GC029 Am I Prescribing The Right Drug
A clinical decision-making framework that guides physicians in evaluating whether a selected medication is the most appropriate choice for a patient based on indication, efficacy, safety profile, patient-specific factors, and evidence-based guidelines.
Am I Prescribing the Right Drug?
This lecture by Prof. Bernard Cheung (Division of Clinical Pharmacology & Therapeutics, HKU) is a foundational clinical pharmacology session that every medical student must internalize before touching a drug chart. The core message is deceptively simple: prescribing is not just about choosing a drug — it's about choosing the right drug, for the right patient, at the right dose, via the right route, at the right time, while weighing efficacy, safety, tolerability, and cost-effectiveness. Errors kill people. This lecture systematically walks through:
- What a prescription actually is (legal document, not a casual note)
- Who can prescribe
- Elements of a valid prescription
- Ten principles of good prescribing (Aronson 2017)
- Prescription errors — who is at risk, who makes them, what types exist
- The 5 Rights of medication administration
- The Swiss Cheese Model of error
- Personalized medicine — pharmacogenetics, benefit-risk analysis
- Cost-effectiveness analysis (including ICER calculation)
- Quality prescribing framework
- The Prescribing Skills Assessment (PSA) tool
This lecture is highly examinable — it covers principles that underpin every drug question in MCQs, SAQs, minicases, and OSCEs. It integrates with GC 070 (Is this the best drug for me?), GC 079 (Prescribing in older people), GC 043/Block A (Drugs and the Kidney), and the Block A Clinical Pharmacology lectures.
A prescription (from Latin praescribere = "to write before") is a written drug order completed BEFORE the drug is prepared for the patient. Rx comes from Latin Recipere (= "to take"). [1]
Why This Matters
A prescription is a legal document. It carries medicolegal weight. An incomplete, ambiguous, or incorrect prescription can lead to patient harm and legal liability. Every element exists for a safety reason.
Who Can Write Prescriptions?
Registered doctors are allowed by law to write prescriptions. Other professionals are allowed, with limited rights, to prescribe. [1]
In Hong Kong, this includes dentists (for dental procedures) and in some jurisdictions, nurse prescribers or pharmacist prescribers with specific training and limited formularies. But for exam purposes: doctors have the broadest prescribing rights.
2. Elements of a Prescription
The elements of a prescription are: Date, Patient's identity, Rx/"please take", Drug name, Formulation, Dosage amount and frequency, Route, Additional directions, and Signature. [1]
| Element | Why It's Required | Common Error |
|---|---|---|
| Date | Establishes when the order was made; legal requirement; important for duration-limited drugs | Missing date → pharmacist cannot dispense |
| Patient's identity | Prevents wrong-patient errors | Using bed number instead of name + DOB/ID |
| Rx / "Please take" | Formal instruction to dispense | Omission |
| Drug name | Must be unambiguous; prefer generic names | Illegible handwriting; brand vs. generic confusion |
| Formulation | Tablet ≠ capsule ≠ syrup ≠ injection | Wrong formulation → wrong bioavailability |
| Dosage + frequency | Determines therapeutic vs. toxic levels | Missing frequency; "daily" vs. "BD" |
| Route | Oral, IV, IM, SC, topical, inhaled, rectal, etc. | Giving IV drug orally or vice versa |
| Additional directions | e.g., "with food," "before meals" | Missing → reduced efficacy or increased side effects |
| Signature | Legal requirement; identifies prescriber | Unsigned prescriptions are invalid |
Formulations include: Tablets, Capsules, Syrup, Injection, Drops, Inhaler, Suppository. [1]
Why this matters: The same drug in different formulations can have vastly different bioavailability. For example, oral morphine has ~30% bioavailability vs. IV morphine at 100%. Prescribing "morphine 10mg" without specifying route and formulation is dangerous.
For PRN (as needed) prescriptions, in addition to the standard elements, you must specify: Indication, Maximum dose to give, Minimum interval between doses, Maximum dose in 24 hours. [1]
Exam Trap: PRN Prescribing
A common exam scenario: "A patient is prescribed paracetamol PRN. What additional information must be on the drug chart?" Answer: indication (e.g., pain/fever), max single dose (1g), minimum interval (4-6 hours), and max 24-hour dose (4g). Without these, a nurse could theoretically give unlimited doses.
