Assessment of cyclosporin A in whole blood and plasma in five patients with different hematocrits.

Measurement of whole blood and plasma levels of cyclosporin A (CsA) by radioimmunoassay in kidney transplant recipients receiving the drug showed that CsA concentrations in plasma increased nonlinearly when whole blood levels of the drug exceeded 1,000 ng/ml. At low plasma levels (less than 200 ng/ml), most of the CsA in the blood was in the nonplasma component, indicating that cellular elements have high affinity for CsA. Comparison of nonplasma CsA concentrations in two patients with hematocrit values of 32.5 and 35% showed that in the patient with a hematocrit of 35% the cellular associated drug was twice as great as that in the other patient, indicating that there may be significant differences in the cellular affinity of CsA in patients with similar hematocrits. Linear regression analysis of the cellular associated CsA versus plasma levels of the drug in a double-reciprocal plot showed a drug saturation capacity of 6,060 ng/ml in the nonplasma component of blood in the patient with a hematocrit of 35%. Similar analyses in the other patients indicated saturation capacities ranging from 4,750 to 10,400 ng/ml.