161. Measurement of total testosterone in women: comparison of a direct radioimmunoassay versus radioimmunoassay after organic solvent extraction and celite partition column chromotography

Abstract

Testosterone (T) has multiple significant physiological effects in women. To date no rapid, simple assay of total T has been shown to produce reliable results in women at the low end of the normal female range. The aim of this study was to evaluate the accuracy of a direct radioimmunoassay (dRIA) for total T by comparing values for total T measured by this assay with values determined by a conventional RIA (cRIA) method that utilizes extraction and chromatographic steps prior to quantification. Methods: Fasting serum samples were obtained from a sub-group of 259 healthy women, aged 18-75 years, randomly recruited from the community and stored at -80°C. Total T was measured by the dRIA method using antibody coated tubes and iodine-labeled T tracer. For comparison, total T levels were also measured using the cRIA after organic solvent (ethylacetate : hexane (3 : 2)) extraction and Celite column partition chromatography prior to RIA. Results: The mean T level by dRIA was 0.76 nmol/L (median 0.70, SD 0.54, min 0.10, max 3.2). The mean difference between the two measurements (dRIA-cRIA) was –0.28 (SD 0.3). The limits of agreement using the Bland-Altman approach on log transformed data showed that, on average, the dRIA value was 63% of the cRIA value and that 95% of the time the dRIA estimate lay between 26% and 155% of the cRIA estimate. However, with respect to clinical application, for classification of values in the lowest 10th centile, agreement between assays was seen in 245/259 women (Kappa = 0.68) Conclusion: The dRIA is a clinically useful assay that provides precise measurements of total T in women, particularly when values are low, and is appropriate for the study of the issue of 'low' T within the female population.