Total prostate specific antigen stability confirmed after long-term storage of serum at -80C

We characterized the long-term stability of total prostate specific antigen in serum samples after storage at -80C from 2001 until 2007.

From the San Antonio Biomarkers of Risk biorepository we chose serum samples from white men 55.2 to 80.5 years old (median age 63.3) in which prostate specific antigen was measured in 2001. These men were not diagnosed with prostate cancer by 2007. A blocked randomization scheme was used to randomly select 47 serum samples with prostate specific antigen values spread over the reference ranges 0.0 to 0.4 (10), 0.5 to 0.9 (10), 1.0 to 1.9 (10), 2.0 to 3.9 (11) and 4.0 to 10.0 ng/ml (6) for repeat measurement by the same assay in 2007. Spearman's correlation coefficient was used to calculate the correlation and the paired t test was used to test the null hypothesis of no average difference between prostate specific antigen measured in 2001 and 2007.

Median prostate specific antigen values were 1.20 ng/ml (range 0.20 to 7.20) in 2001 and 1.30 (range 0.20 to 6.70) in 2007. In 2001 and 2007 mean +/- SD prostate specific antigen was 1.76 +/- 1.59 and 1.84 +/- 1.64 ng/ml, and the coefficient of variation was 0.91 and 0.89, respectively. The correlation between prostate specific antigen values in 2001 and 2007 was high (0.995). Prostate specific antigen values in 2007 were consistently and statistically significantly higher than in 2001 (mean 0.08 ng/ml, p = 0.005). Systematic and random error increased slightly with increasing prostate specific antigen.

Agreement between total prostate specific antigen values measured from serum samples in 2001, stored at -80C for 7 years and then remeasured was highly correlative.