Biological Evidence Screening and Sample Selection
Purpose and hierarchy
Screening asks three questions: what biological material is present, where is it, and which portion should be sampled for DNA. Presumptive tests such as luminol, Kastle-Meyer, TMB, acid phosphatase, alpha-amylase, and PSA indicate a class of substance but are not identity tests. Confirmatory or identity tests, including microscopic sperm, RSID, and immunochromatographic tests, reduce false positives. Traps: luminol reacts with bleach, rust, and plant peroxidases; Kastle-Meyer can react with plant material; acid phosphatase is abundant in vaginal fluid and some plants; PSA is more specific but not absolute. A negative screen does not prove absence, especially after washing, dilution, or on porous substrates.
Sampling decisions
Prefer the most cellular, least contaminated area; cut a portion rather than swab when the substrate allows, because cutting recovers more DNA and preserves a nearby unstained area for a substrate control. Swab with minimal moisture, concentrate the stain, air-dry, and package separately. For sexual assault kits, select sperm-rich fractions and note vasectomy or azoospermia possibilities. Touch DNA requires targeted sampling of likely contact points, not broad swabbing. Always collect reference samples and avoid speaking, coughing, or touching surfaces.
Worked rule
Sample the stain, not the whole object. If a bloodstain is 2 cm across on denim, cut a 5 x 5 mm stained piece plus an adjacent unstained piece; do not elute the entire garment. Document location, appearance, test results, and amount consumed. If two stains are present, sample separately rather than pooling, because pooling can create an uninterpretable mixture and consume evidence.