CLSI C65—Biochemical Tumor Marker Testing discusses preexamination, examination, and postexamination guidance as it relates to 10 commonly measured soluble biochemical tumor markers that circulate in blood and are usually measured in serum or plasma. Preexamination guidance is provided in relation to appropriate test selection for diagnosed cancer patients as well as those with suspected cancer, and also considers patient preparation, specimen collection and handling, and storage and transportation. CLSI C65 provides recommendations for method selection in the examination phase, implementation of internal and external QC (or proficiency testing) procedures, and identification of possible clinically relevant influences and other confounding factors that might affect results. Postexamination recommendations for tumor marker reporting are also presented. These include specifying the method used, cumulating serial results, and advising on clinical decision limits and clinically significant changes in tumor marker concentration.