Suitability of quality control materials for prostate-specific antigen (PSA) measurement: inter-method variability of common tumor marker control materials

Variation in serum prostate-specific antigen levels in men with prostate cancer managed with active surveillance

OBJECTIVE

To describe fluctuations in prostate-specific antigen (PSA) levels in men managed with active surveillance (AS) to determine if a single PSA increase is a consistent measure to use to trigger intervention.

PATIENTS AND METHODS

We evaluated data on 541 patients undergoing AS between 1995 and 2011. PSA variation was described by studying the Kaplan-Meier probability of patients' PSA levels reaching 4 or 7 ng/mL, falling below those thresholds, and then rising to those thresholds again. We also examined PSA variation by calculating the Kaplan-Meier probability of a PSA change followed by an equal or greater change in the opposite direction.

RESULTS

We analysed data on 541 patients undergoing AS with a median (interquartile range [IQR]) of 8 (6-12) PSA measurements and undergoing AS for a median (IQR) of 4 (2-6) years. The 5-year estimate of the probability of reaching a threshold PSA of 7 ng/mL was 40% (95% confidence interval [CI] 35-46%) and the 5-year estimate of subsequently falling below this threshold was 90% (95% CI 82-95%). The 5-year estimate of a PSA direction change was 95% (95% CI 93-97%) overall and 56% (95% CI 51-61%) for PSA direction changes of ≥1 ng/mL.

CONCLUSIONS

We observed a high probability of variability in PSA levels for patients on AS. The probability of changes in PSA, defined by an increase to the specified thresholds or a rise >1 ng/mL within 6 months and subsequent decrease of equal or greater value on a subsequent measurement, increases over time; therefore, a single change in PSA level is not a reliable endpoint for patients on AS.

Analytical performance of a new one-step quantitative prostate-specific antigen assay, the FREND™ PSA Plus

BACKGROUND

We evaluated the analytical performance of a new one-step rapid quantitative sandwich immunoassay for total prostate-specific antigen (tPSA), the FREND™ PSA Plus (FREND PSA) (NanoEnTek Inc., Seoul, Korea).

METHODS

The imprecision, linearity, hook effect, detection limit (LoD), and interference were evaluated and trueness verification and matrix validation were performed. For method comparison, 79 patient specimens were analyzed with FREND PSA and two comparative tPSA assays (Architect® total PSA and cobas® total PSA assay).

RESULTS

Total CVs of the imprecision for low (0.208 ng/mL), medium (4.051 ng/mL), and high PSA levels (5.469 ng/mL) were 15.9%, 6.4%, and 9.1%, respectively. Linearity was observed from 1.01 to 19.15 ng/mL and the hook phenomenon was absent up to 171.48 ng/mL. The LoD was 0.094 ng/mL. The regression equations between FREND (y) and Architect or cobas were as follows: y=0.0133+1.054x (r=0.973), y=-0.2144+1.066x (r=0.977), respectively. Differences between FREND PSA and the comparative methods at a medical decision level of 4.0 ng/mL were less than the optimum specification bias (9.3%). The percentage biases from the trueness verification and interference test were less than the desirable specifications for bias (18.7%). The plasma tPSA level measured with lithium heparin or K2EDTA was comparable to that in the serum.

CONCLUSIONS

The FREND PSA provided reliable analytical performance and test results in comparison to two widely used tPSA assays. It is a simple and rapid test for tPSA and can be applied in point-of-care testing.