The stability of free and bound prostate-specific antigen

Objective To determine if the assay for free prostate specific antigen (fPS A) and the calculated ratio of fPSA to total PSA (f/tPSA) is stable in cond itions likely to be met in routine clinical practice. Materials and methods Two blood samples were obtained from 27 patients atte nding a routine urology clinic. Sample 1 was centrifuged immediately, assay ed for fPSA and tPSA, and the f/tPSA calculated. This sample was then store d at 4 degrees C for 24 h, 48 h and 1 week, or at -20 degrees C for 24 h, 1 week and I month before the assays for fPSA and tPSA were repeated. The se cond sample was left at room temperature for 24 h before assay and processi ng, as for sample 1. Results tPSA is a highly stable analyte; if whole blood samples are process ed immediately, fPSA is stable for 24 h at 4 degrees C and 1 month at -20 d egrees C. There was a significant reduction in the calculated f/tPSA in sam ples stored for greater than or equal to 24 h at 4 degrees C (P<0.01); if t he sample was stored at -20 degrees C the calculated f/tPSA was stable. Aft er 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. Conclusion 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 proces sed correctly. Total PSA is sufficiently stable to permit whole blood sampl es to remain at room temperature for 24 h before serum is separated. If fPS A is to be determined accurately then the whole blood sample must be centri fuged promptly. As the fPSA values in blood samples left at room temperatur e for 24 h are up to 25% lower than those on immediate assay, and the subse quent f/tPSA 29% lower, then for the optimum use of this test, these sample s should also be handled appropriately.