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In this context, we will focus on how to detect a blood presence in the scene of a crime. This test can help to differentiate the blood from other substances, which can save time, effort and even costs by transferring dozens of samples to the lab to examine the blood presence. We will look at how we can identify blood in a crime scene. By using a few tests, we can save time, money and effort by not sending a bunch of samples into the lab.

Three common tests are available at the scene of a crime: Leucomalachite, Kastle Meyer and Luminal. These three tests are able to give us an immediate result on the presence of blood at a crime scene. Let’s explore in depth one of these methods.

Kastle Meyer testKastle Meyer test can be used to test for blood. This test is quick, easy, accurate, and does not cost much. Kastle KM test is a three-stage blood presumptive and rapid test. The test determines the hemoglobin component, which when in contact with hydrogen peroxide causes the indicator phenolphthalein to turn pink.

Materials & Procedures: For the test to be conducted, you will need some materials.

Kastle-Meyer solution

Seventy percent of the solution is ethanol

Deionized or distilled water

3% hydrogen peroxide

Cotton swabs and filter paper

Dropper or Pipette

Procedure

Only a small amount of distilled or swab water is needed to gently rub the dried blood sample onto the paper.

The test will not be affected if you add two drops of 70% ethanol onto the filter paper.

Add 1-2 drops of KM solution (phenolphthalein). The paper should now be pale or colorless.

Add 1-2 drops 3% hydrogen-peroxide solution to the filter or swap paper at this point. This will instantly turn it pink, which is a sign of blood. This chemical reaction will help you to instantly distinguish between blood and other substances.

False positiveFor KM tests, the results must be both positive and negative. As we all know, if a sample changes color to a pink shade it indicates the presence of blood. If the sample does not change color immediately this means that no blood has been detected. However, if you keep the sample for 30 seconds or so there will be fading in color. The indicator solution is affected by hydrogen peroxide, which oxidizes phenolphthalein. This test is also prone to false positives. For example as we said above, even if you don’t have blood present the sample will still turn pink. This test does not distinguish between the molecules of human and animal blood.

Kastle Meyer Blood Test is a sensitive test. Wet or dry stains should not affect the test’s ability to detect blood dilutions down to 1:107. The test results are negative. This is a reasonable indication that there is no heme in the sample. The KM can be used to determine how safe and effective the test is.