External quality assessment of serum hormone determinations in the Nordic countries.

Since 1979 the Nordic Clinical Chemistry Project (NORDKEM) has organized four external quality assessment surveys of serum hormone determinations in the five Nordic countries. Charcoal-stripped control sera with weighed additions of the following hormones have been used: thyrotropin, human placental lactogen, prolactin, estriol, estradiol, testosterone, cortisol, triiodothyronine, thyroxine, and, in the fourth survey, progesterone. Human serum was usually used but in the first survey the serum matrix for the protein hormones was calf serum. In three years no systematic improvement in the coefficients of variation of the results has occurred. The change from calf serum to human serum also had no major effects on the results. Percentage coefficients of bias usually varied on both sides of the true value, but at low hormone concentrations the results were almost always positively biased. It is therefore useful to know the true hormone concentrations of the control samples because all-laboratory means are relatively often biased from these. Variation produced by different methods of calculation of the radioimmunoassay results and the influence of outlying standard points on these data processing routines were studied in the third survey. It is possible that sometimes the variation produced by different methods of calculation is one of the main components of the total variation in external quality assessment of RIA. There were no major differences between the performance of computerized or manual calculation methods. The influence of different calibration standards on serum thyroxine determination was studied in the fourth survey. Of the total coefficient of variation (10.8% to 17.0%) the component of variation produced by different thyroxine calibration standards was 0.8% to 4.6%. In conclusion, standardization of the data processing methods and further standardization of assay reagents are urgently needed to give possibilities to identify more subtle differences in the analytical performance of the laboratories. External quality assessment of hormone determinations needs improvement of the quality of reference materials.