Monitoring Medicines: Blood Tests, Timing and What to Watch For

[HOST] Welcome to ClinicaliQ Clinical Essentials. I'm Emma, and with me is Dr Anzal Qurbain, pharmaceutical physician. Today we are covering monitoring medicines — what to check, how often, and why it matters. There are some drugs where if you don't monitor regularly, patients can end up seriously harmed without any warning. Dr Qurbain, where does monitoring fit into prescribing practice?

[EXPERT] Monitoring is part of prescribing — it doesn't end when you write the prescription. For most routine medicines, you don't need to do much. But for a specific group of drugs, either because they have a narrow therapeutic window or because they cause predictable organ toxicity, regular monitoring is essential for patient safety. The challenge in practice is that these patients often see multiple clinicians and it's easy for monitoring to fall through the gap.

[HOST] Let's work through the main ones. Lithium is probably the one most people think of first.

[EXPERT] Lithium is the classic high-risk monitoring medicine. It has a very narrow therapeutic window — the therapeutic range is typically 0.6 to 1.0 millimoles per litre for maintenance, and toxicity starts appearing above 1.5. The difference between a therapeutic level and a toxic one can be a relatively small change. Levels should be checked every three to six months once stable, and every three months if the patient is elderly or has any renal impairment.

[HOST] What are the signs of lithium toxicity someone might see clinically?

[EXPERT] Early toxicity presents with coarse tremor, confusion, ataxia — the patient looks generally unwell and off-balance. As levels rise further, you get vomiting, seizures, and cardiac arrhythmias. The important point is that levels can rise acutely if a patient becomes dehydrated — exactly the sick day rules problem we discussed in the last episode. Lithium is renally cleared, so any drop in kidney function sends levels up quickly.

[HOST] And beyond levels, what else needs checking with lithium?

[EXPERT] Lithium has long-term effects on the thyroid and kidneys. Hypothyroidism develops in up to a third of patients on long-term lithium. So thyroid function and renal function — eGFR and creatinine — should be checked annually. These checks are separate from the level monitoring and are easy to miss if only the psychiatry team is ordering levels and nobody is thinking about annual organ function.

[HOST] Let's move to methotrexate. This is prescribed for rheumatoid arthritis and psoriasis, not just oncology.

[EXPERT] Yes, and in primary care there are a lot of patients on low-dose methotrexate for rheumatological or dermatological conditions who are being monitored in a shared care arrangement. Methotrexate is hepatotoxic and mylosuppressive. FBC and liver function tests need to be done monthly for the first six months, then three-monthly once the patient is stable on a consistent dose. Missing these tests is a shared care failure that can result in serious harm — bone marrow suppression in particular can be rapid and severe.

[HOST] Is there anything patients on methotrexate should know specifically?

[EXPERT] Two things. First, they must take folic acid alongside it — usually 5mg once a week on a different day to the methotrexate — to reduce side effects including mouth ulcers and nausea. Second, they must understand the weekly dosing. Methotrexate is taken once a week, not daily. There have been serious incidents where patients have taken it daily in error. The dispensing label and the patient information should make the weekly schedule crystal clear.

[HOST] Clozapine is next. I know this has very strict monitoring requirements.

[EXPERT] Clozapine is the most tightly monitored drug in routine psychiatric practice. The risk is agranulocytosis — a potentially fatal drop in neutrophil count — which occurs in roughly one to two percent of patients. The Clozapine Patient Monitoring Service, CPMS, is mandatory. Patients must have weekly full blood counts for the first 18 weeks, then fortnightly for up to a year, then monthly thereafter if counts remain stable. Clozapine can only be dispensed once the monitoring result has been submitted and cleared by the system. It is one of the most robust drug safety systems in UK medicine.

[HOST] What's the signal in the blood count that triggers concern?

[EXPERT] A neutrophil count below 1.5 triggers an amber warning and more frequent monitoring. Below 1.0 is a red result and clozapine must be stopped immediately. The patient must never be rechallenged if they've had a red result. It's one of the absolute contraindications in prescribing.

[HOST] Statins — I often hear that you don't need to monitor these once started. Is that right?

[EXPERT] Largely, yes. A baseline LFT before starting is reasonable, but routine ongoing liver function monitoring is no longer recommended by NICE unless the patient becomes symptomatic. The old concern about statins causing significant liver toxicity has not been borne out in practice at the scale originally feared. If a patient develops unexplained muscle pain or weakness — myopathy — then a CK should be checked. If CK is more than five times the upper limit of normal, statin should be stopped. Rhabdomyolisis is rare but serious.

