Endocrine Abstracts

Introduction: Recently Roche Diagnostics (Rotkreuz, Switzerland) introduced the third generation of its Elecsys free thyroxine (FT4 iii) assay (in Greece from February 2019 onwards). The new assay is considered to have reduced biotin interference compared to its previous version (Ft4 ii). Kits of the FT4 ii assay are still available (and valid) according to the manufacturer’s expiration date. Using the FT4 ii assay per the manufacturer’s instructions we noticed FT4 results that were incongruent both with thyrotropin (TSH) and subsequently with free triiodothyronine (FT3) levels and the clinical presentation of patients.

Aim – Methods – Results: Evaluation with a Bland-Altman plot of FT4 ii results vs the FT4 iii assay’s results in 40 consecutive and unselected subjects showed few outliers (Figure 1). However, the results of twice as many patients (22/40 with FT4 ii vs 10/40 patients with FT4 iii) were in the hyperthyroid range (Mcnemar’s P = 0.005). From the subjects’ history-taking, interference by biotin (or by other substances) was ruled out. Finally the problem was traced to the provided assay’s calibration. This was communicated for assessment to the manufacturer.

Discussion: Knowledge of this is useful, particularly in the quest for recalibration of assays against reference measurement procedures^1,2.

References

1. International Federation of Clinical Chemistry and Laboratory Medicine. Clin Chem Lab Med. 2007 45 (7) 934–936. PubMed PMID: 17617044.

2. Spencer CA. Endotext 2017. http://www.ncbi.nlm.nih.gov/books/NBK279113/.