Misleading results from immunoassays of serum free thyroxine in the presence of rheumatoid factor.
A novel interference with measurements of serum free thyroxine (FT4) caused by rheumatoid factor (RhF) is described. We found misleading, sometimes gross, increases of FT4 results in 5 clinically euthyroid elderly female patients with high RhF concentrations. All 5 patients had high FT4 on Abbott AxSYM or IMx analyzers. "NETRIA" immunoassays gave misleading results in 4 of the 5 patients; Amerlex-MAB in 2 of 4 patients; AutoDELFIA in 2 of the 5; and Corning ACS-180 and Bayer Diagnostics Immuno 1 in 1 of the 5. BM-ES700 system results for FT4 in these women remained within the reference range. Results for serum T4, thyroid-stimulating hormone, free triiodothyronine, thyroid-hormone-binding globulin, and FT4 measured by equilibrium dialysis were normal in all 5 patients. Drugs, albumin-binding variants, and anti-thyroid-hormone antibodies were excluded as interferences. Addition to normal serum of the RhF isolated from each of the 5 patients increased the apparent FT4 (Abbott AxSYM). Screening of 83 unselected patients demonstrated a highly significant positive correlation between FT4 (Abbott AxSYM) and RhF concentrations. Discrepant, apparently increased FT4 with a normal result for thyroid-stimulating hormone should lead to measurement of the patient's RhF concentration.
Norden
A G
AG
Department of Chemical Pathology, Chase Farm Hospitals Trust, Middlesex, UK. 100127.1142@compuserve.com
Jackson
R A
RA
Norden
L E
LE
Griffin
A J
AJ
Barnes
M A
MA
Little
J A
JA
D000368:Aged; D000369:Aged, 80 and over; D001172:Arthritis, Rheumatoid / Q000097:blood; D005260:Female; D006801:Humans; D007118:Immunoassay; D011446:Prospective Studies; D011933:Reagent Kits, Diagnostic; D012217:Rheumatoid Factor / Q000097:blood*; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood*
D011933:Reagent Kits, Diagnostic; D013972:Thyrotropin; D012217:Rheumatoid Factor; D013974:Thyroxine