The most common immunoassays used to measure PSA detect which form of the enzyme?

• A. Total PSA
• B. Free PSA
• C. PSA complexed with α1-antichymotrypsin
• D. PSA complexed with α2-macroglobulin

Answer: (A)

PSA in blood exists in two main forms: free PSA and PSA that is bound to protease inhibitors (such as α1-antichymotrypsin or α2-macroglobulin). The immunoassays most commonly used to measure PSA are designed to quantify total PSA, which reflects the sum of all PSA forms in serum. The antibodies used in these assays recognize PSA epitopes that remain accessible whether PSA is free or complexed, so the assay reports the overall concentration. Free PSA and complexed PSA tests do exist for specific clinical purposes, but they are not the standard screening measure.