Types of Forensic Serology Tests

Molecular component of a DNA sequence

Forensic serology involves the analysis of blood as well as semen, saliva and other bodily fluids to investigate and prosecute criminal acts. While witness testimony can be disputed through various legal maneuvers, blood that connects a perpetrator to a crime scene is much harder to dismiss. In recent years, forensic serology has become much more precise and sophisticated with the advent of DNA testing. However, several other commonly used tests comprise this science.

Presumptive Testing

Forensic serology begins with several basic tests. Using the chemical phenolphthalein, a crime lab conducts a Kastle-Meyer test to determine whether the sample is actually blood. Secondly, forensic scientists use antiserum and gel tests to find out whether the sample is animal blood and, if so, from what species.

Blood Typing

In 1901, serologists discovered the "A-B-O System," which categorized blood into four types: A, B, AB and O. Over the next few decades, they refined the system into many additional subcategories based on the presence of proteins such as Rh (which accounts for the -/+ suffix of a blood type) as well as more than 100 unique antigens. The simplest method of blood typing involves the testing of a sample with anti-serums to identify the four basic groups. However, additional tests can isolate proteins and enzymes to help pinpoint a unique type. For example, an individual with basic Type A blood with protein AK and enzyme PGM may only account for 0.0001 percent of the population. Hence, the odds of a false match between blood samples found at a crime scene with that of a suspect are infinitesimal. Additionally, these tests can sometimes determine age, sex and race from a blood sample, narrowing down the range of possible victims and perpetrators.

Crime Scene Investigation

Very often, criminals attempt to conceal or clean up blood evidence. To uncover their tracks, law enforcement often uses luminol, a chemical that reacts to contact with blood by giving off a slight phosphorescent light in the dark. By spraying the chemical on carpets and furniture, a serologist can identify blood traces and acquire samples. For severely dried bloodstains, forensic scientists use an absorption-elution test that involves heating the sample and adding red cells from standard blood groups to find out what coagulates.

DNA Analysis

Like fingerprints, no two DNA samples are the same except for identical twins (though their blood types may be different). After collecting a sample of blood or other bodily fluids, a crime lab uses state-of-the-art equipment to extract a DNA sample from a human cell. The DNA strand is cut into fragments with a restrictive enzyme, the fragments are placed on a bed of gel, and a radioactive current is run through the gel. Using X-ray film, the lab takes a snapshot of the radiological activity. The resulting DNA print can then be used to identify an individual with less than a one-in-a-billion chance of error.

References

• Austin Peay State University: Forensic Serology
• Forensic Medicine: Forensic Serology
• The Thin Blue Line: Forensic Serology