Clinical Chemistry
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Clinical Chemistry 43: 957-962, 1997;
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(Clinical Chemistry. 1997;43:957-962.)
© 1997 American Association for Clinical Chemistry, Inc.

Articles

Misleading results from immunoassays of serum free thyroxine in the presence of rheumatoid factor

Anthony G. W. Norden1,a, Rodwin A. Jackson2, Lorraine E. Norden1, A. Jane Griffin3, Margaret A. Barnes4 and John A. Little5

1 Departments of Chemical Pathology and
3 Rheumatology, Chase Farm Hospitals Trust, The Ridgeway, Enfield, Middlesex EN2 8JL, UK.

2 Department of the Care of the Elderly, Enfield Community Care Trust, The Ridgeway, Enfield, Middlesex EN2 8JL, UK.

4 Eagle House Surgery, 291 High St., Ponders End, Enfield, Middlesex EN3 4DN, UK.

5 North-East Thames Regional Immunoassay (NETRIA) Laboratory, St. Bartholemew's Hospital & Medical College, 51/53 Bartholemew Close, London EC1A 7BE, UK.
a Author for correspondence. Fax 0181 342 0558; e-mail 100127.1142{at}compuserve.com

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.


Key Words: indexing terms: hyperthyroidism • rheumatoid arthritis • fluoroimmunoassay • polyethylene glycol • affinity chromatography • immunoabsorption • equilibrium dialysis • thyroid-stimulating hormone




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