Discordance in thyroglobulin measurements by radioimmunoassay and immunometric assay: a useful means of identifying thyroglobulin assay interference

BackgroundSerum thyroglobulin (Tg) is useful for monitoring patients with differentiated thyroid cancer (DTC) but is limited by interference from anti-Tg antibodies (TgAb). We determined Tg assay discordance between a radioimmunoassay (RIA) and one of two immunometric assays (IMA) in DTC patients over a 9-year period to gauge assay performance against evidence of recurrent/progressive DTC.MethodsPatients with DTC monitored for >1 year attending local clinics between September 2000 and January 2010 were included. All samples were analysed for Tg using both RIA and IMA. TgAb were measured on all Tg requests made after May 2006. Bias plots comparing RIA against IMA were established to calculate a 2-SD outlier limit. Clinical records were viewed to compare discordant Tg results against clinical evidence of recurrent/progressive DTC.ResultsDiscordant Tg results were observed in 53/433 patients (12.2%). Four were discordant owing to a higher IMA result, one of which demonstrated recurrence. The remaining 49 pat...