State the total duration of treatment or total amount for: Antibiotics, Steroids, Opioids, Hypnotics. [1]
Why these four specifically?
- Antibiotics: Fixed courses to prevent resistance; prolonged use → superinfection, C. diff
- Steroids: Must be tapered; prolonged use → adrenal suppression, Cushing's
- Opioids: Addiction risk; need clear end-point
- Hypnotics: Dependence and tolerance; should be short-term only
Ten Principles of Good Prescribing: [1]
- Be clear about the reasons for prescribing
- Take into account the patient's medication history before prescribing
- Take into account other factors that might alter the benefits and harms of treatment
- Take into account the patient's ideas, concerns, and expectations
- Select effective, safe, and cost-effective medicines individualized for the patient
- Adhere to national guidelines and local formularies where appropriate
- Write unambiguous legal prescriptions using the correct documentation
- Monitor the outcomes of treatment, both beneficial and adverse
- Communicate and document prescribing decisions and the reasons for them
- Prescribe within the limitations of your knowledge, skills, and experience
Let me unpack these because they are directly examinable:
| Principle | First-Principles Explanation | Clinical Example |
|---|---|---|
| 1. Clear reasons | Every drug must have an indication. Don't prescribe "just in case." | Don't give antibiotics for viral URTI |
| 2. Medication history | Know what the patient is already on → avoid duplications, interactions | Adding ACEI when patient already on ARB → dual RAAS blockade → hyperkalemia, AKI |
| 3. Other factors | Comorbidities, pregnancy, renal/hepatic function, age | NSAIDs in CKD → further renal damage [2] |
| 4. Patient ICE | Shared decision-making. Patient may refuse or have concerns | Patient worried about weight gain on insulin → discuss alternatives |
| 5. Effective + safe + cost-effective | The "quality prescribing" triad plus cost | Generic metformin vs. branded → same efficacy, lower cost |
| 6. Guidelines/formularies | Evidence-based practice; don't reinvent the wheel | Following STOPP/START criteria in elderly [3] |
| 7. Unambiguous prescriptions | Prevents misinterpretation | Writing "units" not "U" (U can be misread as 0) |
| 8. Monitor outcomes | Both therapeutic response AND adverse effects | Checking INR on warfarin; checking K+ on ACEI |
| 9. Communicate + document | Other team members need to know your reasoning | Drug chart notes, discharge summaries |
| 10. Prescribe within limits | If unsure, ask a senior. Don't guess. | Junior doctor unsure about chemotherapy → refer to oncologist |
4. Prescription Errors
The likely victims of prescription errors are: Children, Mentally handicapped, Elderly, Nursing homes, ICU patients. [1]
Why these groups?
- Children: Weight-based dosing required; small errors → proportionally large overdoses; limited ability to report symptoms
- Mentally handicapped: Cannot advocate for themselves; may not report adverse effects
- Elderly: Polypharmacy, altered pharmacokinetics, cognitive impairment
- Nursing homes: Multiple carers, communication gaps, polypharmacy
- ICU: Multiple IV infusions, critically ill with altered physiology, high-risk drugs
Elderly patients are on multiple drugs, have chronic illnesses (e.g., hypertension, diabetes, COAD), are more susceptible to adverse effects, may have cognitive problems, and may require assistance in taking medications. [1]
This connects directly to GC 079 (Prescribing in Older People), which gives the prescribing process as:
The right drug → The right dose → The right route → Check allergy → Check RFT → Aware of side effects → Start low & Go slow [3]
High Yield: Prescribing in the Elderly
The principle "Start low, go slow" is a mantra for geriatric prescribing. Elderly patients have reduced renal clearance, reduced hepatic metabolism, increased body fat (affecting Vd of lipophilic drugs), reduced albumin (more free drug), and altered receptor sensitivity. Always check eGFR before prescribing renally-cleared drugs.
The highest rates of Medication Incident Reports (MIRs) were in July (16.5%, 10.2%, and 11.9% in July 2004, 2005, and 2006). [1]
Why July? This is when new house officers (interns) start. They are the least experienced prescribers. This is a well-documented phenomenon worldwide — the "July effect."
In 42.6% of MIRs, house officers were involved. [1]
This is a stark reminder: junior doctors are the most frequent source of prescription errors. The chart shows HOs have the highest absolute number of MIRs. This is why the PSA exists — to train and assess prescribing competence before you start working.
