Erythrocyte mean corpuscular volume as a surrogate marker for 6-thioguanine nucleotide concentration monitoring in patients with inflammatory bowel disease treated with azathioprine or 6-mercaptopurine

Mean corpuscular volume may correlate with erythrocyte 6-thioguanine nucleotide concentrations in patients treated with azathioprine and 6-mercaptourine. We conducted a study of 166 patients with inflammatory bowel disease treated with azathioprine or 6-mercaptopurine to determine the relationship between mean corpuscular volume and erythrocyte 6-thioguanine nucleotide concentrations, disease activity as measured by the Inflammatory Bowel Disease Questionnaire (active disease <170. remission >170), and leukopenia. Blood was submitted for mean corpuscular volume, whole blood 6-thioguanine nucleotide concentration, and leukocyte count. The mean+/-SD mean corpuscular volume during treatment was 94.7+/-6.6 fL and the mean+/-SD change in mean corpuscular volume was 7.5+/-6.3 fL. There were significant correlations between mean corpuscular volume and erythrocyte 6-thioguanine nucleotide concentration (r(s)=0.33, p<0.001) and between change from baseline in mean corpuscular volume and erythrocyte 6-thioguanine nucleotide concentration (r(s)=0.26, p=0.001). There was no correlation between Inflammatory Bowel Disease Questionnaire scores and mean corpuscular volume values (r(s)=0.01, p=0.94). The mean corpuscular volume values in 55 patients with active disease and 111 patients in remission were similar (95.1 vs. 94.5 fL, p=0.57). There was a weak negative correlation between the mean corpuscular volume and the leukocyte count, (r(s)=-0.18, p=0.022). In patients with inflammatory bowel disease treated with azathioprine or 6-mercaptopurine, mean corpuscular volume and change from baseline in mean corpuscular volume correlated with erythrocyte 6-thioguanine nucleotide concentrations and negatively with leukocyte counts, but did not correlate with disease activity as measured by the Inflammatory Bowel Disease Questionnaire. Measurement of mean corpuscular volume is a simple and inexpensive alternative to measurement of 6-thioguanine nucleotide concentrations in patients treated with azathioprine or 6-mercaptopurine.