[Comparison of three quality control investigations on radioimmunological determination of thyrotropin using the "Munich model"].

The results of a thyrotropin external quality assessment survey (EQAS) carried out in 1980 using the "Munich Model" and including a "matched reagent" experiment are presented and are compared with the results from similar surveys carried out in 1974 and 1977. Despite the fact that in 1974 only 4 different kits were used, compared with 12 in 1977 and 17 in 1980, the inter-laboratory precision has continually improved. This was reflected by the inter-laboratory coefficient of variation (CV) for all laboratories, which, for a sample with ca. 15 microU/l thyrotropin were: 65% (1974), 22% (1977) and 12% (1980), and for a sample containing ca. 5 microU/l thyrotropin: 75% (1974), 45% (1977) and 20% (1980). During the same period, the fraction of participants using commercial kits increased from 0.65 to 0.95. There was a definite trend towards shorter assays, even though with such assays there is a danger that the measured values for low thyrotropin concentrations will be too high. In spite of the fact that the "high-blank" effects have to a large extent disappeared, many kits showed an unacceptably high cross-reactivity with human chorionic gonadotropin. A fraction of 0.30 of all participants in 1974 used computer-assisted data-processing, compared with 0.78 in 1980. In 1980, the most commonly used algorithm was a spline function (0.46 of participants using a computer for data-processing). Although the number of kits has increased, the precision has improved, showing that more robust methods are now employed. This has partly come about because of the introduction of human thyrotropin-free serum as matrix for the standards (1974 - 0.16, 1980 - 0.83 of all participants). Further external quality assessment programmes are discussed for the continual monitoring of hormone assay performance.