Two types of errors: (1) Lack of knowledge of what is right or wrong; (2) Has the knowledge but makes an unintentional mistake. [1]
This is a crucial distinction:
- Knowledge-based errors (Type 1): You didn't know metformin is contraindicated in eGFR < 30. Solution: education.
- Execution errors/slips (Type 2): You knew the right dose but wrote the wrong number because you were distracted. Solution: systems (checklists, electronic prescribing, double-checks).
Both types are addressed by the Swiss Cheese Model.
Patients are entitled to 5 rights: Right patient, Right medication, Right dose, Right time, Right route. [1]
| Right | How to Ensure It | Common Failure |
|---|---|---|
| Right patient | Check name + DOB/ID number (at least 2 identifiers); never use bed/room number | Drug given to wrong patient after bed swap |
| Right medication | Use generic name; check allergy status; verify indication | Sound-alike/look-alike drugs (e.g., chlorproMAZINE vs. chlorproPAMIDE) |
| Right dose | Weight-based in children; adjust for renal/hepatic function; check BNF | 10x overdose due to decimal point error |
| Right time | Correct frequency; time-critical drugs (e.g., antibiotics) | Missing doses; wrong timing relative to meals |
| Right route | Verify formulation matches route | IV potassium given too fast → cardiac arrest |
Double-Check Identity
Ask for patient's name and one other detail such as DOB or ID number. Check at least 2 of these (name, DOB, ID number). Do not use bed or room number. [1]
Why Not Bed Number?
Bed numbers change constantly — patients are moved, swapped, transferred. A bed number is an attribute of the location, NOT the patient. Using it as an identifier has caused fatal errors (e.g., giving chemotherapy to the wrong patient). Always use patient-intrinsic identifiers: name, DOB, hospital ID number.
A medication error is a failure in the treatment process that leads to, or has the potential to lead to, harm to the patient. (Ferner et al. Drug Saf. 2006, Reason. BMJ. 2000) [1]
The Swiss Cheese Model (James Reason) conceptualizes safety as multiple layers of defense (like slices of Swiss cheese). Each layer has holes (weaknesses). An error reaches the patient only when the holes in ALL layers align simultaneously.
The layers in medication safety include:
Drug Safety — everyone is a stakeholder: Pharmaceuticals, Hospitals & Clinics, Prescriber, Pharmacist, Nurse or carer, Patient, Regulatory bodies & government, Drug store. [1]
| Layer | Role in Error Prevention |
|---|---|
| Regulatory bodies | Drug approval, safety monitoring, post-marketing surveillance |
| Pharmaceuticals | Drug design, labeling, packaging (tall-man lettering for look-alikes) |
| Hospitals | Protocols, electronic prescribing, formulary restrictions |
| Prescriber | Knowledge, careful prescribing, checking allergies/interactions |
| Pharmacist | Verifying prescriptions, flagging errors, dose-checking |
| Nurse/carer | Final check before administration; verifying 5 Rights |
| Patient | Questioning unfamiliar drugs; reporting side effects |
Key insight: No single layer is perfect. Safety comes from redundancy — multiple checks catching what earlier layers missed.
Severity levels: S0 = error discovered before administration; S1 = did not result in patient injury; S2 = increasing monitoring but no change in vital signs and no injury; S3 = increasing monitoring, change in vital signs but no ultimate injury; S4 = required antidote or reversal or transfer to higher level of care; S5 = permanent injury; S6 = death. [1]
| Level | Description | Example |
|---|---|---|
| S0 | Caught before reaching patient | Pharmacist catches wrong dose |
| S1 | Reached patient, no harm | Extra dose of paracetamol, patient fine |
| S2 | Increased monitoring, no harm | Wrong antibiotic → monitored, no clinical effect |
| S3 | Vital sign change, no permanent harm | Hypotension from antihypertensive overdose |
| S4 | Antidote/reversal needed | Naloxone for opioid overdose; flumazenil for benzo |
| S5 | Permanent injury | Renal failure from nephrotoxic drug |
| S6 | Death | Fatal anaphylaxis to known allergen |
The bar chart in the lecture shows most errors are S0-S1 (caught early or no harm), but even a few S5-S6 events are devastating. The goal is to catch errors at S0.
