Section 7: Drug monitoring

Question 1

What do you monitor for patients on methotrexate?

Answer 1

FBC and LFTs; due to risk of blood dyscrasias (severe anaemia, leukopenia, thrombocytopenia) and hepatotoxicity.
Every 1–2 weeks until therapy stabilised, thereafter patients should be monitored every 2–3 months.

Question 2

What do you monitor for patients on mycophenolate?

Answer 2

FBC and LFTs.

Monitor FBC every week for 4 weeks then twice a month for 2 months then every month in the first year.

Question 3

When and how often do you measure gentamicin levels when monitoring?

Answer 3

For multiple daily dose regimens, blood samples taken 1 hour after administration (‘peak’) and also just before the next dose (‘trough’).

Question 4

When do you measure paracetamol levels in an overdose?

Answer 4

4 hours after administration. Use nomogram to see if value lies above treatment line

Question 5

What are target INRs for patients on warfarin?

Answer 5

Usually 2.5, unless recurrent thromboembolism on warfarin or metallic heart valves (where target is 3.5)

Question 6

At what INR do you give vitamin K PO or IV?

Answer 6

If major bleed, stop warfarin and give 5-10mg IV vit K + pTC

If minor bleed w INR >5, stop warfarin and give IV vit K

If no bleeding but INR >8, stop warfarin and give 1-5mg PO vit K

Question 7

How often do you check INR in patients on warfarin?

Answer 7

Initially weekly then once stabilised, every 4 weeks. In patients in hospital you may want to check up to every 48h.

Question 8

What should be checked before starting patient on amiodarone?

Answer 8

Thyroid function - may precipitate hypothyroidism

Potassium levels - many precipitate hypokalemia

Question 9

Patient on statin has muscle aches with raised CK (>5x ULN). How would you manage him?

Answer 9

Stop the statin. If symptoms resolve and CK returns to normal, reintroduce it at a lower dose.

Question 10

A patient with hypothyroidism on levothyroxine has a TSH of 24.2 (0.4-5.0). What do you do?

Answer 10

Continue dose as per usual. High TSH usually indicates poor adherence rather than inadequate dosing.

Question 11

What do you need to monitor for patients on ciclosporin, and how often?

Answer 11

Monitor renal function and ciclosporin is commonly nephrotoxic. Measure before starting, and monitor serum Cr every 2w for first 3m, then monthly

Question 12

What is the most common side effect of COCP that needs monitoring?

Answer 12

Hypertension. Should be stopped if B > 160/95 mmHg. (no evidence they cause weight gain)

Question 13

What needs to be checked before initiating azathioprine in a patient?

Answer 13

Thiopurine methyltransferase (TPMT) levels - this is required for metabolism of the drug, if patient has low level they need to be treated with a lower dose to reduce risk of toxicity

Question 14

Before prescribing vancomycin, what is an important parameter to check?

Answer 14

Renal function i.e. serum creatinine. Vancomycin, a glycopeptide antibiotic, is almost exclusively renally cleared. In renal impairment, dosage must be adjusted.

Question 15

What parameter is important to check before starting a patient on a statin?

Answer 15

Serum ALT i.e. liver function. Statins are metabolised by the liver - if transaminases ALT/AST are raised > 3x ULN then they are contraindicated.

Question 16

What do you monitor for patients starting on statins, and when?

Answer 16

Check transaminases (ALT/AST) before, 3 and 12 months after starting treatment.

Monitor CK if evidence of myopathy or previous myopathy with statins.

Monitor FBG or HbA1C in patients at high risk of diabetes before starting and after 3 months.

Question 17

When do you measure phenytoin levels in monitoring, and what is the normal reference range?

Answer 17

Measure the trough level i.e. pre-dose.This should be 40-80 micromol/ L. Adjust dose according to therapeutic effect and side effects.

Question 18

What is the normal reference range for serum lithium and at what serum concentration is side effects likely?

Answer 18

0.4-0.8 mmol/L. Toxic effects are likely to manifest at serum concentrations above 1.5 mmol/L?

Question 19

What must you check before starting methotrexate?

Answer 19

FBC, renal and liver function. Risk of blood dyscrasias, liver cirrhosis, and toxicity in presence of renal dysfunction (renally excreted).

Question 20

What is important to check in a patient with no significant PMHx starting on olanzapine?

Answer 20

Fasting blood glucose/ HbA1C, due to increased risk of hyperglycaemia and diabetes (esp olanzapine).

Check prolactin levels due to risk of hyperprolactinaemia.

Baseline ECG indicated if patients have cardiovascular disease or associated RFs - risk of long QT.

Question 21

What do you need to check before commencing amiodarone?

Answer 21

CXR due to risk of pulmonary toxicity. Liver function due to risk of acute liver dysfunction. Thyroid function (T3, TSH, T4) due to risk of hyperthyroidism. Serum K+ as risk of hypokalaemia.

Question 22

What do you need to monitor in patients on amiodarone?

Answer 22

TFTs and LFTs every 6M. If on IV amiodarone, ensure ECG monitoring and resuscitation facilities.

Question 23

What parameter should be checked during Tx with digoxin?

Answer 23

Serum electrolytes and creatinine. Digoxin is primarily excreted renally and patients with renal impairment are at increased risk of toxicity, hyperkalemia.

Question 24

What needs to monitored in patients on sodium valproate?

Answer 24

Liver function, as valproate is a/w hepatotoxicity. Measure baseline and at reg intervals during 1st 6 months, incl prothrombin time (discontinue if this remains prolonged)

Monitor also for pancreatitis.

Question 25

What do you monitor in patients on clozapine and how often?

Answer 25

FBC due to risk of agranulocytosis. Monitor weekly for 18 weeks after starting. Also monitor prolactin at 6 months, lipids 3 monthly.

Question 26

What is the best way to assess the success of fluid replacement?

Answer 26

Blood pressure. This is more reliable than urinary output in early stages

Question 27

What must be monitored in patients on lithium?

Answer 27

Renal function, thyroid function, serum electrolytes (incl. Ca2+). Lithium can lead to renal impairment, nephrotic syndrome, nephrogenic DI. It can cause hypothyroidism, hypoparathyroidism (and hypocalcaemia), hyponatremia.

Question 28

What are the side effects of HRT and how can we monitor this?

Answer 28

Can cause Na+ and fluid retention, leading to rise in BP. Stop if BP >160/95mm Hg and monitor regularly.

Question 29

If a patient on simvastatin has an ALT of 70 (normal 5-35), is this indication to stop the drug?

Answer 29

No. Only stop if more than 3 times ULN or if other signs of hepatotoxicity.

Question 30

How do you decide whether to adjust the dose and dose interval in therapeutic drug monitoring? e.g. lithium, gentamicin

Answer 30

If the pre-dose (‘trough’) concentration is high, the interval between doses must be increased. If the post-dose (‘peak’) concentration is high, the dose must be decreased.

Question 31

What drugs require routine therapeutic drug monitoring?

Answer 31

Aminoglycosides, glycopeptides, phenytoin, tacrolimus, ciclosporin, lithium, unfractionated heparin

Question 32

How do you monitor unfractionated hepatrin?

Answer 32

Sample APTT ratio 6h after starting infusion or change of dose

Question 33

When can you measure plasma digoxin levels after dose?

Answer 33

6 hours after dose when starting/ in secondary care. Take at any time if toxicity suspected.

Question 34

Why do you monitor renal function in patients on digoxin?

Answer 34

Renal impairment would lead to reduced clearance, and dose would have to be reduced.