The 'ultra-free' ultrafiltration technique compared with equilibrium dialysis for determination of unbound thiopental concentrations in serum

We compare a new ultrafiltration technique, involving a unique Millipore membrane, with the classical method of equilibrium dialysis for determining the fraction of thiopental not bound to serum proteins. This fraction, as determined by equilibrium dialysis at 37°C, ranged between 12 and 16% for total concentrations of 50 μg/L to 10 mg/L of serum. In contrast, ultrafiltration at 37°C yielded a 49% higher value for unbound thiopental: 26.3 (SD 2.6)%. Determined at room temperature (24°C), there was no statistically significant difference for results by the two methods: 14.2 and 15.9%, respectively. The discrepancy between results at 37°C may partly be explained by serum proteins penetrating the Ultra-Free® filter. For the routine clinical measurement of unbound drug concentrations, the ultrafiltration membrane at room temperature appears to be sufficiently accurate and less time- consuming than equilibrium dialysis.