Choice of the kit and assay validation

Presently most laboratories choose to measure serum testosterone and DHT by commercially available kits. In all cases the final adoption of a kit for routine determination should depend on an in-house re-validation of the candidate assay. This re-validation includes an assessment of the sensitivity, specificity, accuracy and intra-assay precision of the kit. The specificity and the accuracy of the assay are checked by performing cross-reactivity, parallelism and recovery tests. These tests are usually carried out by the kit manufacturer beforehand and the data are reported in the kit instructions, but it is good practice to repeat them in one's own laboratory, since experience shows that many systems fail to deliver what they promise. Parallelism and recovery tests are crucial and should be performed with several different individual serum samples, since, especially in direct, non-extraction methods, the matrix in which the calibrators are dissolved may be inappropriate, resulting in non-linearity. The serum-matrix effect can be avoided using extraction methods.

Commercial kits contain control sera. Kit controls usually perform quite well and provide results in the expected range. However, care should be taken to use independent, certified control sera, in which the testosterone concentration has been determined by mass spectrometry. Quite often kit controls are in the expected range, while certified control sera are not. Such kits should not be used. Another criterion for choice is comparison of results in a sufficient number of patient sera to those obtained with a validated method. All these tests should be performed even if the candidate testosterone system is part of an automatic multianalyzer. Companies should be asked to contribute data on the calibrators and to allow complete validation before the choice to adopt that system is made. This is particularly relevant considering the overall poor performance in external quality control trials of most kits and automatic analyzers with sera containing low testosterone concentrations, i.e. from children, women and hypogonadal men.

Laboratories must often face the decision whether to adopt a new methodology. This is due to the large number of kits competing for the market or to the launch of an "improved" kit version, to budgetary necessities calling for automation and/or adoption of a multianalyzer, or to more stringent regulatory and environmental laws. All these aspects must be considered, but converting to a new methodology should never be at the costs of accuracy and reproducibility. Sometimes changes in reagent lots cause major differences in the results obtained with a kit; these become evident both from the clinical plausibility of the data and from internal quality control. In this case a full re-validation should be performed, the kit manufacturer approached and, if the problem persists, the methodology changed.

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