Intermethod Differences in Results for Total PSA, Free PSA
Serum prostate-specific antigen (PSA) assays differin calibration and response to different PSA forms.We examined intermethod differences in total PSA(tPSA) and free PSA (fPSA) measurements. We tested157 samples with tPSA concentrations of 2 to 10 ng/mL (2-10 µg/L) using 6 PSA/fPSA method pairs and 1tPSA method: ADVIA Centaur (complexed and total;Siemens Diagnostics, Tarrytown, NY), ARCHITECT i2000_SR(Abbott Diagnostics, Abbott Park, IL), AxSYM(Abbott Diagnostics), IMMULITE 2000 (SiemensDiagnostics), Modular E170 (Roche Diagnostics,Indianapolis, IN), UniCel DxI 800 (Beckman Coulter,Brea, CA), and VITROS ECi (tPSA only; Ortho-Clinical Diagnostics, Raritan, NJ). Regression analysiswas performed for PSA, fPSA, and percentage of fPSAwith the ARCHITECT i2000_SRcomparison method.Differences between test and comparison methodswere estimated at 2.5, 4.0, and 10.0 ng/mL (2.5, 4.0,and 10.0 µg/L) for tPSA and 15%, 20%, and 25% forpercentage of fPSA. Relative differences were morethan 10% at 4.0 ng/mL (4.0 µg/L) tPSA for the Centaur,IMMULITE, ECi, and DxI methods. At 20% fPSA, therelative difference was more than 10% for all methodsexcept the AxSYM. Additional harmonization is neededfor tPSA and fPSA methods.

Prostate cancer is the second leading cause of cancer-related deaths for men in the United States. It has been suggested that screening for prostate cancer may have reduced prostate cancer mortality rates, but this remains controversial. Current American Cancer Society guidelines for prostate cancer screening recommend a digital rectal examination and prostate-specific antigen (PSA) measurement annually for all men 50 or older if they have a minimum life expectancy of 10 years. Although PSA is cleared by the Food and Drug Administration for monitoring patients with prostate cancer for disease recurrence, it is unique in that it is the only tumor marker that is currently approved by the Food and Drug Administration as an aid in the detection of prostate cancer. PSA testing for prostate cancer detection, however, is complicated for many reasons. First, PSA exists in multiple isoforms. Although there are many complexed forms that circulate in low concentrations in blood, the 2 principal forms that are measured by current methods are PSA complexed with α₁-antichymotrypsin (complexed PSA) and uncomplexed, or free, PSA (fPSA). Many automated analyzers have assays to measure total PSA (tPSA), and most can measure at least 1 of the 2 principal forms directly.

Second, screening with PSA is problematic because PSA is really an organ-specific marker for the prostate rather than a specific marker for cancer. Controversy exists regarding the medical decision levels for tPSA and percentage of fPSA within the United States and abroad. For tPSA concentrations, a diagnostic gray zone exists for patients with PSA concentrations between 4.0 and 10.0 ng/mL (4.0-10.0 µg/L). Many patients with PSA concentrations greater than 10.0 ng/mL (10.0 µg/L) have advanced disease, but within this gray zone only 25% have cancer, and controversy exists whether these are the patients who have early-stage disease and would benefit from detection or whether these are the patients who have indolent forms of prostate cancer. Therefore, recommendations indicate that a biopsy should be performed in any patient with a tPSA result that is greater than 10.0 ng/mL (10.0 µg/L), and further investigation is warranted in patients who are in the diagnostic gray zone and have a result between 4.0 and 10.0 ng/mL (4.0-10.0 µg/L) tPSA. In high-risk populations, a biopsy is recommended if the tPSA is 2.5 ng/mL (2.5 µg/L) or more. It has been recently suggested that this lower tPSA cutoff should be adopted for screening men of all ages. Although the medical decision points for percentage of fPSA are also equally controversial, it is generally accepted that measuring fPSA and calculating the percentage of fPSA aids in distinguishing cancer from other benign prostate conditions such as benign prostate hyperplasia, particularly for the population in the diagnostic gray zone. The lower the fPSA/tPSA ratio, the greater the likelihood of cancer. Currently, biopsy is recommended for 15%, 20%, or 25% fPSA.

Third, PSA measurements are further complicated by the fact that different assays measure different PSA isoforms to varying extents. Although a certain amount of variation is inevitable because different methods have antibodies that recognize different epitopes of PSA, ideally, assays that measure tPSA should be equimolar and unbiased in the detection of free and complexed PSA. Historically, however, different assays have produced significantly different results for PSA on the same sample. Although lack of an equimolar response was in part responsible for this phenomenon, the other problem was the lack of calibration against a universal standard. In an effort to standardize PSA methods, the World Health Organization (WHO) developed a standard, the WHO (90:10) (National Institute for Biological Standards and Control 96/670) PSA reference preparation, that consists of 90% PSA complexed to α₁-antichymotrypsin and 10% fPSA. Recent studies evaluated automated methods using this standard, and although bias between different methods has been significantly reduced, results between assays are not interchangeable for tPSA or fPSA, which complicates absolute PSA value recommendations for tPSA cutoffs and interpretation of the fPSA/tPSA ratio.

The aim of our study was to compare 6 commercially available automated methods for PSA concentrations (total and free) by using real patient samples that would minimize matrix effects to determine the degree of method-dependent bias in patient results at critical cutoffs. Our study is unique not only because we evaluated differences using Passing-Bablok analysis but also because statistical differences between methods and concordance at critical cutoffs were determined.