How do you approach a monitoring question?
Work from the main toxicity of the drug to the test that detects it. Ask what could go wrong, which organ is affected and which blood test or observation shows it early.
- Narrow therapeutic index drugs need drug levels (lithium, gentamicin, vancomycin, ciclosporin, tacrolimus, sometimes phenytoin).
- Drugs with end-organ toxicity need organ function tests (methotrexate, amiodarone, azathioprine).
- Drugs that shift electrolytes or renal function need U&E (ACE inhibitors, ARBs, diuretics, spironolactone).
- Drugs with a measurable effect need an effect test (warfarin with INR, levothyroxine with TSH).
- Baseline tests are taken before starting, then repeated at intervals that are most frequent in the first weeks.
The BNF monograph lists monitoring under the drug, and the BNF treatment summaries and NICE guidance give the intervals. Where an interval is not certain, check the BNF or the local shared-care protocol.
Monitoring table: drug, test and reason
| Drug | Monitor | Why |
|---|---|---|
| Lithium | Serum level, U&E/eGFR, TFT, calcium, weight | Narrow index; renal clearance; hypothyroidism and hyperparathyroidism |
| Digoxin | Potassium, renal function; level if toxicity or non-adherence suspected | Toxicity rises with hypokalaemia and renal impairment |
| Gentamicin | Level per local protocol, renal function | Nephrotoxic and ototoxic |
| Vancomycin | Pre-dose (trough) level, renal function | Nephrotoxic; level guides dose |
| Warfarin | INR | Dose response varies with diet, illness and interactions |
| Methotrexate | FBC, LFT, renal function | Marrow, liver and renal toxicity |
| Amiodarone | TFT, LFT, baseline chest X-ray, ECG | Thyroid, liver and lung toxicity |
| ACE inhibitor or ARB | U&E before and 1-2 weeks after starting or increasing the dose | Hyperkalaemia and fall in renal function |
| Loop and thiazide diuretics | Sodium, potassium, renal function | Hyponatraemia, hypokalaemia, volume depletion |
| Statins | LFT at baseline and later; CK only if muscle symptoms | Hepatic effect; myopathy |
| Azathioprine | TPMT status before starting; FBC; LFT | Low TPMT activity risks severe myelosuppression |
| Clozapine | Neutrophil count: weekly at first, then less often | Agranulocytosis |
| Levothyroxine | TSH 8-12 weeks after a dose change, then yearly when stable | Titrate to TSH; over-treatment causes AF and bone loss |
| Ciclosporin, tacrolimus | Trough (pre-dose) level, renal function, BP, potassium | Narrow index; nephrotoxic |
| Antipsychotics | Weight, BP, glucose or HbA1c, lipids, prolactin | Metabolic and endocrine effects |
When should a drug level be taken?
A level is only interpretable if it is taken at the right time after the last dose. The rules below are the ones the exam expects.
- Lithium: 12 hours after the last dose, once the patient is at steady state, which is about 5 to 7 days after a dose change.
- Digoxin: at least 6 hours after the dose, because earlier samples reflect the distribution phase. Routine levels are not needed; take one when toxicity or non-adherence is suspected.
- Gentamicin: a pre-dose (trough) or timed level as the local protocol states. Vancomycin: a pre-dose (trough) sample.
- Ciclosporin and tacrolimus: trough, immediately before the next dose.
- Phenytoin: levels guide dose changes because small increases can cause disproportionate rises; check albumin as it is highly protein-bound.
Timing errors cause false conclusions: a lithium level taken 4 hours post dose will be falsely high, and a digoxin level taken 2 hours post dose will be falsely high.
Which drugs need blood count, liver or thyroid monitoring?
Methotrexate, azathioprine and clozapine are monitored for bone-marrow suppression. Methotrexate and azathioprine need FBC, LFT and renal function at baseline and regularly, with the patient taught to report sore throat, mouth ulcers, bruising or fever at once. Clozapine requires a neutrophil count before starting and at set intervals, and the registration and monitoring system must be followed.
Amiodarone needs thyroid and liver tests at baseline and about every 6 months, with a baseline chest X-ray. Its effects persist for months after stopping because of its very long half-life.
Levothyroxine is monitored with TSH once the patient is stable. Lithium and amiodarone both disturb thyroid function and are monitored with TFT.
When is U&E monitoring needed?
ACE inhibitors, ARBs, spironolactone and diuretics are monitored by U&E because they change potassium and renal function. Check U&E before starting and soon after each dose increase.
A small rise in creatinine is expected after starting an ACE inhibitor. A large rise, or hyperkalaemia, needs the drug reviewed, and potassium-raising co-prescriptions should be stopped. Follow NICE and BNF guidance on thresholds.
Statin monitoring is often misremembered. LFT is checked at baseline and later, but CK is measured only when the patient reports unexplained muscle pain, weakness or dark urine.
Check yourself
- When is a lithium level taken, and why?
- What is the minimum time after a digoxin dose before a level is taken?
- Which blood tests are needed for methotrexate?
- What does TPMT testing predict before azathioprine?
- Which three organs does amiodarone threaten?
- How is levothyroxine dosing monitored?
- When is CK measured in a patient taking a statin?
- Which drugs need a trough level?
Questions people ask
When should a lithium level be taken?
Take it 12 hours after the last dose, once steady state has been reached. A sample taken earlier will read falsely high.
What monitoring does amiodarone need?
Thyroid and liver function at baseline and about every 6 months, and a baseline chest X-ray. Thyroid, liver, lung and eye effects are all possible.
Why is TPMT tested before azathioprine?
Low or absent TPMT activity causes high levels of toxic metabolites and severe myelosuppression, so the dose is reduced or the drug avoided.
Do statins need routine CK monitoring?
No. CK is measured only if the patient has unexplained muscle symptoms. Liver tests are checked at baseline and later.
What is a trough level?
A trough is a sample taken just before the next dose, when the drug level is lowest. It is used for gentamicin, vancomycin, ciclosporin and tacrolimus.
Related revision notes
Sources: BNF; NICE guidance on bipolar disorder, lipid modification and thyroid disease; MHRA Drug Safety Update; Shared-care protocols. These notes are for exam revision and are not patient-specific advice; in the exam and in practice, the BNF is the authority.
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