Urinary hydroxyphenytoin/creatinine ratios as an index of compliance in adult epileptic patients on phenytoin therapy.

The ratio of the concentration of the phenytoin metabolite hydroxyphenyl phenylhydantoin (HPPH) to the creatinine concentration in morning urine samples was used as an index of compliance in adult epileptic patients on phenytoin therapy. For this purpose, the phenytoin dose was estimated from the urinary HPPH/creatinine ratio with the help of a regression equation that took into consideration body weight, height, age, and sex. A high correlation (r = 0.927 for men and r = 0.954 for women) between the prescribed and estimated dose was found for a reference group (127 hospitalized patients on supervised drug regimen). Compliance was judged from the discrepancy between the prescribed and the estimated dose for 45 patients immediately after their admittance to an epilepsy clinic, as well as for 98 outpatients, who had been treated by neurologists. Noncompliance was suspected in 6.7% of patients admitted to the clinic, as well as in 28.6% of the outpatients.