Monitoring azathioprine therapy in myasthenia gravis

Azathioprine (AZA) is used increasingly in the treatment of selected patients with myasthenia gravis (MG). The "usual" dose is 2 to 3 mg/kg/d, but guidelines do not exist to determine a specific dose for an individual patient. We reviewed our previously reported MG patients to determine what laboratory studies correlated with therapeutic efficacy. Among the studies examined, red cell mean corpuscular volume (RBC MCV) was the most useful: in 10 patients who responded to AZA, MCV increased by 15 +/- 2 fl (mean +/- SD), while in 6 nonresponders, MCV increased by only 4.5 +/- 6 fl (p less than or equal to 0.01). RBC MCV may be helpful in monitoring AZA therapy in patients with MG.