BACKGROUND Measurement of 6-thioguanine nucleotide concentrations may be useful for optimising treatment with azathioprine and 6-mercaptopurine.

RESULTS Mean (SD) inflammatory bowel disease questionnaire score was 176 (32). There was no correlation between inflammatory bowel disease questionnaire scores and 6-thioguanine nucleotide concentrations (r _s=−0.09, p=0.24). Median 6-thioguanine nucleotide concentrations in 56 patients with active disease and 114 patients in remission were similar (139v 131 pmol/8×10⁸ red blood cells; p=0.26). There was no correlation between 6-thioguanine nucleotide concentrations and leucocyte counts.

CONCLUSIONS In patients with inflammatory bowel disease treated with azathioprine or 6-mercaptopurine, 6-thioguanine nucleotide concentrations did not correlate with disease activity, as measured by the inflammatory bowel disease questionnaire, or leucocyte count. These findings are discrepant with most previous studies, possibly due to selection of responding patients who tolerated the medications. A prospective, randomised, dose optimisation trial using 6-thioguanine nucleotide concentrations is warranted.