Serum creatinine concentration and the discrepancy between enzyme multiplied immunoassay technique and gas-liquid chromatographic phenytoin levels

In patients on phenytoin therapy and with renal failure, the enzyme multiplied immunoassay technique (EMIT), unlike gas-liquid chromatographic techniques (GLC), can give falsely elevated serum phenytoin concentrations. In 25 patients (creatinine less than or equal to 1.5 mg per dl), discrepancies between the serum EMIT and GLC phenytoin concentrations were accounted for by the inherent imprecision of the methods. However, in 16 of 18 uremic patients (creatinine greater than or equal to 1.7 mg per dl), the discrepancies were greater than could be accounted for by imprecision. For these 18 patients the correlation (r = 0.49) between the discrepancies and the creatinine concentrations appeared to be statistically significant (P less than 0.05). The linear regression relationship was inadequate for accurate correction of the EMIT values using the creatinine concentrations. Another original observation was that in most cases renal dialysis decreases the EMIT to GLC discrepancy. However, post-dialysis serum samples with a creatinine concentration greater than 1.7 mg per dl usually still showed a falsely elevated EMIT phenytoin concentration.