[HOST] Amiodarone seems like an unusually complex one to monitor.

[EXPERT] Amiodarone is genuinely unusual. It's highly effective for arrhythmias, particularly atrial fibrillation when other options have failed, but it has an extraordinary list of potential toxicities. Thyroid — both hyper and hypothyroidism. Liver. Lung — amiodarone-induced pulmonary toxicity is serious and can be insidious. Corneal microdeposits. Peripheral neuropathy. The annual monitoring package is TFTs, LFTs, a chest X-ray, and pulmonary function tests. Some guidelines also include a baseline and annual ECG. The half-life of amiodarone is measured in weeks to months, so even after stopping the drug, monitoring should continue for a period.

[HOST] What about ACE inhibitors and ARBs? These are extremely common prescriptions.

[EXPERT] U&Es — urea and electrolytes — should be checked within one to two weeks of starting an ACE inhibitor or ARB, and then annually if stable. The concern is hyperkalaemia, particularly in patients who also have CKD or who are on potassium-sparing diuretics or spironolactone. A modest rise in creatinine of up to 30 percent after starting an ACE inhibitor is acceptable and expected — the drug is working. More than 30 percent should prompt review and specialist input.

[HOST] And anticoagulants — INR for warfarin is well known, but what about the direct oral anticoagulants?

[EXPERT] Warfarin requires INR monitoring — frequency depends on stability, but typically every 8 to 12 weeks once within range, more often if unstable. The target INR is usually 2.0 to 3.0 for most indications, or 2.5 to 3.5 for mechanical heart valves. DOACs — apixaban, rivaroxaban, dabigatran, edoxaban — don't require routine INR monitoring, which is part of their appeal. But that doesn't mean no monitoring. Renal function should be checked annually in all patients on DOACs, because these drugs are renally cleared to varying degrees. Dabigatran in particular — 80 percent renal clearance — needs dose adjustment or is contraindicated at low eGFRs.

[HOST] What about weight? Can weight change affect DOAC dosing?

[EXPERT] It's a good question that often gets overlooked. Some DOACs — apixaban is the main example — have dose criteria partly based on body weight and age. At the time of prescribing, these criteria may be checked, but a patient who loses significant weight over the following years might have their clinical profile change. Annual review should include checking that the prescribed dose still matches the current criteria.

[HOST] What's the overarching challenge here for clinicians in practice?

[EXPERT] The biggest challenge is coordination. A patient might be on lithium monitored by psychiatry, methotrexate monitored in a shared care arrangement with rheumatology, an ACE inhibitor started by their GP, and warfarin monitored at an anticoagulation clinic. Nobody has a complete picture unless someone is actively joining the dots. Shared care protocols exist precisely to define who is responsible for what — but they only work when there's a named lead at each point.

[HOST] How should a practice approach this systematically?

[EXPERT] The best approach is a recall system — either within the practice clinical system or through a dedicated monitoring register — that flags when monitoring is overdue. Many GP systems can do this. The other thing that helps is structured medication reviews, which are now a funded activity in primary care. A good SMR should include checking that monitoring is up to date for every high-risk medicine.

[HOST] What's your clinical pearl for monitoring?

[EXPERT] Patients on multiple monitored medicines are the ones most likely to have a monitoring gap. If someone is on lithium and methotrexate and warfarin, they're three separate monitoring streams that need coordination. The practical tip is to use the consultation when you're doing one check to also verify the others are up to date. And if a shared care protocol is in place, make sure there's a named clinical lead who owns the overall picture — not just the bit their specialty prescribed.

[HOST] Really useful. To summarise today's episode: key monitored medicines include lithium — levels every 3 to 6 months, annual thyroid and renal function; methotrexate — FBC and LFTs monthly then 3-monthly; clozapine — mandatory CPMS blood counts; amiodarone — annual TFTs, LFTs, chest X-ray and pulmonary function; ACEi and ARBs — U&Es at 1 to 2 weeks and annually; warfarin — INR regularly; and DOACs — no routine INR but annual renal function. And the clinical pearl — patients on multiple monitored drugs are the ones most at risk of a gap. Thank you, Dr Qurbain.

[EXPERT] Thanks, Emma. Good monitoring saves lives — it really is that straightforward.

[HOST] That's all for this episode of ClinicaliQ Clinical Essentials. We'll be back with more practical prescribing guidance next time.