Identification of interferences in free thyroxin assay—establishment of acceptable recovery limits for the dilution test

Abstract

: One of the simplest and most effective methods to verify the presence of analytical interferences in immunoassays is the dilution test. Use of this test is generally not recommended in free T4 (fT4) immunoassays, but it has been previously successfully used for the detection of interferences. The aim of this study was to confirm the data previously published and to determine an acceptable range for recovery, beyond which the presence of interference can be considered probable. fT4 were measured with Access Free T4 assay (Beckman Coulter Inc., Brea, CA, USA). After preliminary evaluation of this method’s performance in a dilution test of 13 samples of patients with different binding capacities, the acceptable range of recovery of the dilution test was evaluated using 121 analytically and clinically plausible specimens within the measurement range of the assay. Moreover, 44 cases were evaluated with inconsistent results among the measured thyroid hormones. The acceptability range (2.5th–97.5th percentile) of recovery was between 77.4% and 121.7%. In 23 of the 44 suspected cases evaluated, the result of dilution showed recoveries within the established acceptability range, and the value of fT4 was also confirmed on another analytical platform (Liason XL). In the 21 cases where the recoveries were outside the acceptability range, the concentrations measured on Liaison XL were instead markedly different from the Access method and showed probable interferences. In conclusion, it has been verified that the Access fT4 method allows to obtain an excellent recovery at the dilution test, at least up to the 1/10 dilution. The reference ranges established for the acceptability of recovery under non-interference conditions allow to identify a possible analytical interference in a simple and reproducible way.