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 Ax...

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Bibliographic Details
Published in:Clinical chemistry (Baltimore, Md.) Md.), 1997-06, Vol.43 (6), p.957-962
Main Authors: Norden, Anthony G. W, Jackson, Rodwin A, Norden, Lorraine E, Griffin, A. Jane, Barnes, Margaret A, Little, John A
Format: Article
Language:English
Subjects:
Aged
Aged, 80 and over
Arthritis, Rheumatoid - blood
Biological and medical sciences
Endocrinology
Female
Humans
Immunoassay
Investigative techniques, diagnostic techniques (general aspects)
Medical sciences
Pathology. Cytology. Biochemistry. Spectrometry. Miscellaneous investigative techniques
Prospective Studies
Reagent Kits, Diagnostic
Rheumatoid Factor - blood
Thyrotropin - blood
Thyroxine - blood
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Summary: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.
ISSN:0009-9147
1530-8561
DOI:10.1093/clinchem/43.6.957