Blood evidence

Blood evidence

• The examination of blood is divided into two parts –– biological (serology)– physical (spatter)

What can blood evidence tell you?

• manner of blood loss• position of victim and suspect• animal or human• male or female• blood type• medical condition• identify of injured

is it blood?

Is it blood? There are several tests

1. Kastle Meyer• hemoglobin + phenolphthalein + hydrogen

peroxide = deep pink color• it is a presumptive test so a positive result

should be confirmed• false positives can come from some

vegetables but they are unlikely to be at a crime scene!

Is it blood tests . . .

2. Hemastik3. Luminol• positive test produces light• very sensitive – stains diluted up to 300,000

times• false positives possible• covers large areas• will not interfere with DNA testing

Miscellaneous info

• as blood dries it changes from red to brown• spectrophotometric analysis of blood stains

looks at color differences and can pinpoint the age of the blood from 1 day to 3 weeks

Let’s look at the Biology of blood

• plasma = liquid part; – mainly water; – 55% of the blood– contains antibodies (proteins that are part of the

immune response)

blood layers

• solid materials = blood cells and platelets– red blood cells (RBC) – have antigens on cell

surface for recognition– white blood cells (WBC) – have a nucleus so

important in DNA typing– Platelets (blood clotting)

Collection and Preservation of Blood Evidence

• In order for blood evidence to be used, a reference sample MUST be collected (from the victim, from the suspect, etc.)

• “vacutainer” tubes are used for collection –allows the blood to stay liquid and fresh; blood is then useful for tests (blood type, DNA, etc.)

• After collection the blood reference sample must be refrigerated

Collection• Start with moist blood first

– Collect in clean, unused plastic containers– Use sterile cotton squares or swab– Store wet in plastic for no more than 2 hours– Once in lab dry the sample

Collection

• If dry . . .– If surface is small and absorbent, collect in paper

container– If surface is large and absorbent – cut out stains

and place in paper container– If surface is non-absorbent, scrape blood into a

paper container– If surface is non-absorbent, moisten stain with

distilled water and absorb onto a piece of cotton

Collection and Preservation

• Dried blood stains left on clothing may not be useful – difficult to determine when the blood was left and under what circumstances

Is the blood human or not

human?

Human or non-human?

1. Precipitin test• Every animal produces

antibodies that react with each other’s blood

• What to do . . . Addantibodies for suspected animal to blood sample; if a precipitate appears then the antibody “matches” that blood

Human or non-human?

2. Gel Diffusion• Similar to precipitin

test • The container is

changed• Allows you to test

several blood sources at a time

Human or non-human?• The precipitin test is

sensitive – a small amount of blood is needed

• The blood can be 10-15years old (mummified tissues 4000 years old have given positive results!)

• Diluted blood stains (washed) can still test positive

What is the blood type?

Determining blood type

• Blood type is based on an antigen-antibody reaction

• The RBCs have several antigens on the surface; the three most familiar are– A antigen– B antigen– D antigen (responsible for positive or negative)– In reality there are over 256 antigens for more

than 23 blood groups!

Determining blood type

• The antibodies in the blood plasma will be the opposite of the antigen on the RBC

Blood type Antigen Antibody A A anti-BB B anti-AAB AB noneO none anti-A, anti-B

Determining blood type

• When typing blood . . .– The antibody has two binding

sites, one on each end – Two antigens can attach to one

antibody, and one RBC can be attached to several antibodies

– Clumping (agglutination) will occur

Determining blood type

1. The blood sample is placed into three wells, labeled A, B, Rh (antigen D)

2. The appropriate antibody is added to eachwell (anti-A to well “A”, anti-B to well “B” and anti-D to well “Rh”)

3. Stir and wait a minute4. If the blood clumps in a well then that

antigen was on the surface

What is the blood type?

Generalities with blood types

• Type O is common in indigenous populations (Native American, Aborigines, Latin America)

• Type A is common in Caucasian populations• Type B is common in African American and some

Asian populations (Thai)• Type AB is common in Japanese and Chinese

populations• Middle Eastern populations have nucleated RBC• Males tend to have more RBCs than females

Frequency tableABO PGM EAP Hp

A 40% 1- 1.8% A 11.2% 1 16.5%

B 10% 1+ 40.7% B 39.5% 2 35.0%

O 45% 1+1- 17.1% BA 41.4% 2-1 47.9%

AB 5% 2+ 3.1% C 0.2%

2- 0.3% CA 3.2%

2+2- 1.9% CB 4.6%

1+2+ 22.3%

1-2+ 4.7%

1+2- 6.8%

1-2- 1.4%

Genetics of Blood Type

• There are 3 alleles that determine the ABO blood types – IA, IB, i

• Type A is either IAIA or IAi• Type B is either IBIB or IBi• Type AB is IAIB

• Type O is ii

Genetics problems

• What are the possible blood types for the children of a father with Type A blood and a mother with Type O blood?

• What are the possible blood types for the children of a father with Type A blood and a mother with Type B blood?