Kandavel et al (1) retrospectively investigated the value of mean corpuscular volume to white blood cell (MCV/WBC) ratio as estimates for 6-TG levels, and further as surrogate marker for thiopurine efficacy in pediatric inflammatory bowel disease (PIBD) patients treated with thiopurine in their center. Their analysis was based on 440 PIBD patients with complete blood cell count, 441 patients with ESR or CRP values, 111 patients with physician global assessment (PGA) evaluation, but only 53 patients with 6-TG levels available. No information on concomitant drugs, endoscopic findings or disease activity scores like the wPCDAI were given. The MCV/WBC ratio was poorly related to ESR and CrP and not significantly associated with the 4 categories of PGA. The concluding AuROC analysis showed poor results for prediction of quiescent disease (defined by normal PGA and ESR or CrP) by either MCV/WBC (n = 107) or 6-TG (n = 14!). In spite of these findings and major limitations of the study as pointed out in the Editorial by Bousvaros (2), the authors conclude “that the MCV/WBC ratio provides an accurate, easy, and low-cost alternative method for therapeutic monitoring of thiopurine medications.”
To test the reliability of MCV/WBC as “a poor man’s drug level” for thiopurine efficacy, we analyzed data from the PIBD registry of the Society for Paediatric Gastroenterology of German-speaking countries; www.gpge.eu). The registry includes data on more than 5000 children and adolescents with IBD with >50,000 documented contacts reported by >50 PIBD outpatient clinics from 2004 onwards.