Introduction
Multi-kallikrein immunoassays are widely used to overcome the limitations of PSA in identifying potentially aggressive prostate cancer. In this study, we report the diagnostic performance of a mutant antibody-assisted assay for intact free PSA (iPSA) in combination with the commonly used four-kallikrein approach.

Methods
Immunoassays for total PSA (T), free PSA (F), iPSA using wild-type (I-W) or mutant 4D4 antibody (I-MC), total hK2 (T-hK2), and free hK2 (F-hK2) were used to assess perioperative plasma samples (n = 310). The individual parameters alone, in different ratios or in the four-kallikrein combinations were specifically analyzed in patients with prostate gland volume lower or equal and above the median (38 mL).

Results
In the low prostate gland volume group, T and the two hK2́s alone provided superior separation of the two groups over all other parameters. The ratio of calculated nicked PSA (F-I) to T using I-MC separated the two groups significantly. In the higher gland volume group, calculated nicked/T using both I-MC and I-W provided identical separations superior to the conventional F/T ratio. In the whole material, F/T and CN/T ratios performed similarly superseding any single parameter.
In the four-kallikrein Logistic Regression analysis, the I-MC (AUC, 0.75) provides clearly better results than I-W (AUC, 0.71) in the group of lower gland volumes. In terms of Odds Ratios, I-MC overall provides improvements over I-W in the whole cohort and the two groups.

Conclusions
The I-MC assay demonstrates modest but clear improvement in differentiating benign and low-grade prostate cancer from clinically significant cancers, particularly in patients with less than median gland volume. This is an effect of the mutant antibody and its improved affinity enabling an alternative assay format providing robust and accurate analytical test performance.