FDA had approved method Hybritech “Total PSA” for monitoring prostate cancer in 1986. In 1994 this method was approved by FDA for early diagnostics of prostate cancer. Later the most of manufacturers of alternative test- systems used method Hybritech “Total PSA”, including the internal standard Hybritech, as the reference, often without performing fullscale researches on definition of clinically significant PSA levels. Differences in antibodies, which are used for production of test-systems, and in equimolarity of methods lead to significant variability of PSA results. In 1999 WHO approved the primary reference calibrator – the standard of WHO 96/670 which consists of free and total PSA in ratio 10:90. The method of mass spectrometry was used as primary reference technique of measurement by manufacturing of WHO 96/670, instead of method Lowry which was used for Hybritech standard. It was found that the extinction factor of WHO 96/670 standard was approximately 26 % higher than internal Hybritech standard. Cut-off value was detected as 3,1 ng/ml for method Hybritech “Total PSA” by using WHO calibration. It is 22 % lower than cut-off value 4 ng/ml which was detected basing on full-scaled prospective investigation using Hybritech calibration. These changes are especially important by monitoring patients’ PSA level. Ignoring of changes in cut-off values by introduction of traceability to WHO standard leads to false-positive/negative results. It was impossible to reach full interchangeability between different test-systems of different manufacturers. According to recommendation of the European group on tumor markers, together with PSA result, the physician should receive the information regarding method of detection and reference ranges which correspond to this method.