Yes, azathioprine commonly causes macrocytosis
Macrocytosis is a well-recognized and common hematologic effect of azathioprine therapy that occurs frequently during treatment and does not require dose adjustment or discontinuation. 1
Clinical Significance
According to the British Association of Dermatologists' guidelines, macrocytosis is explicitly described as "a common finding" during azathioprine therapy 1. Importantly, the guidelines make a critical distinction: unlike other hematologic adverse effects (leukopenia, anemia, thrombocytopenia, pancytopenia), macrocytosis is the only one that should NOT prompt dose adjustment 1.
The FDA drug label confirms that macrocytic anemia can occur as part of azathioprine's hematologic toxicity profile 2.
Practical Clinical Use
Macrocytosis can actually serve as a useful clinical marker for medication adherence 1. The guidelines specifically state that macrocytosis "can be used to assess patient noncompliance" - meaning if a patient is taking azathioprine regularly, you should expect to see macrocytosis develop.
Recent data from myasthenia gravis patients showed that macrocytosis was one of the three most frequent laboratory abnormalities during azathioprine therapy 3. In pediatric inflammatory bowel disease, macrocytosis typically develops after leukopenia, making it a poor predictor of impending bone marrow suppression 4.
Monitoring Implications
- Regular FBC monitoring is essential throughout azathioprine therapy 1
- Weekly monitoring for the first 8 weeks, then at least every 3 months once stable 1
- When macrocytosis is detected, continue current dosing - this is the expected response 1
- Only adjust doses for leukopenia, thrombocytopenia, or anemia 1
Important Caveats
While macrocytosis itself is benign and expected, severe megaloblastic anemia with pancytopenia is a rare but serious complication that can occur even after decades of therapy 5. In one case, this developed 22 years after transplantation. Additionally, macrocytosis that persists despite vitamin B12 replacement should raise concern for myelodysplastic syndrome, particularly in patients with inflammatory bowel disease on long-term azathioprine 6.
The presence of macrocytosis with MCV >101 fL has a 92% positive predictive value for therapeutic 6-thioguanine levels in IBD patients, making it a useful surrogate marker for drug efficacy 7.