This lecture is about thinking before you order, and thinking again when you read the result. It covers the entire journey of a laboratory test — from the clinical question that prompts the order, through pre-analytical pitfalls, analytical interferences, to post-analytical interpretation traps. The overarching philosophy is "Primum non nocere" — even lab tests can harm if misused, misinterpreted, or ordered without purpose. ^[1]

1. Understand the Laboratory Testing Cycle and its phases.
2. Know the reasons for ordering laboratory tests (confirm, aid, refine, monitor, assess, detect, stratify, optimize).
3. Appreciate reference intervals and their statistical basis — why "out of range" ≠ disease.
4. Recognise pre-analytical, analytical, and post-analytical pitfalls that produce spurious results.
5. Understand specific interference mechanisms: haemolysis, in vitro glycolysis, high-dose hook effect, biotin interference, heterophilic antibodies, macro-complexes, drug interferences.
6. Understand the role and caveats of Point-of-Care Testing (POCT).
7. Apply precision medicine concepts: pharmacogenomic testing (HLA alleles, DPD deficiency, molecular markers for targeted therapy).

This lecture is a favorite source for MCQs testing:

• Identification of spurious results
• Pre-analytical errors (e.g., EDTA contamination, drip-arm sampling, delayed specimen processing)
• Knowledge of reference intervals and their statistical definition
• When POCT is appropriate and its pitfalls
• Pharmacogenomics examples (HLA B15:02, HLA B58:01)

Past paper Q7 (2021) directly tested assay interference recognition. Q6 (2025) tested sensitivity vs. specificity in screening vs. confirmatory contexts. ^[6][7]

• ~5% of normal subjects will have results outside the reference interval — this is by definition, not disease
• Deviation from the reference interval does NOT necessarily mean disease; the converse is also true (i.e., a "normal" result does not exclude disease)
• Reference intervals vary by age, gender, race, and test methodology → values from literature may not be locally applicable
• Always interpret in the proper clinical context

Why this is exam-critical: A common MCQ trap is presenting a result slightly outside the reference interval and asking whether the patient has disease. The answer depends on clinical context, not the number alone.

Rule of thumb: If results from specimens collected on different occasions differ by more than 2.77 × total SD (analytical + biological variation combined), there is > 95% chance of a genuine change. ^[1]

Why this matters: When monitoring a patient (e.g., serial creatinine in AKI), small fluctuations may be within normal analytical/biological variation. Only changes exceeding this threshold should prompt clinical action.

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A 46-year-old woman on steroids for RA, blood sample taken at the Rheumatology Clinic on a Saturday morning: Na 128, K 6.7, Cl 89, Urea 5.0, Creatinine 105, Glucose 2.3. ^[1]

Looks like Addisonian crisis (low Na, high K, low glucose). BUT — the clue is "Saturday morning at the Rheumatology Clinic." This was likely an EDTA-contaminated specimen (e.g., blood drawn into EDTA tube then decanted into a biochemistry tube, or cross-contamination). EDTA causes spuriously low Ca and high K. The low glucose is from in vitro glycolysis due to delayed processing over the weekend.

How to spot this in exams: Whenever Na is low, K is very high, and Cl is low but urea/creatinine are normal → think EDTA contamination + delayed specimen processing, not genuine electrolyte disorder.

In "one-stage 2-site sandwich immunometric assays," gross antigen excess can saturate both the capture and detection antibodies independently (rather than forming the intended sandwich), resulting in a falsely LOW or NORMAL measured value despite extremely high actual concentration. ^[1]

AFP, Ferritin, Prolactin, β-hCG, and other tumour markers ^[1]

55-year-old man, AFP was 24.0 ng/mL pre-operatively, then "rose" to 136 ng/mL 2 weeks after left hepatic lobectomy for HCC. Paradoxical? Actually, the pre-operative AFP was falsely low due to hook effect. The true pre-operative AFP was astronomically high, and the post-operative 136 was the declining (but still elevated) true value after tumour debulking. ^[1]

Exam trap: When a tumour marker is "unexpectedly low" in the setting of known high-volume malignancy, always consider the hook effect. Request a diluted specimen to unmask the true value.

Many modern immunoassays use the biotin-streptavidin interaction as part of their chemistry (biotin binds streptavidin with extremely high affinity, used to anchor reagents). Exogenous biotin in the patient's blood competes with the assay's biotin-labelled reagents for streptavidin binding sites.

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Why opposite directions?

• In sandwich assays: Biotin displaces the biotin-labelled detection antibody from the streptavidin-coated surface → less signal captured → falsely low reading
• In competitive assays: Biotin displaces the biotin-labelled analyte analogue from streptavidin → less analogue bound → system interprets this as more patient analyte present → falsely high reading

A patient taking high-dose biotin supplements (common in hair/nail health products, or therapeutic doses in multiple sclerosis) could present with:

• Low TSH + High fT4 → mimicking thyrotoxicosis when the patient is actually euthyroid
• This has led to inappropriate thyroid treatment in real cases

Rapid screening kits are commercially available (e.g., VeraTest Biotin®) — serum biotin > 15 ng/mL gives a positive screen result ^[1]

Management: Ask patients to stop biotin supplements for at least 48-72 hours before blood tests.

