An interfering component in cardiac troponin I immunoassays—Its nature and inhibiting effect on the binding of antibodies against different epitopes

Objectives: We recently reported on the occurrence of a common interfering factor (IF) that negatively affects determinations of cardiac troponin I (cTnI). The aim of the present investigation was to extend our initial finding by a detailed epitope-based determination of the location of IF and to reveal the approximate size and characteristics of IF. Design and Methods: Two-site immunoassays using combinations of 16 monoclonal and 2 polyclonal cTnI antibodies and 1 monoclonal troponin C (TnC) antibody were used to measure the analytical recovery of cTnI or cTnI–TnC in serum samples. Gel filtration of serum samples containing IF was performed and the analytical recovery of cTnI in the fractions was determined. EDTA-plasma samples to which cTnI had been added and serum samples containing endogenous cTnI were also separated by gel filtration. Results: The mean analytical recoveries of cTnI were 28.3% (range 7.5–55.1%) and of cTnI–TnC were 23.5% (range 8.7–51.8%) in samples containing IF when antibodies against midfragment epitopes of cTnI were used. The mean recovery of cTnI was 65.1% and 73.3% for antibodies with N- and C-terminal epitopes. Gel filtration of samples with low recovery of cTnI showed that the approximate molecular mass of IF was 50–200 kDa and that the cTnI elution profiles of samples containing IF were shifted towards higher molecular mass compared with samples with less IF. Conclusions: Antibodies against midfragment epitopes of cTnI are affected by IF to a considerable but variable extent, and the presence of IF can be demonstrated by gel filtration.