The Forensic Scientists in the Biology section collect trace evidence and utilize serology screening tests to detect and then identify stains during the examination of physical evidence. Serology is the detection and identification of bodily fluids. Most often, the stains of interest for detection and identification include bodily fluids such as blood or semen. Homicides, sexual assaults and burglaries are some of the more common cases in which this type of screening may be necessary. The scientists perform both presumptive and confirmatory tests for blood and semen. A presumptive or indicative test is one that establishes the possibility that a specific body fluid may be present. A confirmatory test provides a more specific identification of a bodily fluid.


Blood spattered and dripping on a white background.

Presumptive Testing


For blood, the presumptive test used in the Biology section is the phenolphthalein or Kastle-Meyer test. This test produces an immediate bright pink color in the presence of blood due to a peroxidase reaction catalyzed by hemoglobin present in the blood.

Confirmatory Testing

ABAcard® HemaTrace®

ABAcard® HemaTrace® is the confirmatory test used for blood. This test works by using antibodies to human hemoglobin to allow detection of blood. Although the ABAcard® HemaTrace® test is confirmatory for the presence of blood, it is considered indicative for the presence of human blood. This is because there are certain animals (i.e. higher primates, etc.) that may also give a positive reaction with this test.


An unseen scientist's hands with gloves on swabbing a carpet section with a cotton swab,

Presumptive Testing

Acid Phosphatase (AP)

For semen, one presumptive test used in the Biology section is the acid phosphatase or AP test. This test produces a dark purple color in the presence of the enzyme acid phosphatase, which is found in high concentrations in human semen.

ABAcard® p30 or Prostate Specific Antigen (PSA)

ABAcard® p30 is another presumptive test used for semen. This test works by using antibodies to a protein called p30 or PSA that is found in human semen.

Confirmatory Testing

Christmas Tree Stain

The confirmatory test for semen involves the search for spermatozoa or sperm cells. A combination of two different stains is used on a sample to be analyzed for spermatozoa. This stain combination (called “Christmas Tree” stain) gives sperm cells a unique red and green color combination that makes them easier to identify under microscopic examination.

Alternate Light Source

The Biology section also uses the alternate light source or ALS. This specialized lighting uses ultraviolet and near ultraviolet light to reveal stains of bodily fluids that may not be visible to the unaided eye. Some bodily fluids like semen and saliva actually glow under these lighting conditions making them much easier to locate on clothing or other evidence. Blood absorbs this light and so will appear dark under the ALS.


Samples that require DNA, hair and fiber, gunshot residue or paint analysis will be forwarded to an accredited laboratory with those capabilities. Evidence is usually forwarded to the Texas Department of Public Safety Laboratory system, unless other arrangements are made with the submitting agency or prosecuting official.