Measurement of lidocaine free concentration

Since lidocaine exhibits significant variation in serum protein binding, the availability of a practical method for measuring free lidocaine concentration could contribute to the optimization of individual lidocaine dosage regimens. Fifty serum samples from patients receiving lidocaine were partitioned by ultrafiltration and equilibrium dialysis. The lidocaine concentration in the ultrafiltrate was measured using an enzyme multiplied immunoassay (EMIT) and a gas-liquid chromatographic assay (GLC). The lidocaine concentrations in dialysates and filtered retentates were measured by EMIT. Ultrafiltrate concentrations measured by EMIT correlated well with those measured by GLC (r2 = 0.77), but the EMIT results were approximately 10-20% higher than the GLC measurements (GLC = 0.09 + 0.79 EMIT). At least a portion of this difference could be attributed to minor calibrator differences. The concentrations in dialysate and filtered retentate agreed well (r2 = 0.93; filtered retentate = -0.05 + 1.12 X dialysate). The fraction free values obtained by ultrafiltration were slightly lower than those obtained by equilibrium dialysis (0.301 +/- 0.086 vs. 0.345 +/- 0.137; p less than 0.05). It can be concluded that sample partitioning with ultrafiltration and measurement of free lidocaine concentration by EMIT yields results similar to those obtained by equilibrium dialysis or a GLC assay procedure.