Assessment of qualitative tests.

For qualitative (2-class) tests, which provide binary (yes/no) information, the correctness of specimen classification remains the most important criterion for performance evaluation. However, a more informative picture emerges from the relationship between percentage of positive results and analyte concentration, which allows some inherent test characteristics to be derived (the positive/negative discrimination concentration and the "grey zone" around it). The information content of evaluation approaches is decidedly improved by the availability of numerical results (counts per minute, absorbance) in most situations of clinical interest. The concentration/response functions underlying the quality of individual tests may thus be derived and compared, and laboratory staff given a more objective criterion to judge individual performance. Examples are drawn from the authors' experience in running external quality assessment programs for tests for infectivity markers.

Immunoassay external quality assessment in Italy: HBsAg and anti-HBs tests.

The mode of operation of the CNR/Tecnostandard external quality assessment scheme for HBsAg and anti-HBs assays is outlined, and the relevant results reported. Emphasis is given to the retrospective evaluation of data in an attempt to derive a picture of the state of the art in Italian laboratories. The approaches followed for this evaluation include analysis of the rate of correct results and inspection of the relationships between analyte concentration and either the percentage of positive classifications of samples or the numerical test responses. Information is given on the "average" performance of individual kits as actually used in participants' laboratories.