Performance characteristics of four free phenytoin immunoassays

The measurement of the unbound or free phenytoin concentration is indicated in several situations, including uremia. In patients with uremia, metabolites of phenytoin and other substances accumulate and can displace phenytoin from its protein binding sites, with a consequent increase in the free fraction of drug. Some of the phenytoin metabolites that accumulate in uremia can cross-react with phenytoin immunoassays. In this study the authors evaluated four free phenytoin immunoassays compared with a high-performance liquid chromatography (HPLC) method: the Roche COBAS Integra, the Syva EMIT 2000, the Opus INNOFLUOR, and the Abbott TDx. All four methods demonstrated good precision, with interday coefficients of variation of ≤5% and comparable recoveries using quality control material. Two of the methods, the EMIT 2000 and COBAS Integra, showed excellent agreement with the HPLC method using samples from patients both with normal renal function and with renal insufficiency. The other two methods, the INNOFLUOR and TDx, showed average positive biases for the therapeutic range of 3-7% and 21-22%, respectively, compared with the HPLC method for samples from patients with normal renal function, and average positive biases of 24-32% and 75-81%, respectively, with samples from patients with uremia.