Two-year stability of free and total PSA in frozen serum samples

Objectives. To investigate the stability of free and total prostate-specific antigen (PSA) in serum stored at -20 degrees C and -70 degrees C for 2 years. Methods. Fresh blood from 29 patients was drawn and allowed to clot at room temperature. Serum was prepared by centrifugation 1 hour later and tested to determine the free and total PSA levels. Samples were aliquoted, frozen at -20 degrees C or -70 degrees C, and monitored for long-term stability over the course of 2 years. Results. On average, 91% of the free PSA, 92% of the total PSA, and 89% of the percent free PSA values were recovered after 2 years of storage at -20 degrees C; and 96% of the free PSA, 101% of the total PSA, and 95% of the percent free PSA values were recovered after 2 years of storage at -70 degrees C. Seventy percent of the samples stored at -20 degrees C and 100% of the samples stored at -70 degrees C showed no statistically significant decline in free PSA values over the 2-year period. Only one total PSA sample (-20 degrees C) showed a statistically significant loss after 2 years of storage. Conclusions. Our results suggest that samples stored frozen for up to 2 years at -70 degrees C should give acceptable percent free PSA results. UROLOGY 52: 247-251, 1998. (C) 1998, Elsevier Science Inc. All rights reserved.