Blood Evidence

Unknown Stain at a Scene Questions to be answered: Is it blood? Is it human blood? Whose is it? –Determine blood type, alcohol content, drugs present –Determine the method(s) in which blood may have been deposited

Screening Tests Based on the “peroxidase-like” activity of the blood  Linked to the hemoglobin’s heme portion Basic reaction: H 2 O 2 + Heme  2[OH - ] Colorless substrate + 2[OH - ]  Colored product + 2H 2 O

Screening Tests Extremely Sensitive  Presence of blood detectable even when there is no visible staining EX: phenolphthalein – can detect 1 part blood in million parts of dilutent

Kastle-Meyer Color Test Mixture of phenolphthalein and hydrogen peroxide on blood cause the mixture to turn pink  Because hemoglobin reacts like a peroxidase (enzyme that breaks down hydrogen peroxide), reaction will give off oxygen in presence of hydrogen peroxide

Hematest® Tablet reacts with the heme group in blood causing a blue-green color

Luminol  Reaction with blood produces light Very sensitive – can detect blood diluted up to 10,000 times –Does not interfere with later DNA tests

False Positives  A test result that is read as positive but actually is negative Must make sure that the test is giving a true result of blood –Will get positive results from: Grass, apples, potatoes, onion, corn Saliva, spinal fluid, mucus Rust, iodine, bleach

Confirmation Tests Required to prove the presence of blood Must be done in the lab Tests are: –Tekayama Test –Telchmann Test

Confirmation Tests  Addition of specific chemicals to the blood will create characteristic crystals with hemoglobin derivatives Not very sensitive Positive results mean: –Establishes presence of heme does not establish the species origin

Blood origins Once stain is determined to be blood, the origins of the blood must be determined –What animal did it come from? precipitin testStandard to identify origin is the precipitin test  Standard for testing blood is the Anti-human test

Precipitin test An antibody that reacts with its corresponding antigen will be used on the unknown stain to see if it will precipitate (solidify or crystallize)

Human vs Animal Blood Microscopic observation Precipitin test—blood is injected into a rabbit; antibodies are formed; the rabbit’s blood is extracted as an antiserum; the antiserum is placed on sample blood. The sample will react with human proteins if human blood is present. This test is very sensitive and requires only a small amount of blood.

Anti-human test Human blood is injected into an animal –Usually a rabbit –Animal will produce antibodies against human blood Serologist will extract antibodies from the rabbit’s serum  Called Human anti-serum

Anti – Human Test Layer an extract of the blood stain on top of the human anti-serum in a capillary tube  If blood is human, there will be a reaction between the human anti-serum and the unknown blood and a precipitate will form

False positives –Contamination of the blood sample –Blood is from higher order apes Close relatives of humans –Orangutan –Chimpanzee –Gorilla

Other origin tests Gel diffusion  Same basic technique as a precipitin test except it is done in a gel plate Antigens and antibodies will be attracted towards each other Electrophoretic method  Apply an electrical charge to accomplish same results

Secretor An individual who secretes his or her blood-type antigen in body fluids –Found saliva, semen, vaginal secretions, gastric juices  Approximately 85% of people are secretors