Pitfalls in the determination of unbound carbamazepine concentrations in plasma

The usefulness of three simple, commercially available ultrafiltration systems [ Centriflo CF25 (A), Centriflo CF50A (B) and Ultrafree (C)] in determining concentrations of unbound carbamazepine in plasma was tested. The concentrations of carbamazepine measured in the first fractions of the ultrafiltrate were low, probably as a result of binding of the drug to the ultrafiltration system. The amount of this binding decreased in the order A greater than B greater than C. When using A and B, the first portion of ultrafiltrate obtained after centrifuging for 15 minutes must therefore be discarded. Using the Ultrafree filters this was not necessary. Plasma proteins were found to pass through the Ultrafree filter membrane when more than 0.7 ml ultrafiltrate was collected. Comparison of the ultrafiltration methods with equilibrium dialysis showed a good correlation: r = 0.96 (A), r = 0.97 (B) and r = 0.98 (C). However, in the carbamazepine plasma concentration range 2.5-10.0 micrograms/ml, significantly lower free fractions were measured with the Centriflo cones. The free fractions measured were: 0.28-0.30 (equilibrium dialysis), 0.17-0.18 (A), 0.22-0.24 (B) and 0.29-0.30 (C). The time required to collect the ultrafiltrate was 30 minutes (A and B) and 2 hours (C).