Misleading thyroid function tests in congenital dysfibrinogenemia

The presence of latent fibrin clots is a recognised pre-analytical factor that causes inaccurate immunoassay results. This report details a case of a patient with Graves' disease and congenital dysfibrinogenemia (CD) that had serum thyroid function test results (TFTs) that were not in keeping with clinical signs or symptoms. Analysis of plasma samples taken from the patient was shown to provide more accurate results than those obtained using serum samples. Further cases of patients with CD, all sharing the same genetic mutation of fibrinogen, and discordant TFTs are described, where TFTs measurement in serum samples proved to be unreliable. Despite evidence of fibrin effecting immunoassays, this is the first report of its kind linking CD to erroneous immunoassay results. The mechanism is postulated to be related to atypical forms of fibrinogen resulting in latent fibrin in serum samples blocking the antigen binding site and leading to incorrect results. Congenital dysfibrinogenemia is asymptomatic in most patients and therefore abnormal, albeit inaccurate, TFTs may be the first finding. Recognition of CD as a cause of discordant results is important when interpreting TFTs to avoid unnecessary investigations and inappropriate clinical interventions to those with the disorder and potentially identify undiagnosed cases.