Presumptive Blood Test

Introduction

Crime scene investigators employ many tests that are fast and easy to use at the crime scene even if the results are only “presumptive” and not considered hard evidence or proof. The presumptive tests can often lead the investigators in the right direction to search for additional proof. This kit is a simulation of the Kastle-Meyer Presumptive Color Test for the presence of blood.

Concepts

• Presumptive test

• Oxidation

Background

The Kastle-Meyer Color Test is often used to presumptively indicate if blood is present on an object. The test is fast and easy and is thus ideal for use in the first moments on a crime scene. The test detects certain chemical components in blood. The heme, or iron-containing portion of the red blood cell, reacts with hydrogen peroxide to oxidize phenolphthalein (a colorless indicator) and give a pink product.

Heme iron + phenolphthalein (clear) + hydrogen peroxide → oxidized phenolphthalein (pink) + reduced iron

Phenolphthalein is normally clear and colorless, but in the presence of blood and hydrogen peroxide, it turns pink. If the results of the Kastle-Meyer Color Test are positive, then investigators know that blood may be present and that further analysis of the residue may provide additional useful information (e.g., DNA analysis).

The disadvantage of the test is that it is presumptive. In the presence of chemical and plant oxidants from sources other than blood, the test will produce false positive results. (It is because of this false positive characteristic that this simulation works. Real blood is usually not allowed in classrooms. Check your local safety guidelines before doing tests using real blood.)

In this simulated activity, simulated blood plasma will be spread onto various articles from a simulated crime scene. Spraying test solution (phenolphthalein) on these items from the crime scene will reveal whether blood may be present on the item.

Materials

Safety Precautions

The simulated blood plasma solution is a corrosive liquid and is dangerous to skin and eyes. The phenolphthalein solution is an alcohol-based solution and is moderately toxic by ingestion or inhalation. Spraying of phenolphthalein should be done in a fume hood or a well-ventilated area. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information. Wash hands thoroughly with soap and water before leaving the laboratory.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Phenolphthalein may be disposed of according to Flinn Suggested Disposal Method #18b and the simulated plasma by Method #10.

Prelab Preparation

Use a Beral-type pipet to spatter items in the crime scene with the simulated blood plasma prior to class. Allow time for the solution to dry on the contaminated surfaces. Absorbent items work and dry best. If a crime scene is not created, spatter four pieces of filter paper with simulated plasma and allow to dry. Label the four spattered filter papers—“Door Knob,” “Chair,” “Floor” and “Knife.” Use another unspattered filter paper and label it “Fork Handle.”

Procedure

1. Create a crime scene story. Relate it to your total crime scene (if you are doing an entire unit) or create a mini-crime scene scenario similar to the one that follows. (Be creative but tasteful.)

A death is reported at an apartment on South Main Street. When the crime scene investigators arrive, they find John Ross (dead victim) on the floor beneath a kitchen table with a fork in one hand and a newspaper in the other. It has been determined that John died at least several hours ago and there is little obvious evidence of blood anywhere in the area, although it looks like something might be on the shirt of the victim. A magazine is also knocked down to the floor next to the victim. Stains are noticed on the floor and on the door leaving the kitchen. A knife was found in the bushes in the next door neighbor’s yard. All stains and items are tested to determine if blood might be on them in an attempt to determine what else to test for and where else to look.

2. Have students observe and record the results of the Kastle-Meyer Color Test as you spray the phenolphthalein on each item or filter paper swab from the item.
3. Discuss possible implications of the presumptive test results. Is there any reason to suggest foul play?
4. Have students write scenarios of possible occurrences in the kitchen of John Ross’ house and detailing what other evidence should be sought.

Teacher Tips

• This kit contains enough materials to conduct many simulated Kastle-Meyer Color Tests on crime scene items. The kit includes—100 mL of phenolphthalein solution, a spray bottle, 100 mL of simulated blood plasma and 50 sheets of filter paper.

• Do not spatter the simulated plasma on anything of value. The solution is a corrosive liquid. Try using an old T-shirt, sock or dish towel.