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Suspect spurious results when: ^[1]

1. Result is clinically unexpected and inconsistent with other correlates
2. Inconsistent with other parameters in multi-parameter testing
3. Significant unexpected change from previous test
4. Grossly and permanently "abnormal" in an apparently healthy subject
5. Increased likelihood of interference: recent immunisation, blood transfusion, immunotherapy, imaging with fluorescent compounds, prolonged contact with animals (heterophilic antibodies)

POCT is clinical laboratory testing conducted close to the site of patient care, providing rapid turnaround of test results for prompt patient management. ^[1]

Why POCT exists: In conventional laboratory testing, specimen delivery to the lab constitutes a significant proportion of the total turnaround time (TAT). For time-critical decisions (e.g., DKA management, blood gas in respiratory failure, glucose in hypoglycaemia), waiting for central lab results could be dangerous.

Errors in POCT diagnostics can lead to misdiagnosis, improper treatment, costly follow-up, death, and medico-legal consequences. ^[1]

Context: CF patients require sweat chloride testing (sweat test). POCT conductivity-based sweat analyzers can give misleading results if sweat volume is insufficient or if the skin is contaminated. Always ensure adequate sweat stimulation (pilocarpine iontophoresis) and volume.

Context: Pre-eclampsia workup includes urine protein (spot protein:creatinine ratio or 24-hour urine), LFT, CBP/platelets, LDH, urate. Be aware that haemolysis in the specimen may affect results and that pregnancy itself alters reference intervals (e.g., ALP is physiologically elevated from placental isoenzyme, creatinine is lower due to increased GFR).

Context: In neonatal jaundice, the key test is total and direct bilirubin. Be aware that POCT transcutaneous bilirubinometry may be inaccurate in certain skin tones or after phototherapy. Always confirm with laboratory serum bilirubin if values are near exchange transfusion thresholds.

Context: After IV iron (isomaltoside) administration, serum ferritin rises dramatically (iron is being stored). A dramatically elevated ferritin post-IV-iron does NOT indicate iron overload — it reflects the administered dose. Wait at least 7-14 days after IV iron before rechecking iron studies.

Context: HbA1c is unreliable in the presence of anaemia, particularly if there is active/recent blood loss, haemolysis, or recent transfusion. This is because HbA1c reflects average glucose over the RBC lifespan (~120 days). If RBC lifespan is shortened (haemolysis, bleeding) or if new non-glycated RBCs flood the circulation (transfusion, iron replacement after bleeding), HbA1c will be falsely low. Use fructosamine or glycated albumin as alternatives. ^[1]

Context: In patients with heavy proteinuria, proteins lost in urine include albumin → low serum albumin → altered total calcium (but ionised calcium may be normal). Also, in progressing nephropathy, creatinine-based eGFR becomes less reliable and cystatin C-based equations may be preferred. Additionally, the HbA1c may be affected by uraemia (carbamylated haemoglobin interferes with some assays).

• When investigating anaemia, CBC parameters (Hb, MCV, MCH) are the starting point; reticulocyte count helps distinguish decreased production from increased destruction/loss ^[3]
• Iron studies (serum iron, TIBC, ferritin) are interpreted together — TIBC is the most useful test to distinguish iron deficiency from anaemia of chronic disease ^[3]

• Reinforces the POCT concepts covered in this lecture
• Emphasises governance: POCT must have quality assurance, regular calibration, and operator training ^[1]

• Coagulation tests: PT (extrinsic pathway, Factor VII), APTT (intrinsic pathway, Factors VIII, IX, XI, XII), TT (fibrinogen → fibrin), fibrinogen, D-dimer ^[4]
• Mixing study: 1:1 mix with normal plasma → if clotting time corrects, it's factor deficiency; if not, it's an inhibitor ^[4]

• LFT interpretation: hepatocellular (↑ALT, ↑AST) vs cholestatic (↑ALP, ↑GGT) vs synthetic failure (↓albumin, ↑PT) ^[5]
• AST:ALT ratio > 2:1 → alcoholic liver disease ^[5]
• AKI vs CKI differentiation: CKD has ↑PO₄, ↓Ca²⁺, normocytic anaemia, small kidneys on USS ^[2]

1. "Normal" result excludes disease → FALSE. 5% of normal people fall outside reference intervals; conversely, early disease may have "normal" results.
2. EDTA tube used for chemistry → Gives falsely low Ca²⁺, Mg²⁺, ALP and falsely high K⁺. A very common exam scenario.
3. Drip-arm blood → Dilution artefact. Results are meaningless. Always collect from the opposite arm.
4. Hook effect → Low/normal tumour marker in advanced cancer = hook effect. Request serial dilutions.
5. Biotin → Low TSH + High fT4 ≠ thyrotoxicosis. Ask about supplements.
6. HbA1c in anaemia/haemolysis/transfusion → Unreliable. Use fructosamine or glycated albumin.
7. POCT glucose in peritoneal dialysis patients → GDH-PQQ meters cross-react with icodextrin → falsely elevated glucose → masked hypoglycaemia.
8. Stress-induced prolactin elevation → Not a prolactinoma. Repeat after rest, or measure macroprolactin.

When asked "What are the possible causes of this spurious result?", structure your answer:

1. State the phase of the lab testing cycle where the error occurred (pre-analytical > analytical > post-analytical)
2. Name the specific factor (e.g., EDTA contamination, haemolysis, delayed processing)
3. Explain the mechanism (e.g., EDTA chelates divalent cations; K₂-EDTA releases K⁺ into specimen)
4. State the expected direction of change (e.g., ↓Ca, ↓Mg, ↑K)