Serum phenytoin concentration in multi-racial Singapore adult patients

Abstract

Serum phenytoin concentrations were determined in 1139 samples obtained from adult epileptic patients from different ethnic groups in Singapore; namely Chinese (881), Malays (98) and Indians (160). The data was analysed using statistical analysis system (SAS). One of the parameters used for comparison was the C/D ratio which is the ratio of concentration (mg L-1) per dose (mg kg-1). With the mean daily dose of 5 to 6 mg kg-1, the serum concentration (mean ± SEM) attained in the Indian group was 17.1 ± 1.35 mg L-1, which was significantly (p < 0.001) higher than that attained in the Chinese (10.5 ± 0.39 mg L-1) and in the Malays (11.2 ± 1.3 mg L-1; p < 0.005). The mean C/D ratio of 3.56 obtained in the Indians was also significantly (p < 0.001) higher than 1.85 in the Chinese and 2.26 (p < 0.01) in the Malays. This trend of inter-ethnic differences was also observed in patients who were at steady-state phenytoin and with (a) other concurrent medications, (b) no other medication or (c) phenobarbitone. Concurrent phenobarbitone generally increased the mean serum concentration of phenytoin. In the Chinese group, 56% of the requests for phenytoin monitoring was due to poor seizure control and the mean serum concentration was 8.6 mg L-1, while 15% was due to toxicity and the mean serum concentration in this group was 22.9 mg L-1.