Interpretation of results for tumor markers on the basis of analytical imprecision and biological variation.

Interpretation of results for CA 15.3, carcinoembryonic antigen (CEA), and tissue polypeptide antigen (TPA) during breast cancer monitoring requires data on intra- (CVP) and inter- (CVG) individual biological variation, analytical imprecision (CVA), and indices of individuality. The average CVP and CVG obtained from 22 healthy women were, respectively, 6.2% and 62.9% (CA 15.3), 9.3% and 86.8% (CEA), and 28.3% and 133% (TPA). The indices of individuality were all < 0.6: 0.2 (CA 15.3), 0.15 (CEA), and 0.2 (TPA). CVA depended on the concentration of the analytes. CVP and CVA determine what constitutes a significant difference between sequential results. Assuming a CVA of 11.2% (CA 15.3), 9.5% (CEA), or 11.9% (TPA), results must differ by 30%, 31%, or 72%, respectively, for P < or = 0.05. We found that CVP and CVA contribute considerably to the variation during breast cancer monitoring. Consequently, both CVP and CVA should be considered in criteria for marker evaluation. Because of low indices of individuality, conventional cutoff limits are inappropriate both for initial identification and for follow-up of breast cancer.