KLK3 and rheumatoid arthritis: The unwanted interaction of the analyte with bulk proteins of the biofluids may significantly reduce the effective target concentration due to hindering of its binding site, as demonstrated by the fraction bound to the free prostate-specific antigen (PSA) in serum samples [39], or by the interference in the detection of the chemokine receptor CXCL12 [40], following the binding with glycosaminoglycans found in urine samples from rheumatoid arthritis patients, thus lowering the sensitivity compared to a running buffer solution.