AbstractIntroduction of preanalytical automation in our laboratory required the use of plastic blood collection tubes. Because of possible interference caused by adsorption of components to the plastic wall and because there is virtually no literature on this subject, we investigated the influence of collection of serum in plastic tubes on the results of nearly all our immunoassays for hormones and tumour markers. Blood from healthy volunteers was collected simultaneously in glass and plastic tubes, or sera prepared from blood collected in glass tubes were brought into the plastic tubes under investigation. Hormone and tumour marker levels were measured in the pairs thus obtained. Results were analysed using paired t-tests or Wilcoxon signed rank tests. We found small but statistically significant differences (p<0.05) between glass and plastic for free triiodothyronine, progesterone, prolactin, prostate-specific antigen and pregnancy-associated plasma protein-A. Non-significant trends (0.05<p<0.15) were seen with α-foetoprotein, androstenedione, BR (an assay for carbohydrate antigen 15.3), human chorionic gonadotropin and testosterone. Passing and Bablok regression analysis, however, revealed no clinically relevant differences (slopes ranged from 0.89 for progesterone to 1.07 for pregnancy-associated plasma protein-A, while intercepts were all small). We concluded that a switch to plastic blood collection tubes could occur without any implications for the interpretation of results.