(8, 9). Although we did not investi-gate collection tubes from various manufacturers, available evidence, including our data, indicates that these problems may be common. The more rapid coagulation process in clot-activator tubes may be associ-ated with more extensive proteolysis in the specimen, potentially leading to greater protein fragmentation that is subsequently detected by mass spectrometry. Nonbiological changes, observed repeatedly in the low-mo-lecular-weight serum proteome pro-files, raise the question of whether serum is the specimen of choice for major protein- and/or peptide-type clinical analytes such as hormones and tumor markers (8). In summary, clot activator-con-taining collection tubes may lead to preanalytical artifacts in proteomic studies. In our experience, these tubes can be effectively substituted with Li-heparin plasma tubes for chemistry analytes or plain serum tubes used for immunoassay and specimen banking. This study was supported by Inter
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