Problems of validity in steroid immunoassays.

A valid steroid immunoassay is the one which correctly describes the contents in unknown samples. Such an assay has to be accurate (devoid of bias) and precise. Assay accuracy can be assessed by a comparison of the results with those obtained by a reference method, e.g. gas liquid chromatography-mass spectrometry. Alternatively, the accuracy assessment can be made in terms of the purity of the compound assayed. The test of parallelism or the test of radiochemical purity can be used for this purpose. The former test, however, is not fully reliable. The degree of precision is of great importance especially when validity of individual measurements is considered. A low reproducibility (a high between-assay variation) leads to results of uncertain validity. An increased standardization and simplification of assays may result in a substantial improvement of the precision, and thus validity.