Blood Tests for Medication Monitoring
Which blood tests your medications quietly demand - and why skipping them is one of the most common preventable problems in adult medicine.
Why Medications Need Blood Monitoring
Every medication does something useful AND something unintended. Routine blood tests catch the unintended effect before it causes harm - rising liver enzymes on a statin, sinking potassium on a diuretic, accumulating drug on failing kidneys.
For some drugs the blood test IS the dosing tool. Warfarin doses are titrated entirely by INR. Lithium doses are titrated by serum level. Skipping these tests is not just risky - it makes the treatment impossible to manage properly.
And for medications taken long-term, some quietly deplete nutrients - metformin takes B12, PPIs take magnesium, diuretics take potassium. These are easy to track and easy to fix when caught.
The 15 Most Commonly Monitored Medications
| Medication | Tests | Frequency | Why |
|---|---|---|---|
| Statins (atorvastatin, rosuvastatin, simvastatin) | ALT, CK | Before starting, 3 months, then annually | Rare but real liver enzyme rise and muscle injury (myopathy) |
| Metformin | eGFR, B12, HbA1c | eGFR every 6-12 months, B12 yearly | Reduced kidney clearance can accumulate the drug; long-term use depletes B12 |
| Warfarin | INR | Daily initially, then weekly to monthly | Narrow therapeutic window - too low risks clots, too high risks bleeding |
| Lithium | Lithium level, TSH, eGFR, calcium | Lithium 3-monthly, kidneys & thyroid 6-monthly | Narrow safe range; long-term kidney and thyroid damage |
| Methotrexate | FBC, LFTs, eGFR | Fortnightly for 3 months, then monthly, then 3-monthly | Bone marrow suppression and hepatotoxicity |
| ACE inhibitors / ARBs (perindopril, irbesartan) | Potassium, eGFR, creatinine | Before starting, 1-2 weeks after, then 6-12 monthly | Can raise potassium and worsen kidney function |
| Diuretics (frusemide, indapamide) | UEC (especially K, Na) | Before starting, 1-2 weeks after, then 6-12 monthly | Electrolyte loss; risk of low sodium or potassium |
| Levothyroxine (thyroid replacement) | TSH (and FT4) | 6 weeks after each dose change, then yearly when stable | Under-replacement leaves symptoms; over-replacement risks osteoporosis and AF |
| PPIs (omeprazole, pantoprazole - long-term) | Magnesium, B12 | Yearly if used >1 year | Long-term use depletes magnesium and B12 absorption |
| Antiepileptics (carbamazepine, valproate) | Drug level, FBC, LFTs, sodium | After starting, then 6-monthly | Bone marrow effects, liver toxicity, hyponatraemia |
| Allopurinol (gout) | eGFR, uric acid, FBC, LFTs | 4-6 weeks after start, then 6-12 monthly | Rare hypersensitivity, kidney clearance affects dose |
| Biological / immunosuppressants (azathioprine, infliximab) | FBC, LFTs, hepatitis B/C, TB screen | Pre-treatment screen, then fortnightly to monthly | Bone marrow suppression, infection reactivation |
| Oral contraceptive pill (combined) | BP (not blood), lipids if family history of clots | Yearly review | Increased clotting risk; minimal routine bloods unless other risk factors |
| Hormone Replacement Therapy | Lipids, glucose, LFTs (if oral) | Before starting, yearly review | Oral oestrogen has more metabolic effects than transdermal |
| Spironolactone | Potassium, eGFR | 1-2 weeks after start, then 6-monthly | Risk of dangerously high potassium |
Statins Deep-Dive
More than 2 million Australians take statins. They cut cardiovascular events by about 25% over 5 years. The monitoring is straightforward but routinely missed.
ALT (liver)
Check before starting, at 3 months, then yearly. A 3x rise above upper limit usually prompts a dose review. A mild rise of 1-2x rarely requires action.
CK (muscle)
Check only if muscle symptoms occur. Mild aches without CK rise are common and not a reason to stop. CK above 10x normal needs urgent review.
Warfarin and INR - Why It Cannot Be Skipped
Warfarin has a narrower therapeutic window than almost any other common drug. INR below target means clot risk; above target means bleeding risk - and the difference can be a single missed test.
Most warfarin patients in Australia attend a community INR clinic for finger-prick testing - results in 30 seconds, dose adjusted on the spot. If you have moved to a DOAC (apixaban, rivaroxaban), INR is no longer needed - but kidney function still is.
If you take warfarin, treat every INR appointment like a hospital appointment. Cancel and reschedule rather than skip.
Medications That Quietly Deplete Nutrients
| Medication | Depletes | What to test |
|---|---|---|
| Metformin | Vitamin B12 | Annual B12 level |
| PPIs (omeprazole, pantoprazole) long-term | Magnesium, B12, calcium | Annual Mg, B12; consider DEXA |
| Statins | Coenzyme Q10 (theoretical) | Clinical only - CK if muscle symptoms |
| Diuretics (loop and thiazide) | Potassium, magnesium, sodium | Regular UEC plus magnesium |
| Oral contraceptive | Folate, B6, B12 (mild) | B12, folate if symptomatic |
| Long-term corticosteroids | Calcium, vitamin D, potassium | Vitamin D, calcium, DEXA |
| H2 blockers (ranitidine, famotidine) | B12, iron absorption | B12, iron studies if symptomatic |
What to Ask Your Doctor or Pharmacist
Ready-to-use script:
“I have been on [medication X] for [X months/years]. What blood tests should we be doing to monitor it, and how often? Are there any nutrients this medication depletes that we should also check?”
For people on multiple medications, ask your community pharmacist for a Home Medicines Review (HMR) - Medicare funds it, and the pharmacist creates a list of monitoring tests for your GP. It is one of the most underused free services in Australian primary care.
Track Medication-Related Markers Over Time
Upload every blood test you have ever had - SmarterBlood charts liver enzymes, kidney function, INR and electrolytes year by year so you can see whether your medications are agreeing with you.