Low Baseline IsoPSA Index is Associated With a Prolonged Low Risk of Clinically Significant Prostate Cancer Diagnosis in Men With an Elevated PSA.

Objective: To compare the rate of diagnosing clinically significant prostate cancer (csPCa) in men with elevated prostate-specific antigen (PSA) stratified by baseline IsoPSA Index, thus assessing IsoPSA's intermediate-term predictive ability for csPCa.

Material And Methods: Single-center retrospective review of consecutive patients (n = 1578) who underwent IsoPSA testing from November 2016-August 2022. Data dichotomized into patients with low (≤6) and high IsoPSA Indices (>6). Most recent subsequent IsoPSA and PSA tests, prostate biopsy, and magnetic resonance imaging (MRI) collected. Time-to-event Kaplan-Meier estimates generated for the risk of csPCa stratified by baseline IsoPSA Index.

Results: Among 541 patients with initial low IsoPSA Indices (≤6), 23 (4.3%) were diagnosed with csPCa on a subsequent biopsy. Also, among these 541 patients, 204 had an MRI, of which 48/204 (23.5%) showed suspicious lesions (PIRADS ≥4). Among 1037 patients with initial high IsoPSA Indices, 366 (35.3%) were diagnosed with csPCa on a subsequent biopsy. Also, among these 1037 patients, 712 had an MRI, of which 342/712 (48.0%) showed suspicious lesions (PIRADS ≥4). After 12, 24, and 30 months, respectively, the risk of developing csPCa was 0.4% (95% CI 0.1%-1.6%), 2.5% (1.4%-4.4%), and 6.3% (4%-9.6%) in patients with low IsoPSA Indices, compared to 5.9% (4.6%-7.6%), 31.7% (28.3%-35.4%), and 49.5% (45.3%-53.9%) in patients with high IsoPSA Indices. Limitations include the retrospective review of prospectively collected data and unknown true csPCa rates as not all patients were biopsied.

Conclusion: The risk of developing csPCa was smaller in patients with initial low vs high IsoPSA Indices over the ensuing 30 months, which supports using IsoPSA to safely avoid follow-up testing.