Quantitative determination of phenytoin and phenobarbital in capillary blood by Ames Seralyzer

We evaluated the use of serum obtained from capillary blood specimens with the Seralyzer ARIS (AMES Division Miles Laboratories) assay for phenytoin (PHT) and phenobarbital (PB). The Ames Seralyzer method is capable of measuring PHT and PB concentrations in 10 microliter of serum. Therefore, it may be used to measure concentrations of these drugs in small blood samples, such as obtained by skin puncture and capillary tube collection. In this study, simultaneous venous and capillary samples were obtained from 90 patients with epilepsy. Seralyzer results were compared with serum values from venous blood determined with a gas chromatographic (GC) method. Comparison of PHT values obtained by Seralyzer (Ys) versus GC by linear regression gave the equation: Ys = 0.36 + 0.88 X (r2 = 0.93, n = 63). The mean difference between the Seralyzer and GC was -0.96 micrograms/ml (p less than 0.001). For PB, the equation was: Ys = -0.53 + 1.07 X (r2 = 0.93, n = 31) with a mean difference of +1.28 micrograms/ml (p = 0.02).