Endocrine Abstracts

Introduction: Falsely high FT4 and FT3 with concurrent falsely low TSH have been reported in patients taking biotin and in a few case reports of patients with IgG anti-streptavidin antibody. However, other isotypes and effects on other immunoassays have not been reported.

Case: A 77 year female presented with worsening fatigue with a background of hyperthyroidism on carbimazole. Clinically she appeared hypothyroid. Her results on Roche Cobas platform were: TSH 0.75 mIU/l (R.I. 0.27–4.2), FT4 12 pmol/l (12–22) and FT3 8.1 pmol/l (3.9–6.8). However, tests repeated on Siemens Centaur platform were: TSH 37 mIU/l (0.3–4.0), FT4 7 pmol/l (10–20) and FT3 3.0 pmol/l (3.0–6.5), suggesting Cobas results were falsely low for TSH and falsely high for FT4 and FT3. To identify the cause of interference a second sample from the patient was preincubated with streptavidin microparticles. After incubation TSH on Cobas platform increased from 0.2 to 0.7 mIU/l, while FT4 reduced from 28 to 16 pmol/l, and FT3 from 9.0 to 4.7 pmol/l. Siemens results were unchanged. Elution of streptavidin microparticles with 0.1M citric acid identified a 75 kDa band on reducing SDS–PAGE. Purification and sequencing of peptide fragments by LC-MSMS of this band identified this protein as IgM heavy chain. This demonstrates IgM anti-streptavidin antibody as the cause of interference. The patient’s hypothyroid symptoms resolved when carbizamole was stopped. The effect of preincubation with streptavidin microparticles was also tested on Roche Cobas immunoassays for FSH, LH, oestradiol and testosterone in this patient’s serum. Sandwich immunoassays were falsely low and competitive immunoassays were falsely high. Oestradiol exhibited the largest difference, at 240 pmol/l initially but becoming undetectable following incubation.

Conclusions: This is the first case describing IgM anti-streptavidin antibody as the cause of interference on thyroid function tests and also shows its effects on other immunoassays.