Comparison of fresh frozen serum to proficiency testing material in College of American Pathologists surveys: alpha-fetoprotein, carcinoembryonic antigen, human chorionic gonadotropin, and prostate-specific antigen.

CONTEXT: Most proficiency testing materials (PTM) contain an artificial matrix that may cause immunoassays to perform differently with this material than with clinical samples. We hypothesized that matrix effects would be reduced by using fresh frozen serum (FFS). OBJECTIVE: To compare the performance of an FFS pool to standard PTM for measurement of alpha-fetoprotein, carcinoembryonic antigen, human chorionic gonadotropin (hCG), and prostate-specific antigen (PSA). DESIGN: One FFS specimen and 4 different admixtures of PTM were distributed in the 2003 College of American Pathologists K/KN-A (for alpha-fetoprotein, carcinoembryonic antigen, hCG, and total and free PSA) and C-C (hCG only) Surveys. PARTICIPANTS: The number of laboratories that participated in the surveys varied from a low of 288 (free PSA, K/KN-A Survey) to a high of 2659 (hCG, C-C Survey). MAIN OUTCOME MEASURES: Method imprecision and method bias were compared between the FFS specimen and the standard PTM specimen with the closest value. Method imprecision was determined by calculating the coefficients of variation for each method and for all methods combined. Bias was defined as the proportional difference between peer-group mean and the median of all method means. RESULTS: The FFS specimen gave significantly higher imprecision than PTM for the analytes alpha-fetoprotein, carcinoembryonic antigen, total PSA, and free PSA. For hCG, no substantial imprecision differences were observed in both surveys. Bias was significantly greater for the alpha-fetoprotein, carcinoembryonic antigen, and total PSA assays and significantly lower for the hCG and free PSA assays when comparing the FFS with the PTM. CONCLUSIONS: Fresh frozen serum did not provide consistently lower imprecision or bias than standard PTM in a survey of commonly ordered tumor markers.