The lecture slide shows a pie chart of error types. Based on the prescribing literature and lecture context, common types include:
| Error Type | Proportion | Example |
|---|---|---|
| Wrong dose | Most common | 10x dose due to decimal point |
| Wrong drug | Common | Confusing drug names |
| Wrong frequency | Common | QD written instead of BD |
| Wrong route | Less common but dangerous | Oral drug given IV |
| Wrong patient | Dangerous | Same surname, wrong patient |
| Omission | Common | Missing a regular medication |
| Wrong duration | Common with antibiotics | 3 days instead of 7 days |
| Drug interaction | Knowledge-based | Adding clarithromycin to patient on statins |
9. Personalized Medicine
The benefits and the risks of a drug may be different in different patients. "What's one man's meat is another man's poison." [1]
This is the fundamental concept underlying personalized/precision medicine. A drug that works well in one patient may be ineffective or toxic in another due to:
Complex interplay between genetic and environmental factors affects the fate and effect of drugs. Factors include: Genetics, Demographics (sex, age, body weight), Concomitant conditions (e.g., renal failure), Interactions with food and other drugs — all affecting pharmacokinetics and pharmacodynamics. [1]
| Factor | Pharmacokinetic Effect | Pharmacodynamic Effect |
|---|---|---|
| Age | ↓ renal/hepatic clearance in elderly | ↑ sensitivity to benzodiazepines in elderly |
| Body weight | ↑ Vd for lipophilic drugs in obese | — |
| Renal failure | ↓ clearance of renally-excreted drugs | ↑ sensitivity to CNS effects (e.g., gabapentin accumulation) |
| Genetics | CYP2C19 poor metabolizer → ↓ clopidogrel activation | HLA-B*5801 → allopurinol hypersensitivity |
| Drug interactions | CYP3A4 inhibition by ketoconazole → ↑ statin levels | Additive sedation: opioid + benzodiazepine |
| Food | Grapefruit juice inhibits CYP3A4 → ↑ drug levels | High-vitamin-K diet → ↓ warfarin effect |
Using pre-treatment characteristics including sex, age, etc.; Using initial treatment response as a guide; Genetics/mutations; Receptor status. [1]
Examples:
- Pre-treatment characteristics: ACEI works better in younger Caucasians; CCBs work better in elderly/Black patients (lower renin states)
- Initial response: If a patient doesn't respond to an SSRI after 4-6 weeks, switch rather than persist
- Genetics: HER2-positive breast cancer → trastuzumab; EGFR-mutant lung cancer → gefitinib
- Receptor status: ER-positive breast cancer → tamoxifen
Key pharmacogenetic tests: [1]
| Drug | Gene Test | Why |
|---|---|---|
| Carbamazepine | HLA-B*1502 (mandatory) | Risk of Stevens-Johnson Syndrome/TEN, especially in Han Chinese/Southeast Asian populations |
| Allopurinol | HLA-B*5801 | Severe cutaneous adverse reactions (SCAR); very high prevalence in Han Chinese (~6-8%) |
| Abacavir | HLA-B*5701 | Hypersensitivity reaction; testing virtually eliminates the risk |
| Azathioprine / 6-Mercaptopurine | TPMT | Deficiency → accumulation of toxic 6-TGTP → severe myelosuppression [4] |
| Clopidogrel | CYP2C19 | Poor metabolizers cannot activate the prodrug → reduced antiplatelet effect → ↑ cardiovascular events [5] |
| Warfarin | VKORC1, CYP2C9 | Determines dose sensitivity; VKORC1 variants → lower dose needed; CYP2C9 poor metabolizers → slower clearance |
| Statins | SLCO1B1 | Variants → ↑ risk of myopathy, especially with simvastatin |
Mandatory Testing: HLA-B*1502 for Carbamazepine
In Hong Kong (and all Southeast Asian populations), HLA-B*1502 testing is MANDATORY before prescribing carbamazepine. The risk of SJS/TEN is dramatically higher in carriers. This is a direct exam question.
TPMT and Azathioprine — deeper explanation from supporting notes [4]:
- Azathioprine is metabolized by three pathways. TPMT converts active metabolite 6-MP into inactive 6-MMP.
