Blood is by far the most common form of serological evidence used in forensics.
In forensic science, serology testing involves the analysis of not only serums from the blood, but other bodily fluids such as saliva, semen and urine with the intent to find and identify physical evidence from a crime. Forensic serologists must visually examine evidence that may contain these bodily fluids and perform presumptive tests to identify the presence of the fluid. Further tests are performed to confirm the identity of the fluid. Blood is by far the most common form of serological evidence used in forensics.
Presumptive bloodstain analysis
Even in crime scenes where blood is present but not visible, traces can always be found. Initial tests to find blood involve using an ultraviolet light which causes the blood to appear black against a fluorescent background. If blood is suspected, a substance called Luminol is sprayed on the area. Luminol is a presumptive test that uses the oxidation of certain organic compounds to react with hemoglobin, causing the blood to fluoresce. However, it is only considered presumptive because other substances that contain heme-like structures or peroxidases, like onions and potatoes, may also fluoresce. Another presumptive test is called the Kastle-Meyer test, which uses a phenolphthalein and hydrogen peroxide solution that will turn pink in the presence of blood. If an area or stain is positive for these tests, more testing follows to further identify the blood.
Bloodstain identification
To confirm the presence of blood, scientists may use crystalline tests called the Teichman or Takayama tests, which both add chemicals to the blood that will react with hemoglobin and cause crystallization. After confirmation, forensic investigators want to know if it is human, and if so, what type. To identify the species, scientists may use an immunological precipitin test. This test involves adding antibodies against human proteins and the unknown blood sample into a gel matrix and using an electric current to allow them to migrate toward each other. If they combine and form a line (the precipitin), the sample is human. Scientists then use the ABO blood typing test to determine the bloodstain's blood type and Rh factor. For blood typing tests, anti-A and anti-B serums are added to the bloodstain and observed for clotting, or agglutination.
Semen
A common presumptive test used to identify the possible presence of semen evidence is the AP test, during which a chemical substrate will produce a color when it reacts with the enzyme acid phosphatase (AP) that is present in semen. It is only a presumptive test because the AP enzyme is also present in vaginal secretions, so the test cannot distinguish between the two. To confirm semen identification, a test for the Prostate Specific Antigen, p30 (PSA) protein in semen is usually performed, using immunological based methods. Also, spermatozoa can be stained and visualized microscopically to confirm the presence of semen.
Saliva
There are no quick tests to locate saliva stains so visually identifying them is the most difficult problem. However, if a suspicious stain or fluid is found, laboratory tests may analyze it for the presence of amylase, an abundant enzyme in saliva. If positive, the concentration of amylase can be determined with tests such as the radial diffusion test or the Phadebas test.
Urine
As with saliva, urine stains are difficult to find and investigators must rely on the yellow color and odor to find a possible match. To confirm the presence of urine, scientists use tests that identify urea, uric acid, creatinine, and other substances commonly found in urine.
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