PSA assay and reagent therefor

A prostate specific antigen (“PSA”) assay comprising:

• contacting a sample with a Reagent A to form a mixture, said Reagent A including a mixture of insoluble carriers with two kinds of anti-PSA monoclonal antibodies respectively immobilized thereon, and thereby, reacting two kinds of anti-PSA monoclonal antibodies at one time, and
• wherein
• the two kinds of anti-PSA monoclonal antibodies can react with both free PSA and complex of free PSA and α1-antichymotrypsin (PSA-ACT) and recognize different epitopes,
• each of the two kinds of anti-PSA monoclonal antibodies has a ratio (fKd/cKd) of a dissociation constant (fKd) for free PSA to a dissociation constant (cKd) for complex of free PSA and α1-antichymotrypsin (PSA-ACT) of more than 0.1 and equal to or less than 2.0 in the absence of said agglutination accelerator,
• each of the two kinds of anti-PSA monoclonal antibodies has a dissociation constant (fKd) for free PSA of 10 nM or less, and
• the insoluble carriers have an identical average particle size that is more than 0.20 μm and equal to or less than 0.40 μm;
• and
• reacting the mixture of the sample and Reagent A with a Reagent B, said Reagent B including one or more agglutination accelerators selected from polyethylene glycol, a polysaccharide, polyvinylpyrrolidone, polyvinyl chloride, a poly-γ-glutamate, and poly(2-methacryloyloxyethylphosphorylcholine), wherein said agglutination accelerator excludes PEG6000,
• wherein the concentration of the agglutination accelerator has been set so that an equimolar response to free PSA and complex of free PSA and α1-antichymotrypsin (PSA-ACT) is obtained, said equimolar response obtained when c/f ratio is 90 to 115%, said c/f ratio being the ratio (PSA-ACT/fPSA) of the reactivity with PSA-ACT to the reactivity with fPSA.