Question

A detective found some body fluids on a victim's clothes. The body fluids were sent to the crime lab for analysis. on the basis of the antibody analysis, could the crime lab determine whether the sample is blood plasma or semen? Explain your answer.

Answer 1

The body fluid from a victims cloth can be analysed based on antibody analysis by several methods.

One such method is PSA Test which is used to detect Prostrate Specific antigen (PSA) in the given sample.

PSA is an antigen present specifically in semen and can be detected by monitoring the interaction between PSA and anti human PSA antibody. Specific colours can be visualied if these antibodies are conjugated with fluorescent enzymes.

PSA test is a confirmatory test because other body fluids do not contain or contains very less amounts of PSA which will not interfere with PSA semen test.

But male urine may at times give a false positive result.

Advantage of PSA test is that

• It gives result even with azoospermic males (males devoid of sperm production) as seminal plasma will still contain PSA.
• Semen diluted 10⁶ times will even give the result.

Some common PSA tests are

• ELISA
• Membrane Aspiration Test (MAT)
• Thin layer Immuno Assay

So, a body fluid that contains PSA can be confirmed to be semen using the above mentioned methods ecause PSA will be present only in semen or semen plasma.

Rapid Stain Identification semen test (RSID semen test) is another method by which a semen specific semenogelin can be detected by using a monoclonal antibody (an antibody which binds to a single epitope) named anti human Sg (seminogelin) antibody.

This method does not show false positives with other body fluids and takes only ten minutes.