Stability of serum total and free prostate specific antigen under varying storage intervals and temperatures

Measurement of total serum prostate specific antigen (PSA) is widely used as an aid to early detection of prostate cancer. Measurement of the ratio of free-to-total PSA (percentage of free PSA) may help increase specificity of PSA testing. We prospectively studied the effects of varying the storage temperature and interval on total and free PSA levels.

We measured the baseline total and free serum PSA levels in 36 volunteers (mean age 66 years) and then retested aliquots of these serum samples after varying storage intervals (24 hours, 2 weeks and 9 months) at 3 different temperatures (4C, -20C and -70C). Volunteers represented a spectrum of prostatic conditions (PSA levels 2.0 to 4.0 ng./ml., PSA levels greater than 4.0 ng./ml. without cancer and PSA levels greater than 2.0 ng./ml. with prostate cancer). We used repeated measures analysis of variance to test for changes in total and free PSA levels as a function of time and temperature. We also evaluated the impact of storage at different temperatures and times on the percentage of free PSA.

Across groups total and free serum PSA decreased from the baseline level differentially as a function of longer storage interval and higher temperature (p <0.05). No significant difference was found for change in total PSA at 24 hours, 2 weeks or 9 months for storage temperatures of -20C compared with -70C. A significant change from baseline level was found for free PSA when stored in -20C compared with -70C for 2 weeks but the magnitude of the change was modest.

For storage intervals up to 9 months total PSA is more stable than free PSA under temperature conditions ranging from 4C to -70C. This differential stability has important implications for the clinical evaluation of percentage of free PSA to distinguish between benign and malignant diseases of the prostate.