STABILITY OF SERUM TOTAL AND FREE PROSTATE SPECIFIC
ANTIGEN UNDER VARYING STORAGE INTERVALS AND
TEMPERATURES

CARLOS G. ARCANGELI, DEBORAH S. SMITH, TIMOTHY L. RATLIFF AND
WILLIAM J. CATALONA
From the Division of Urologic Surgery, Department of Surgery, Washington University School of Medicine, St. Louis, Missouri, and Department of Urology, University of Iowa College of Medic·ine, Iowa City, Iowa

ABSTRACT
Purpose: 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.
Materials and Methods: 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,
Results: Across groups total and free serum PSA decreased from~the baseline level differen-
eialiy 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.
Conclusions: 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. J Urol 158:2182-2187, December 1997.

The authors conclude "Presently, some reference laboratories may receive samples that have not been appropriately refrigerated, and measurement of free and total PSA may not occur on the same day the sample is drawn. Our data suggest that significant amount of free PSA immunoreactivity is lost even when serum is stored for as little as 24 hours at temperatures warmer than -20C."

In the real world, specimens often sit at room temperature and are refigerated at 4C. Thus many men who have their free PSA measured at distant labs risk receinving an artificially low value and hence are falsely told they have a higher risk for malignancy. - JRO