Urine and plasma free phenytoin concentration correlation

In an attempt to find a simple, noninvasive method for estimating the plasma free concentration (Cpf) of phenytoin (PHT), the relationship between urine PHT concentration (Cu) and Cpf was studied in 40 epileptic patients who were 6 to 50 yr old. Cu was determined by gas-liquid chromatography and Cpf by equilibrium dialysis at 37°. Cu was generally greater than Cpf, but correlation between the two was strong (r2 = 0.876) and in the same range as previously published results for saliva and Cpf. Correlation of Cu with Cpf was not influenced by patient age, urine pH, or urine flow rate. In 24 patients timed urine collections were obtained and calculated renal clearances of PHT were shown to increase in proportion to urine flow rates. Although it is not known if renal impairment and albuminuria will alter the relationship between Cu and Cpf, it appears that Cu may be useful in estimating the concentration of pharmacologically active PHT in plasma. Clinical Pharmacology and Therapeutics (1982) 32, 628–634; doi:10.1038/clpt.1982.213