The question
Work it with the BNF open before you show the answer. At exam pace this item has 1 min 12 s.
An 84-year-old woman attends her general practice 2 days after a diarrhoeal illness settled. During a norovirus outbreak at her sheltered housing scheme she had 4 days of vomiting and watery diarrhoea, managed at home; she is now eating again but still feels washed out. She took all her tablets throughout, including on the days she could keep little else down. Her next anticoagulation review, with blood tests, is booked in 6 weeks.
PMH: atrial fibrillation; hypertension; chronic kidney disease.
DH: apixaban 2.5 mg PO twice daily (12-hrly) (atrial fibrillation); ramipril 2.5 mg PO daily (hypertension); amlodipine 5 mg PO daily (hypertension); paracetamol 1 g PO 6-hrly as required. No known drug allergies.
SH: widowed; lives alone in sheltered housing; lifelong non-smoker; no alcohol.
On examination: Temperature 36.6°C, HR 78/min and irregularly irregular, RR 14/min, O2 sat 97% breathing air. BP 126/74 mmHg sitting, 108/66 mmHg after 1 minute standing, with light-headedness on standing. Weight 56 kg, recorded 58 kg at her last review. Dry lips; abdomen soft and non-tender; no bruising and no bleeding.
Investigations: At her last anticoagulation review 6 weeks ago: Na 138 mmol/L (135–145), K 4.6 mmol/L (3.5–5.3), creatinine 110 µmol/L (45–84), eGFR 43 mL/min/1.73 m² (>60); creatinine clearance 31 mL/min by the Cockcroft–Gault equation at her then weight of 58 kg Hb at that review 121 g/L (115–165); no blood tests have been taken since
- anti-factor Xa activity, before the next apixaban dose
- creatinine clearance (Cockcroft–Gault), at the booked review in 6 weeks
- creatinine clearance (Cockcroft–Gault), recalculated today
- international normalised ratio, today
- no blood monitoring at any stage of treatment
Answer and marking
creatinine clearance (Cockcroft–Gault), recalculated today
Marks2
ExplanationTwo separate pieces of knowledge are being marked. The first is which parameter follows a direct oral anticoagulant: not the INR (BNF, apixaban, Monitoring requirements: 'No routine anticoagulant monitoring required (INR tests are unreliable)', read 22 August 2026), not a routine anti-factor Xa level — the drug has no target range to titrate to — but the kidney function that determines its dose and its licence. Apixaban is avoided below a creatinine clearance of 15 mL/minute and dose-reduced at 15–29 mL/minute, and the clearance is calculated by Cockcroft–Gault, not read off an eGFR.
The second is when. Her stable-state schedule is genuinely the booked one: NICE CKS (Anticoagulation — oral, apixaban scenario, last revised April 2026, read 22 August 2026) repeats the bloods yearly for most people, every 4 months over 75, and by 'CrCl divided by 10' months below 60 mL/minute — about every 3 months at her 31 mL/min. The check booked in 6 weeks respects all of that. What overrides it is the same page's event clause: 'If the person has an intercurrent illness that may impact renal or hepatic function, repeat renal and liver function tests as needed.' Four days of vomiting and diarrhoea, a 2 kg weight loss, a postural blood pressure drop and a continued ACE inhibitor, in a woman whose clearance already sits 2 mL/min above the 15–29 mL/minute band, is exactly the illness that clause describes. If today's clearance has fallen below 15 mL/minute, she is taking a drug she should not be taking at all — and nobody would know until the booked bloods in 6 weeks.
The wrong answers each carry a real fact one step too far: anti-factor Xa assays exist but are reserved for exceptional situations, not reviews; the booked interval is correct arithmetic applied to a schedule the illness has suspended; the INR belongs to her neighbour on warfarin; and 'no monitoring' over-reads the no-INR-clinic message into a claim the dosing criteria on her own chart refute.
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How to approach a drug monitoring item
The repeatable method from the Drug Monitoring section guide.
- Identify the drug and its riskWhat organ or system does this drug threaten, or what narrow range must it stay in?
- Classify the monitoringBaseline, scheduled, symptom-triggered, or a timed drug level. The question usually fits one.
- Find the timingThe monograph's monitoring requirements state when. For levels, the relationship to the dose is the whole point.
- Interpret the resultAgainst the stated target or threshold, not against the general reference range alone.
- Decide the actionContinue, reduce, hold, stop, or re-sample. The BNF often states the trigger.
Before you submit
- Which of the four kinds of monitoring is this?
- Is the timing right relative to the dose?
- Am I judging against the drug's target, not the lab's range?
- Does the result actually change management?
- Is the action aimed at the drug, not the number?
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