Lot-to-lot variation in three thyroid-related radioimmunoassay kits

We examined lot-to-lot variation in triiodothyronine, thyroxine, and thyrotropin radioimmunoassay kits over 14-20 months. For the thyroxine assay, less than 20% of the overall variation was attributable to lot-to-lot changes. In contrast, for the triiodothyronine and thyrotropin assays half to two-thirds of the overall variation was attributable to lot-to-lot differences. The shifts seen in control results were also observed in results for patients. These shifts produced disturbing deviations on quality-control charts but did not appear to compromise the clinical utility of the results. We conclude that lot-to-lot variation can account for a major part of the long term variability of radioimmunoassay kit results and should be considered in evaluating kits and monitoring their performance.