- If TPMT is deficient (1 in 300 people), most azathioprine is shunted toward the toxic 6-TGTP pathway → bone marrow suppression, neutropenic fever.
- NUDT15 testing is also important, especially in Hong Kong/Asian populations (more common deficiency).
- Also check if patient is on xanthine oxidase inhibitors (allopurinol, febuxostat) → these block another clearance pathway (XO), further increasing toxic metabolite accumulation.
11. Benefit-Risk Analysis
Benefit-risk matrix: [1]
- Good efficacy + Low risk of harm → PRESCRIBE
- Good efficacy + High risk of harm → PRESCRIBE (consider carefully)
- Poor efficacy + Low risk of harm → DON'T PRESCRIBE (X)
- Poor efficacy + High risk of harm → DON'T PRESCRIBE (X)
For severe disease (e.g., cancer), even drugs with low benefit-to-risk ratio may be worth prescribing. For mild disease (e.g., dermatitis), only high benefit-to-risk drugs should be used. [1]
| Severe Disease (e.g., Cancer) | Mild Disease (e.g., Dermatitis) | |
|---|---|---|
| High benefit-to-risk | Prescribe | Prescribe |
| Low benefit-to-risk | Prescribe with caution + discuss with patient | Do NOT prescribe (X) |
Key principle: The acceptable level of risk scales with the severity of the disease. You'd accept more side effects from chemotherapy for metastatic cancer than from a cream for mild eczema.
The approach to informing patients depends on severity AND frequency of adverse effects: [1]
| Common | Rare | |
|---|---|---|
| Severe | Prescribe but inform patient | Prescribe with caution and warn patient |
| Mild | Prescribe — no need to inform | Should not be prescribed (cost-benefit unfavorable for rare but unexplained mild effects) |
Wait — the lecture's matrix for rare + mild says "should not be prescribed." This seems counterintuitive. The logic is: if a side effect is mild but genuinely rare, it probably means the drug itself has other issues that make it not worth using. However, in practice, most interpretations focus on: always warn about common effects and severe effects, regardless of frequency.
Practical Interpretation
For exams, remember:
- Common + severe → Must inform the patient before prescribing
- Rare + severe → Must warn (e.g., SJS with carbamazepine, agranulocytosis with clozapine)
- Common + mild → Mention but reassure (e.g., GI upset with metformin)
- Rare + mild → Often not clinically significant enough to mention routinely
12. Cost-Effectiveness Analysis
Costs are not just drug price. They include: cost of administering the drug, cost of monitoring, and cost of adverse effects. Costs depend on perspective (who is paying). [1]
Benefits depend on perspective. Future benefits are less valuable than immediate ones (discounting). A useful measure of benefit is QALY (Quality-Adjusted Life Year). [1]
QALY explained from first principles:
- 1 QALY = 1 year of perfect health
- If a treatment gives you an extra year of life but at 50% quality (e.g., bedridden), that's 0.5 QALY
- Treatment may improve survival (extend life) AND/OR increase quality of life (better function even if no extra years)
The Incremental Cost-Effectiveness Ratio (ICER) = ΔCost / ΔEffect
Example: Antibiotic A costs 80 with 80% efficacy. Per 100 patients: A costs 8,000 → 80 cured. Extra 2,000/20 = $100 per extra cure. [1]
The exam question answer is C: $100 per additional chest infection cured.
ICER Calculation — Exam Must-Know
ICER = (Cost_B - Cost_A) / (Effect_B - Effect_A)
In this example: (60) / (0.80 - 0.60) = 100 per additional cure
Or equivalently per 100 patients: (6000) / (80 - 60) = 100
A treatment is considered cost-effective if ICER falls below the "willingness-to-pay threshold" (varies by country/healthcare system).
