Red cell mean corpuscular volume (MCV) correlates with 6 thioquanine nucleotide (6TG) levels during azathioprine or 6-MP therapy for Crohn's disease

Systematic review: managing anaemia in Crohn's disease

“…Both drugs increase the MCV and thereby obstruct one of the most sensitive measures of iron deficiency. On the other hand, the MCV increase is considered a good marker for azathioprine drug monitoring 78, 79 . In the presence of iron deficiency this marker cannot be used as well.…”

“…In fact, the increase in MCV is commonly used as marker of drug efficacy. 78,79 The coexistence of iron deficiency may obstruct this feature and vice versa. This MCV increase is typically not associated with significant anaemia and rarely needs dose adaptation.…”

“…As treatment recommendations changed over the past decades, the relative importance of drug‐induced anaemia shifted from sulfasalazine to 6‐MP and azathioprine. In fact, the increase in MCV is commonly used as marker of drug efficacy 78, 79 . The coexistence of iron deficiency may obstruct this feature and vice versa.…”

Change in Erythrocyte Mean Corpuscular Volume During Combination Therapy with Azathioprine and Infliximab Is Associated with Mucosal Healing

“…6,7 However, thiopurine therapeutic drug monitoring is relatively underused because of limited availability of the thiopurine metabolite test and the absence of reimbursement for the test in many clinical practice settings. [9][10][11] Patients who achieve clinical remission and higher 6-TGN concentrations after treatment with thiopurine are more likely to have higher absolute MCV values and an increase of at least 7 fL in the MCV value after starting thiopurine therapy. [9][10][11] Patients who achieve clinical remission and higher 6-TGN concentrations after treatment with thiopurine are more likely to have higher absolute MCV values and an increase of at least 7 fL in the MCV value after starting thiopurine therapy.…”

Management of refractory inflammatory bowel disease

“…The standard dose of 6-MP or AZA may be cautiously increased in selected refractory patients to higher doses in the absence of leukopenia (the primary dose-limiting factor) or other side effects [29,30]. Measurement of thioguanine nucleotide metabolites may be helpful when increasing the dosage beyond the standard dose [30][31][32], although following changes in the mean corpuscular volume of red blood cells may be equally helpful [33].…”