INFLUENCE OF SERUM SEPARATOR TUBES ON TOTAL AND FREE PHENYTOIN CONCENTRATIONS AND DOSAGES

The influence of serum separator tubes (SSTs) on total and free phenyt oin concentrations and phenytoin dosages was evaluated in patients tre ated with phenytoin. Thirty blood samples were obtained from 18 patien ts. Equal volumes of blood were placed into SSTs and plain tubes. Samp les were centrifuged, and serum analyzed for total and free phenytoin by fluorescence polarization immunoassay (FPIA) at 2, 24, and 48 h. To tal phenytoin concentrations collected in SSTs were significantly lowe r than those collected in plain tubes at 2 (12.3 vs 12.8 mg/L, p < 0.0 1), 24 (11.4 vs 12.9 mg/L, p < 0.01), and 48 h (10.9 vs 12.9 mg/L, p < 0.01). These differences resulted in significantly higher calculated maximum rates of metabolism (V(max)) and daily phenytoin dosages (R0) for SSTs at 24 (V(max): 10.4 vs 10.1 mg/kg/day; R0: 8.2 vs 8.0 mg/kg/d ay, p < 0.01) and 48 h (V(max): 10.8 vs 10.3 mg/kg/day; R0: 8.5 vs 8.1 mg/kg/day, p < 0.01). Free phenytoin concentrations from SSTs were si gnificantly lower at 48 h (1.56 vs 1.61 mg/L, p < 0.05). However, ther e were no significant differences in calculated dosages. Observed stat istical differences in total phenytoin concentrations can be clinicall y important for making dosage adjustments, especially in patients unde rgoing nonlinear elimination. Thus, SSTs should not be used to collect blood for total serum phenytoin determination.