Procalcitonin interference in an immunometric calcitonin assay

Background. Procalcitonin (PCT) increases in septic patients, and is not transformed into calcitonin (CT). We found in septic patients, a significant increase of CT, as determined by an immunoassay using polyclonal antibodies. We compare determination using polyclonal and monoclonal AB. Methods. We included 34 patients: 17 with clinical signs of sepsis, a positive haemoculture (PCT > 0.5 mu g/L) and 17 without them (PCT < 0.1 mu g/L). CT was determined by two above-mentioned methods. The influence on CT levels was observed after using the high-concentration PCT calibrator addition to a mixed serum sample with a low concentration of CT. The dilution test of the high-concentration calibrator PCT was performed by an IBL calibrator, with a zero calcitonin concentration. Results. In the septic patients we found an interference in calcitonin determination using the polyclonal AB (IRMA); 24.1-718 mu g/L, proportional to the PCT levels (r = 0.814, p < 0.0001). When using the monoclonal AB (ELISA), the calcitonin levels < 6.5-46.3 ng/L, and no interference of PCT was observed. In the non-septic group, we did not record any PCT interference using either the polyclonal or the monoclonal AB, and the CT levels were within the reference ranges using the two methods (r = 0.997, p < 0.0001). The recovery and dilution tests confirmed interference by PCT on the calcitonin determination with the polyclonal antibody. Conclusions. Results show that in septic patients there is visible interference of PCT in the calcitonin determination, principally in the IRMA method (polyclonal AB); while no such relationship was observed in the ELISA method (monoclonal AB).