Azathioprine Initiation and Monitoring Guidelines

Pre-Treatment Assessment

The British Society of Dermatology recommends measuring TPMT activity in all patients before initiating azathioprine therapy, with a strength A recommendation and level 1+ evidence 1
Consider NUDT15 genotyping, particularly for Asian patients, as genetic variants increase myelosuppression risk 2
Obtain baseline complete blood count (CBC), liver function tests (LFTs), and renal function 1, 4
Screen for hepatitis B, hepatitis C, and HIV; refer if positive 2
Check varicella zoster virus (VZV) immunity and vaccinate if seronegative 2
Ensure cervical cancer screening is current in women 2
Provide written and verbal information about the medication 2

Start azathioprine at 50 mg daily (or 1 mg/kg/day) with food, for patients with normal TPMT activity, and increase to target maintenance dose of 2-2.5 mg/kg/day based on TPMT status while tapering steroids 1, 2, 3
For patients with intermediate TPMT activity, start at 50% of standard dose: 1-1.5 mg/kg/day 1, 2, 3
Avoid azathioprine entirely in patients with absent TPMT activity (homozygous/compound heterozygous) 1, 2, 5

Monitor CBC and LFTs weekly for the first 4 weeks, then every 3 months for duration of therapy 4, 5
Return to weekly monitoring if dose is increased 4
More frequent monitoring is required for patients with intermediate TPMT activity, higher doses, hepatic or renal impairment 4

Increase to full maintenance dose of 2-2.5 mg/kg/day after 4 weeks at initial dose, for patients with normal TPMT activity 1, 2, 3
Therapeutic effects often take several months to become apparent after initiation 1
If no improvement occurs within 3 months, consider withdrawing azathioprine and reconsidering diagnosis or adherence 4, 5

Begin tapering prednisone once azathioprine reaches maintenance dose and patient shows clinical response 3
Reduce prednisone by 2.5 mg each month as azathioprine dose increases 3
Goal is complete prednisone withdrawal while maintaining disease control on azathioprine monotherapy 3

Elderly patients: use doses at the lower end of the range (1 mg/kg/day) 1, 4
Renal impairment: use 75% of usual dose for creatinine clearance 10-50 mL/min, and 50% of usual dose for creatinine clearance <10 mL/min 2
Hepatic impairment: use lower doses and monitor more frequently 1