Stability of free prostate-specific antigen in serum samples under a variety of sample collection and sample storage conditions.

Objectives: Four studies were conducted to characterize the stability of free and total prostate specific antigen (PSA) under various sample collection and storage conditions.

Methods: In the first study, fresh blood from 11 patients was drawn and allowed to clot at room temperature (RT). Serum was prepared by centrifugation 1, 3, 5, or 8 hours after the blood draw and tested to determine the free and total PSA levels. In the second study, serum specimens from 12 individuals were stored at RT or 4 degrees C and were tested on days 0, 1, 2, and 7. In the third study, four fresh serum samples were subjected to five freeze-thaw cycles and tested after each cycle. In the fourth study, 29 fresh samples were aliquoted, frozen at -20 degrees C or -70 degrees C, and monitored for long-term stability.

Results: Approximately 1% of the free PSA was lost per hour of clotting time. Between 2% and 3% of the free PSA was lost per day of storage at 4 degrees C or 23 degrees C. About 0.9% of the free PSA was lost per month of storage at -20 degrees C compared with about 0.4% per month at -70 degrees C. Total PSA appeared to be stable throughout these studies.

Conclusions: Our results suggest that routine serum preparation and refrigerated storage of samples for up to 24 hours is acceptable for the measurement of both free and total PSA. Samples that are to be retained for longer than 24 hours should be frozen. Samples stored for extended periods should be kept at -70 degrees C.