Presumptive Tests for Blood
Before DNA analysis, serological testing identifies the type of biological fluid present. Presumptive tests for blood exploit the peroxidase-like activity of hemoglobin. The Kastle-Meyer test applies phenolphthalin reagent followed by hydrogen peroxide to a sample swab—a pink color indicates possible blood. The leucomalachite green test works similarly, producing a green color. These tests are highly sensitive but not specific to human blood—they can react with animal blood, plant peroxidases (horseradish, potatoes), and some chemical oxidants. Luminol, sprayed in a darkened room, produces blue chemiluminescence in the presence of blood, revealing cleaned or diluted stains invisible to the naked eye. All presumptive positive results must be confirmed with more specific tests before being reported as blood.