Blood Evidence

Unknown Stain at a SceneQuestions 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 bloodLinked to the hemoglobin’s heme portion

• Basic reaction:

H2O2 + Heme 2[OH-]

Colorless substrate + 2[OH-] Colored product + 2H2O

Screening Tests

• Extremely SensitivePresence of blood detectable even when

there is no visible staining

• EX: phenolphthalein – can detect 1 part blood in 1-10 million parts of dilutent

Kastle-Meyer Color Test

• Mixture of phenolphthalein and hydrogen peroxide on blood cause the mixture to turn pinkBecause 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?

• Standard to identify origin is the precipitin precipitin testtestStandard 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 serumCalled 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 diffusionSame basic technique as a precipitin test

except it is done in a gel plate• Antigens and antibodies will be attracted towards

each other

• Electrophoretic methodApply 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