Immunometric assay interference: incidence and prevention.
BACKGROUND: The primary aim of the study was to reduce interference in an in-house two-site, two-step immunometric assay. METHODS: In the running laboratory routine, 11 261 samples were tested with a carcinoembryonic antigen (CEA) assay with bovine immunoglobulin but no murine immunoglobulins in the buffer, in parallel to our routine CEA assay, using 15 mg/L heat-treated nonspecific murine immunoglobulin (MAK33) in the buffer and with the Fc fragments removed from the capture antibody. RESULTS: The frequency of interference was estimated to be 4.0% (95% confidence interval, 3.3-4.7%). The addition of 15 mg/L native MAK33 had little effect (frequency, 3.9%; 95% confidence interval, 3.2-4.6%), whereas adding 15 mg/L heat-treated MAK33 reduced interference to 0.86% (0.61-1.12%), and adding 50 mg/L reduced it further to 0.06% (0-0.13%). Removing the Fc fragments by itself reduced interference to 0.10% (0.02-0.19%). There were no statistically significant differences for age (P <0.23) or gender (P <0.40) between patients with interference (n = 210) and a randomly selected interference-negative control group (n = 186). Interference was not constant in patients: 15 of 25 individuals positive for interference and with four or more samples screened for interference had an interference-negative sample either before or after the peak of interference. CONCLUSIONS: In a two-site, two-step immunometric assay using mouse monoclonal antibodies, use of heat-treated nonspecific murine immunoglobulin in the buffer or removal of the Fc fragment from the capture antibody could improve performance.