The stability of free and bound prostate‐specific antigen

Abstract

Objective To determine if the assay for free prostate specific antigen (fPSA) and the calculated ratio of fPSA to total PSA (f/tPSA) is stable in conditions likely to be met in routine clinical practice. Materials and methods Two blood samples were obtained from 27 patients attending a routine urology clinic. Sample 1 was centrifuged immediately, assayed for fPSA and tPSA, and the f/tPSA calculated. This sample was then stored at 4°C for 24 h, 48 h and 1 week, or at −20°C for 24 h, 1 week and 1 month before the assays for fPSA and tPSA were repeated. The second sample was left at room temperature for 24 h before assay and processing, as for sample 1. Results tPSA is a highly stable analyte; if whole blood samples are processed immediately, fPSA is stable for 24 h at 4°C and 1 month at −20°C. There was a significant reduction in the calculated f/tPSA in samples stored for ≥24 h at 4°C (P<0.01); if the sample was stored at −20°C the calculated f/tPSA was stable. After 24 h storage at room temperature, fPSA decreased by 6.3% and f/tPSA by 6.4%. Subsequent storage of serum at 4°C for 1 week resulted in a 25% decrease from the baseline value. After 1 month at −20°C the fPSA value was 13% lower than the baseline value. Conclusion These results indicate that if there is to be confidence in the accuracy of the f/tPSA value, then blood samples must be handled and processed correctly. Total PSA is sufficiently stable to permit whole blood samples to remain at room temperature for 24 h before serum is separated. If fPSA is to be determined accurately then the whole blood sample must be centrifuged promptly. As the fPSA values in blood samples left at room temperature for 24 h are up to 25% lower than those on immediate assay, and the subsequent f/tPSA 29% lower, then for the optimum use of this test, these samples should also be handled appropriately.

Major inter-laboratory variations in PSA testing practices: results from national surveys in Ireland in 2006 and 2007

“…The stability of PSA, especially fPSA has been shown to decrease upon storage, in a time-and temperature-dependent manner [22,23]. Therefore, differences in sample handling could have consequences on the tPSA and fPSA levels measured and may lead to additional disparity between laboratories as well as having potentially harmful clinical consequences.…”

Five-Year Stability Study of Free and Total Prostate-Specific Antigen Concentrations in Serum Specimens Collected and Stored at – 70°C or Less

“…There are numerous studies that have examined the stability of serum tPSA, fPSA, and PSA-ACT while on clotted blood (15,17,19,20,24). The consensus from these studies is that serum tPSA and PSA-ACT are stable on the clot for up to 24 h at room temperature (19,20,24) but to prevent loss of fPSA immunoreactivity, serum must be harvested from the clot within 3 to 4 hours after the blood draw (15,19,20), at which time it can be safely stored at 4 o C for up to 24 h (19,20,24) or up to 1 month at -20 o C (24). In our study the clot- ted blood samples were centrifuged at the collection site (i.e., physician's office) within 1 hour of the blood draw and then shipped overnight to DOCRO (transit time of about 16 h at approximately 4 o C).…”

“…Numerous investigators have evaluated the effect of time and temperature on the stability of the various forms of PSA in serum and plasma and found that tPSA and PSA-ACT generally were more stable than fPSA (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25); fPSA was more stable in plasma stored at -20 o C than in serum stored at the same temperature (17,22); serum fPSA was quite stable when stored between -70 o C (16) and -80 o C (19)(20)(21). The parameters shown to be most critical for PSA stability over time are related to specimen collection, processing, and storage temperature (15-17, 19, 20, 24, 26).…”

Modeling of the pH- and the temperature-dependant deviations of the free to total PSA (prostate specific antigen) ratios for clinical predictability of prostate cancer and benign prostate hyperplasia

“…Substituting the initial concentration (C, as 100%) and the averaged documented values of decay rates Cartledge et al, 1999;Jung et al, 1998Jung et al, , 2000Kioukia-Fougia et al, 1999) into equation (39) Similar treatment gave the corresponding decay rate coefficients for t-PSA and PSA-ACT complexes, respectively. Table A.7 shows the thermal decay rate coefficients and their related activation energies for various PSA species at different temperatures.…”

“…The ratio of the fraction of uncomplexed f-PSA (free prostate specific antigen, denoted as S f hereafter) to total PSA (denoted hereafter as S t ) has been proposed clinically as a diagnostic tool to enhance the predictability to distinguish early stages of prostate cancer (PCa) from benign prostate hyperplasia (BPH) (Stenman et al, 1991;Christensson et al, 1993). Currently, some documents (Christensson et al, 1993;Pettersson et al, 1995;Arcangeli et al, 1997;Jung et al, 1998Jung et al, , 2000Cartledge et al, 1999) described that the inherent highly pH-and temperature-sensitive character of all PSA species either in vivo or in vitro could result in considerable apparent false reports. Hence a mathematical model is required to quantify such possible deviations.…”