The cost-effectiveness plane has 4 quadrants:
| Quadrant | ΔCost | ΔEffect | Decision |
|---|---|---|---|
| NE (upper right) | More costly | More effective | Depends on ICER vs. threshold |
| SE (lower right) | Less costly | More effective | Dominant — always adopt |
| NW (upper left) | More costly | Less effective | Dominated — never adopt |
| SW (lower left) | Less costly | Less effective | Depends on ICER vs. threshold |
Quality prescribing considers: Efficacy, Safety, Tolerability, and Cost-effectiveness. [1]
| Pillar | Definition | Example |
|---|---|---|
| Efficacy | Does the drug work for the intended indication? | Ramipril reduces mortality in HFrEF |
| Safety | What are the serious adverse effects? | Clozapine → agranulocytosis (requires monitoring) |
| Tolerability | What are the common, bothersome side effects? | Metformin → GI upset (many patients stop it) |
| Cost-effectiveness | Is the benefit worth the cost? | Generic simvastatin vs. branded rosuvastatin |
Efficacy ≠ Tolerability: A drug can be very effective but poorly tolerated (e.g., full-dose aspirin for analgesia — effective but GI bleeding). Conversely, a drug can be well-tolerated but have poor efficacy for a given indication.
The lecture emphasizes that drug safety is a shared responsibility:
Drug Safety stakeholders: Pharmaceuticals, Hospitals & Clinics, Prescriber, Pharmacist, Nurse or carer, Patient, Regulatory bodies & government, Drug store. [1]
This maps directly onto the Swiss Cheese Model — each stakeholder is a "slice" that can catch errors.
The PSA is an online training and assessment package to promote better prescribing skills, bridging the gap between basic pharmacology knowledge and clinical application, using realistic clinical cases with assessment-scoring-feedback cycles. [1]
The PSA has 8 sections across 60 items in 120 minutes (200 marks):
| Section | Content | What It Tests |
|---|---|---|
| 1 | Prescribing | Writing prescriptions correctly |
| 2 | Prescription Review | Identifying errors in existing prescriptions |
| 3 | Planning Management | Selecting appropriate drug therapy |
| 4 | Providing Information | Counseling patients about drugs |
| 5 | Calculation Skills | Drug dose calculations |
| 6 | Adverse Drug Reactions | Recognizing and managing ADRs |
| 7 | Drug Monitoring | Interpreting drug levels, lab monitoring |
| 8 | Data Interpretation | Using clinical data to guide prescribing |
Specialties covered: MED, SURG, ELD (elderly), PED, PSYCH, O&G, GP [1].
16. Integration with Related Lectures
The elderly prescribing checklist from GC 079 directly extends the principles in this lecture:
- STOPP criteria — drugs that should be STOPPED in elderly (potentially inappropriate)
- START criteria — drugs that should be STARTED in elderly (potentially beneficial but often omitted)
- Beers Criteria — AGS list of potentially inappropriate medications in older adults
Key renal thresholds from STOPP/START V3 [3]:
| Drug | eGFR Threshold | Risk |
|---|---|---|
| Digoxin ≥ 125µg/day long-term | < 30 | Digoxin toxicity |
| Dabigatran | < 30 | Bleeding |
| Rivaroxaban/Apixaban/Edoxaban | < 15 | Bleeding |
| NSAIDs | < 50 | Renal deterioration |
| Metformin | < 30 | Lactic acidosis |
| Spironolactone/Eplerenone | < 30 | Hyperkalemia |
| Bisphosphonates | < 30 | Acute renal failure |
| Nitrofurantoin | < 45 | Drug toxicity |
| Methotrexate | < 30 | Methotrexate toxicity |
Three principles of drug prescribing in renal impairment:
- Avoid further nephrotoxic insult (e.g., NSAIDs, aminoglycosides)
- Attention to correct dose (adjust for eGFR)
- Beware of side effects amplified in renal failure (e.g., ethambutol → optic neuritis; acyclovir, quinolones, imipenem → CNS side effects)
- CYP450 enzyme interactions (CYP3A4 metabolizes ~50% of drugs)
- Type A (augmented, dose-dependent) vs. Type B (bizarre, idiosyncratic) adverse drug reactions
- CYP2C19 and clopidogrel — poor metabolizers need alternative antiplatelet therapy
17. Exam Intelligence
| Question Type | Example Stem |
|---|---|
| MCQ | "Which of the following is NOT one of the 5 Rights?" |
| MCQ | "A 70-year-old with eGFR 25 is prescribed metformin. What is the concern?" |
| SAQ | "List the elements of a valid prescription" |
| SAQ | "Calculate the incremental cost-effectiveness ratio given..." |
| SAQ | "Name 3 pharmacogenetic tests and the drugs they relate to" |
| Minicase | "A new HO prescribes amoxicillin to the wrong patient. Discuss using the Swiss Cheese Model" |
| OSCE | "Write a prescription for this patient" / "Review this prescription and identify errors" |
| Trap | Why Students Fall For It | Correct Answer |
|---|---|---|
| Using bed number to identify patient | Seems convenient | Never — use name + DOB/ID |
| Forgetting PRN parameters | Write drug + dose only | Must include indication, max dose, min interval, max 24h dose |
| Confusing cost-effectiveness with cheapest drug | Cheapest isn't always best | ICER considers both cost AND effectiveness |
| Thinking pharmacogenetic testing is optional for carbamazepine | Recall that it's just "recommended" | Mandatory for HLA-B*1502 before carbamazepine in Asian populations |
| Ignoring renal function when prescribing | "The drug is commonly used" | Always check eGFR — many common drugs need dose adjustment |
From past Fourth Summative papers, prescribing questions commonly test:
- Prescription writing errors (SAQ/minicase format)
- Dose adjustment in renal impairment
- Drug interactions and ADRs
- Pharmacogenetics (HLA-B1502, HLA-B5801, TPMT)
- STOPP/START criteria in elderly
- Cost-effectiveness calculations
-
"List the 5 Rights of medication administration."
