Peripheral detection of S100beta during cardiothoracic surgery: what are we really measuring?

BACKGROUND: S100beta has been used in cardiac surgery to identify patients with postoperative neurologic complications. However, extracranial proteins may falsely elevate measurements of serum S100beta;. Objectives of this study were (1) to quantify S100 beta levels in serum and pericardial cavity during coronary artery bypass grafting (CABG), and (2) to identify proteins recognized by standard immunodetection as S100beta.
METHODS: Systemic and pericardial cavity blood from 5 patients undergoing CABG were sampled before, during, and after cardiopulmonary bypass (CPB). A commercially available enzyme-linked immunosorbent assay (ELISA) kit was used to quantify S100beta. Two-dimensional gel electrophoresis, Western blot, and mass spectroscopy were also performed to identify S100a and other proteins.
RESULTS: Mean S100beta levels measured by ELISA, systemic and pericardial cavity blood were (in ng x mL(-1)) 1.0 +/- 0.46 and 111 +/- 71 before CPB, 0.6 +/- 0.11 and 113 +/- 54 during CPB, and 1.7 +/- 0.64 and 101 +/- 42 after CPB, respectively. However, gel electrophoresis and Western blot analysis revealed proteins other than S100beta to be present in the pericardial cavity giving a falsely elevated serum S100a levels measured by immunoassay. Mass spectroscopy of identified potential candidates revealed contaminants including haptoglobin I precursor, apolipoprotein A-1 precursor, complement factor B precursor, and complement C3 precursor.
CONCLUSIONS: S100beta immunoassays are not specific for S100a and give a falsely elevated reading due to contaminants from the surgical field that cross react with the assay's antibody. This does not appear to be an issue in nonsurgical patients. Caution must be exerted when evaluating immunodetection results for low-abundance proteins under conditions where contamination of the sample is likely.