THE EFFECTS OF MODIFYING PROFICIENCY TESTING MATERIALS ON THYROID-FUNCTION TEST-RESULTS

Objective.-To gain insight on the matrix effects, and possible clinica l implications, resulting from diluting and concentrating proficiency testing survey material used for the measurement of thyroid function t ests. Design.-To the standard set of five proficiency survey samples, three supplementary ''Wildcard'' samples were added. These additional samples were manufactured by overfilling and underfilling vials prior to lyophilization so as to vary the thyroxine-binding protein concentr ations. Survey participants measured thyroxine, free thyroxine, and th e triiodothyronine uptake and related tests on the Wildcard samples. I n addition, free thyroxine indices were calculated. Setting.-The first mailing of the 1995 College of American Pathologists (CAP) Ligand Ass ay-Series 1 Survey. Main Outcome Measures.-Results obtained from the r egular set of survey samples and the Wildcard set were compared to val ues expected by the laws of conservation of matter and mass action. Pa rticipants.-The approximately 2000 participants of the first mailing o f the 1995 CAP Ligand Assay-Series 1 Survey. Results.-Numerous assays systems did not give the predicted results, including all of the singl e-step radioimmunoassays for free thyroxine and over three quarters of free thyroxine index determinations. Conclusions.-Varying the dilutio n of proficiency survey material produced results that were not predic ted by the laws of conservation of matter and of mass action. Although these observations may have been the result of matrix effects, one ca nnot rule out the possibility that certain thyroid assays may not work in clinical situations having abnormal thyroxine-binding protein conc entrations.