The stability of free and bound prostate-specific antigen

To determine if the assay for free prostate specific antigen (fPSA) and the calculated ratio of fPSA to total PSA (f/tPSA) is stable in conditions likely to be met in routine clinical practice.

Two blood samples were obtained from 27 patients attending a routine urology clinic. Sample 1 was centrifuged immediately, assayed for fPSA and tPSA, and the f/tPSA calculated. This sample was then stored at 4 degrees C for 24 h, 48 h and 1 week, or at -20 degrees C for 24 h, 1 week and 1 month before the assays for fPSA and tPSA were repeated. The second sample was left at room temperature for 24 h before assay and processing, as for sample 1.

tPSA is a highly stable analyte; if whole blood samples are processed immediately, fPSA is stable for 24 h at 4 degrees C and 1 month at -20 degrees C. There was a significant reduction in the calculated f/tPSA in samples stored for >/=24 h at 4 degrees C (P<0.01); if the sample was stored at -20 degrees C the calculated f/tPSA was stable. After 24 h storage at room temperature, fPSA decreased by 6.3% and f/tPSA by 6.4%. Subsequent storage of serum at 4 degrees C for 1 week resulted in a 25% decrease from the baseline value. After 1 month at -20 degrees C the fPSA value was 13% lower than the baseline value.

These results indicate that if there is to be confidence in the accuracy of the f/tPSA value, then blood samples must be handled and processed correctly. Total PSA is sufficiently stable to permit whole blood samples to remain at room temperature for 24 h before serum is separated. If fPSA is to be determined accurately then the whole blood sample must be centrifuged promptly. As the fPSA values in blood samples left at room temperature for 24 h are up to 25% lower than those on immediate assay, and the subsequent f/tPSA 29% lower, then for the optimum use of this test, these samples should also be handled appropriately.