What laboratory tests are recommended for patients on Azothioprine (Immunosuppressant)?

Laboratory Monitoring for Patients on Azathioprine

For patients on azathioprine therapy, regular monitoring of complete blood count (CBC) and liver function tests (LFTs) should be performed weekly for the first 4 weeks of treatment, then monthly until maintenance dose is achieved, followed by monitoring every 3 months for the duration of therapy. 1, 2

Pre-Treatment Laboratory Assessment

• Thiopurine methyltransferase (TPMT) activity or genotyping should be checked prior to initiating therapy to guide dosing and identify patients at high risk for toxicity 1, 3
• NUDT15 genotyping should be considered, especially in patients with severe myelosuppression 4
• Baseline complete blood count (CBC) including platelet count 1, 4
• Baseline liver function tests (LFTs) 1, 2
• Consider hepatitis screening prior to treatment if using with other hepatotoxic agents 5

Monitoring Schedule

Initial Phase

• Weekly CBC and LFTs for the first 4 weeks of therapy 1, 2
• Continue weekly monitoring until maintenance dose is achieved 1, 2
• Return to weekly monitoring following any dose increase 1

Maintenance Phase

• Once stable on a fixed dose, reduce monitoring to a minimum of once every 3 months for the duration of therapy 2, 4
• More frequent monitoring (weekly) is required for patients with:
  • Low TPMT activity 1
  • Hepatic or renal impairment 1
  • Elderly patients 1
  • Those treated with higher doses of azathioprine 1

Specific Parameters to Monitor

Complete Blood Count (CBC)

• White blood cell count (WBC) - watch for leucopenia (WBC < 3.0 × 10⁹/L) 6
• Platelet count - monitor for thrombocytopenia (platelets < 100,000 × 10⁶/L) 6
• Hemoglobin - check for anemia 6
• Macrocytosis - common finding that can be used to assess patient compliance 2

Liver Function Tests (LFTs)

• Transaminases (ALT, AST) 1, 2
• Alkaline phosphatase (ALP) - decreasing levels may indicate successful treatment of the underlying condition 7
• Bilirubin 2

Special Considerations

• Myelosuppression can occur at any time during treatment (from 2 weeks to 11 years after starting therapy) 6
• Bone marrow suppression is uncommon but potentially severe, with leucopenia being the most common and important hematological complication 6, 8
• Severe pancytopenia has been reported in patients with TPMT deficiency 8, 9
• Patients with two mutant TPMT alleles typically develop myelosuppression within 1.5 months of starting therapy, while those with one mutant allele or normal genotype may develop toxicity later 9

Patient Instructions

• Report immediately any evidence of infection, unexpected bruising or bleeding, or jaundice 1
• Report any neurologic symptoms (headache, dizziness, numbness, tingling, or weakness) 5
• Check temperature frequently and report fever immediately 5
• Report signs of infection such as cough, aches, fever, chills, wounds with redness/discharge, burning with urination, nausea, vomiting, and diarrhea 5

Drug Interactions Requiring Additional Monitoring

• For patients also taking allopurinol or febuxostat (xanthine oxidase inhibitors), reduce azathioprine dose to approximately 1/3 to 1/4 the usual dose and increase monitoring frequency 4
• Use caution with aminosalicylate derivatives (e.g., sulphasalazine, mesalazine) as they may inhibit TPMT enzyme 4
• Monitor more closely if using with drugs affecting myelopoiesis, including co-trimoxazole 4
• Watch for anemia and severe leukopenia if used with angiotensin-converting enzyme inhibitors 4

By following this monitoring protocol, clinicians can minimize the risk of serious complications while maximizing the therapeutic benefits of azathioprine therapy.