Reduced stability of prostate-specific antigen after long-term storage of serum at -20 degrees C

The measurement of serum prostate-specific antigen (PSA) is widely used for the detection and management of patients with prostate cancer. Many studies on the validity of PSA as a marker for prostate cancer are performed on clinical samples that have been stored frozen for years. We have studied the stability of free (F), total (T) and complexed (C) PSA immunoreactivity and the proportion of free to total PSA (F/T) in serum after melting sera stored at -20 degrees C for 2 years and 2 weeks, respectively. In contrast to the decrease in PSA-F and F/T observed in fresh samples, PSA-C decreased and PSA-F increased in a time-dependent fashion after thawing samples that had been kept frozen for 2 years. This caused a net decrease in PSA-T and an increase in F/T. These results suggest that even though serum PSA is fairly stable during short-term storage, long-term storage at -20 degrees C reduces the stability of PSA immunoreactivity. Thus, results obtained on samples stored for prolonged times at -20 degrees C should be interpreted with caution. Because of the changes in PSA-F and F/T in both fresh and archival samples stored unfrozen, it is recommended that sera are melted only for the period required for pipetting the samples.