hemoglobin determinations. Cliii Chem 1986;32:358-60. Free Cortisol in Serum Assayed by Temperature-Controlled Ultrafiltrationbefore

A method is described for a temperature-controlled ultra-filtration procedure to measure free cortisol in serum. A special thermometer with a sensor was developed, mea-suringthe temperature directly in the ultrafiltrationdevice. The sensor is screwed on the axis of the centrifuge rotor, and the centrifuge is placed in a temperature-controlled box so that the temperature of the sample is kept at 37#{176}C ± 0.1 #{176}C.The overall CV of the free cortisolassay ranges from 2.2 % to 11.4%, of which the ultrafiltrationcontributes only 2.2-3.6%. Increasing amounts of cortisol-binding protein,as found inwomen usingestrogen-containingoral contraceptives, have minor but significant effects on the free cortisol concentrations in serum. Serum free cortisol concentrations in a reference population (n = 114; central 95 percentiles) were 12-43 nmol/L (4-9.5 % of total cor