[Thiopurine methyltransferase activity in patients with autoimmune hepatitis]

The measurement of the activity of thiopurine methyltransferase (TPMT) is useful to monitor, on an individual basis, the dose of azathioprine in order to identify patients at risk of myelotoxicity. However, the distribution of the enzymatic activity in patients with autoimmune hepatitis is unknown. Our objective was to analyze the activity of TPMT in a group of 200 patients with autoimmune hepatitis and to evaluate the possible effect of some variables such as the treatment with azathioprine on this activity.

The activity of TPMT was determined by a radiochemical method; the activity was determined in the erythrocytes of patients with autoimmune hepatitis from 31 Spanish hospitals. We studied the relationship between TPMT levels and several demographic variables as well as its correlation with azathioprine treatment.

We included 209 patients (80% females, mean age 50 years, 39% on azathioprine). The mean value of TPMT was 20.7 U/ml erythrocytes (from 0 to 39). TPMT levels were adjusted to a normal distribution. 1%, 9% and 90% patients had low (< 5 U/ml), intermediate (5-13.7 U/ml) and high (>= 13.8 U/ml) TPMT levels, respectively. In the multivariate analysis, there were no differences when comparing the mean TPMT values according to age, gender or previous azathioprine treatment.

TPMT activity in patients with autoimmune hepatitis displays a similar distribution to that reported in other populations (approximately 1% of patients have low levels and 9% have intermediate levels) and it does not depend on the age, gender or simultaneous azathioprine treatment.