Our experience with quality control in current growth hormone assays

For many years now the German Society for Clinical Chemistry has organized interlaboratory surveys by order of the Federal Medical Association. Human growth hormone (GH) is one of 20 endocrine parameters included in a set of two control specimens and offered to survey participants at least 4 times a year. Since no reference method exists for GH, the 'true value' remains unknown. Thus, evaluating survey results is limited to a presentation of how far one participant's results agree with those of other participants using the same or different methods. Medians and percentiles are calculated for all participants and, when possible, for subgroups using the same kit. Over the last years, 60-80 laboratories have participated in each GH survey with 12-16 different kits. The participants' results have shown considerable scatter with differences in GH levels of several hundred percent. The largest discrepancies have occurred with specimens of low GH concentrations, those especially interesting for pediatricians. An evaluation of subgroups using the same kit has shown a relatively good group agreement but large method-dependent differences between these groups. As regards the median, the results of different kits varied by as much as 250%. Accordingly, different reference values have to be applied to different GH kits. For the last 3 years not only have several companies changed their methods, but also one third of the laboratories participating in the survey have switched to other GH kits. This leads us to wonder if all the clinicians served by these laboratories are being informed of this and of any resulting discrepancies in their GH parameters.