Most tissues possess characteristics that are typical of the specific material but not unique to it. For example, semen has a high concentration of the enzyme acid phosphatase, but the enzyme is found at lower levels in other body fluids, including vaginal secretions. Screening or presumptive tests make use of a target chemical to establish the possibility that a specific body tissue or fluid is present. Confirmatory tests are then used to identify the specific biological material, which can then be typed.

The line between screening and identification is not always clear. For example, while examining the clothing of a suspect, a forensic biologist might visually locate a brown stain that presumptively tested positive for blood and was then DNA typed.  The DNA type is found to match the victim. Knowing that the loci tested are higher primate specific, what conclusions can be drawn?

The only unqualified conclusion that can be offered is that the stain contains DNA that matches the victim. It has not been proven to be blood.

If asked "Could the results have arisen because the material tested was the blood of the victim?" then an answer of "Yes" is justified. However, it would be wrong to report that the material was human blood with a DNA type that matched the victim. The material was not subjected to confirmatory testing for blood or proven to be human in origin.

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