwide; Hong Kong has an expanding program.
- Primary CoQ10 deficiency is a rare but treatable mitochondrial disorder — high-dose CoQ10 can limit progression but cannot reverse established damage.
Active Recall - Paediatric Chemical Pathology
[1] Lecture slides: GC 157. Paediatric Chemical Pathology.pdf [2] Lecture slides: GC 154. Biochemical Screening for Early Detection of Diseases.pdf [3] Lecture slides: Chemical Pathology Seminar_Inherited metabolic disease 2025.pdf [4] Lecture slides: Chemical Pathology Data interpretation.pdf [5] Lecture slides: GC 078. Polyuria and polydipsia glucose metabolism, diabetes mellitus, diabetic ketoacidosis [Update 2025].pdf [6] Lecture slides: MBBS IV Electrolytes_2024.pdf [7] Past papers: 2024 Fourth Summative MCQ.pdf (Q67) [8] Past papers: 2016 Fourth Summative SAQ.pdf (Q5) [9] Past papers: 2018 Fourth Summative SAQ.pdf (Q5) [10] Past papers: 2024 Fourth Summative MCQ.pdf (Q92)
GC156 Many Of My Family Members Have Cancers Cancer Genetics And Cytogenetics (notes)
Cancer genetics and cytogenetics is the study of hereditary gene mutations, chromosomal abnormalities, and familial cancer syndromes that predispose individuals and their relatives to an increased risk of developing various malignancies.
GC158 Point-of-care Testing (POCT)
Point-of-care testing is diagnostic testing performed at or near the site of patient care, outside the central laboratory, to provide rapid results that facilitate immediate clinical decision-making.