- Right patient, right medication, right dose, right time, right route
-
"Why should bed numbers never be used to identify patients?"
- Bed numbers are attributes of location, not the patient; patients are moved/transferred; using bed numbers has caused fatal medication errors
-
"An antibiotic costing 90 and treats 60%. Calculate the ICER."
- ICER = (50)/(0.6-0.4) = 200 per additional cure
-
"Name 4 drug classes for which total duration must be stated on the prescription."
- Antibiotics, steroids, opioids, hypnotics
-
"List 3 pharmacogenetic drug-gene pairs with clinical significance."
- Carbamazepine/HLA-B1502; Allopurinol/HLA-B5801; Azathioprine/TPMT
-
"What are the two types of prescribing errors?"
- Knowledge-based errors (don't know what's correct) and execution errors (know what's correct but make an unintentional mistake)
-
"Explain the Swiss Cheese Model in the context of medication safety."
- Multiple layers of defense (prescriber, pharmacist, nurse, patient, system) each have weaknesses; errors reach the patient only when weaknesses in all layers align simultaneously; safety depends on redundancy
-
"A junior doctor wants to prescribe paracetamol PRN. What must be specified?"
- Indication, maximum single dose, minimum interval between doses, maximum 24-hour dose
High Yield Summary
Prescribing is a LEGAL act with LIFE-OR-DEATH consequences. Every prescription must contain: date, patient identity, drug name, formulation, dose, frequency, route, additional directions, and signature. Patients have 5 Rights (patient, medication, dose, time, route). Identity must be verified with at least 2 identifiers — NEVER bed number. PRN prescriptions need indication, max dose, min interval, and max 24h dose. The 10 Principles of Good Prescribing (Aronson 2017) are directly examinable. Errors are commonest in July (new HOs) and in vulnerable groups (children, elderly, ICU, cognitively impaired). The Swiss Cheese Model explains how multiple safety barriers prevent errors from reaching patients. Pharmacogenetics is HIGH YIELD: HLA-B1502/carbamazepine (MANDATORY in HK), HLA-B5801/allopurinol, TPMT/azathioprine, CYP2C19/clopidogrel, VKORC1+CYP2C9/warfarin, SLCO1B1/statins, HLA-B*5701/abacavir. Quality prescribing = Efficacy + Safety + Tolerability + Cost-effectiveness. ICER = ΔCost/ΔEffect. Always check renal function before prescribing. Start low, go slow in the elderly.
Active Recall - Am I Prescribing the Right Drug?
[1] Lecture slides: GC 029. Am I prescribing the right drug.pdf [2] Senior notes: Block A - Drugs and the Kidney.pdf [3] Lecture slides: GC 079. Prescribing in older people.pdf; GC 079 (supp-2)STOPP-START-V3.pdf [4] Senior notes: Block A - Chronic diarrhoea_ irritable bowel syndrome and inflammatory bowel disease.pdf (Azathioprine/TPMT section); Block A - Patients with non-viral chronic liver diseases.pdf [5] Senior notes: Learning_Points_All_Lectures.txt (Clinical Pharmacology sections)
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GC030 An Old Man With Bone Pain And Anaemia
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