MISLEADING RESULTS FROM IMMUNOASSAYS OF SERUM-FREE THYROXINE IN THE PRESENCE OF RHEUMATOID-FACTOR

A novel interference with measurements of serum free thyroxine (FT4) c aused by rheumatoid factor (RhF) is described. We found misleading, so metimes gross, increases of FT4 results in 5 clinically euthyroid elde rly female patients with high RhF concentrations. All 5 patients had h igh FT4 on Abbott AxSYM(R) or IMx(R) analyzers. ''NETRIA'' immunoassay s gave misleading results in 4 of the 5 patients; Amerlex-MAB(R) in 2 of 4 patients; AutoDELFIA(R) in 2 of the 5; and Coming ACS-180(R) and Bayer Diagnostics Immune 1(R) in 1 of the 5. BM-ES700(R) system result s for FT4 in these women remained within the reference range. Results for serum T-4, thyroid-stimulating hormone, free triiodothyronine, thy roid-hormone-binding globulin, and FT4 measured by equilibrium dialysi s were normal in all 5 patients. Drugs, albumin-binding variants, and anti-thyroid-hormone antibodies were excluded as interferences. Additi on to normal serum of the RhF isolated from each of the 5 patients inc reased the apparent FT4 (Abbott AxSYM). Screening of 83 unselected pat ients demonstrated a highly significant positive correlation between F T4 (Abbott AxSYM) and RhF concentrations. Discrepant, apparently incre ased FT4 with a normal result for thyroid-stimulating hormone should l ead to measurement of the patient's RhF concentration.