Effects of haemolysis, lipaemia and bilirubinaemia on an enzyme-linked immunosorbent assay for cortisol and free thyroxine in serum samples from dogs.

The possible interference of haemolysis, lipaemia and bilirubinaemia on commercially available enzyme-linked immunosorbent assays (test kits Enzymun-Test; Boehringer Mannheim) for cortisol and free thyroxine (FT4) in canine plasma samples was studied. Serum samples from 20 clinically normal dogs were enriched in vitro with different amounts of fresh haemolysate, lipid and bilirubin and compared with the original sera. The tests were used in connection with the analyser system Enzymum-Test (Boehringer Mannheim) System ES300. Haemolysis significantly (P = 0.039) interfered with the accuracy of FT4 determination independent of haemoglobin concentrations. Thus, haemolysis should be avoided in FT4 testing by competitive enzyme immunoassay. Lipaemia significantly interfered (P = 0.0015) with cortisol determination but not with FT4 determination (P = 0.41). Regression analysis showed that FT4 concentration significantly increased as triglyceride levels increased, especially at concentrations greater than 125 mg dL-1. The positive bias observed was of no diagnostic importance for cortisol results and the increase of FT4 levels could be precisely predicted by linear regression analysis. The addition of bilirubin significantly (P = 0) interfered with cortisol testing but, as with lipaemia, the magnitude of the differences were of no clinical significance.