Prostate Specific antigen (PSA) is a serine protease with chymotrypsin-like activity. The protein is a single chain glycoprotein with a molecular weight of 28.4 kDA. PSA derives its name from the observation that it is a normal antigen of the prostrate but is not found in any other normal or malignant tissue. PSA is released from the normal prostate and appears at low serum concentrations in healthy men. Studies with reverse transcription-PCR have shown that PSA also is expressed at a low concentration in peripheral blood cells and other tissues. High serum concentrations can be detected in patients with advanced prostate cancer (PCA). Therefore PSA is applied as a tumor marker for the clinical management of PCA. However, increased PSA concentrations in serum also occur in patients with benign prostate hyperplasia (BPH). Hence the goal is to discriminate clearly between BPH and PCA in the clinicallaboratory to spare the patient invasive diagnostic procedures, such as a prostate biopsy.

In human serum PSA occurs in two forms: free PSA (f-PSA) and complexed PSA. The major form is a complex of PSA and antichymotrypsin (ACT). The fraction of f-PSA was shown to be substantially smaller in patients with untreated PCA than in patients with BPH. Therefore combined measurements of f-PSA and total PSA (t-PSA) may lead to a better discrimination between BPH and PCA. Some recent studies have already shown that the f-PSA/t-PSA ratio is helpful in the differential diagnosis of BPH and PCA.

PSA is found in benign, malignant and metastatic prostrate cancer. Since prostate cancer is the second most prevalent form of male malignancy, the detection of elevated PSA levels plays an important role in the early diagnosis. Serum PSA levels have been found to be more useful than prostatic acid phosphatase (PAP) in the diagnosis and management of patients due to increased sensitivity.

May be diagnosed by:

• Determination of specific antibodies based on the ELISA technique
• Determination of specific antibodies based on the RIA-technique

Free PSA ELISA:

Free PSA kit is a direct solid phase enzyme immunoassay for quantitative determination of free Prostatic Specific Antigen (fPSA) in human serum.

Principle of the Assay:

In this method, standards, patient specimens and/or controls (containing the native fPSA antigen) are first added to streptavidin coated wells. Biotinylated monoclonal and enzyme labelled antibodies are then added and the reactants are mixed: these antibodies have high affinity and specificity and are directed against distinct and different epitopes of fPSA.

Reaction between the various fPSA antibodies and native fPSA occurs in the microwells without competition or steric hindrance, forming a soluble sandwich complex.

The interaction is illustrated by the following equation:

BtnAb(m) = Biotinylated Monoclonal Antibody (Excess Quantity)
AgtPSA = Native Antigen (Variable Quantity)
EnzAb = Enzyme labelled Antibody (Excess Quantity)
EnzAb-AgtPSA-BtnAb(m) = Antigen-Antibodies Sandwich Complex
ka = Rate Constant of Association
k-a = Rate Constant of Dissociation 

Simultaneously, the complex is fixed to the well through the high affinity reaction of streptavidin and biotinylated antibody. This interaction is illustrated below: 

Streptavidin CW = Streptavidin immobolized on well.
Immobilized Complex = Antibodies-Antigen sandwich bound.

After equilibrium is attained, the antibody-bound fraction is separated from unbound antigen by decantation or aspiration. The enzyme activity in the antibody-bound fraction is directly proportional to the native antigen concentration. The activity of the enzyme present on the surface of the well is quantified by reaction with a suitable substrate to produce colour. By utilizing several different calibrators of known antigen values, a dose response curve can be generated from which the antigen concentration of an unknown can be ascertained.

Specific performance characteristics: 

Correlation with RIA performed on 167 samples is r = 0.957

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