Rapid fluorometric analysis of unbound salicylate in whole blood

A method has been developed for the analysis of unbound salicylate by membrane cone ultrafiltration of either whole blood or plasma with quantitation by spectrofluorometry. The method was used to study the distribution of salicylate in whole blood over a wide range of concentrations. It was found that at a concentration of 50 ug/ml in whole blood only 9% of the salicylate is present as free drug in the plasma water, while at a concentration of 500 ug/ml, 23% is free. At concentrations exceeding 500 ug/ml the plasma protein binding sites appear to be fully satrated since the free drug concentration increases linearly with respect to the whole blood concentration. Changes in hematocrit have a relatively minor effect on the ultrafiltrate salicylate concentration. The blood/plasma salicylate concentration ratio was found to vary widely over the blood salicylate range studied, 50-1000 ug/ml.