The Phenolphthalein blood test is used to screen for the presence of the hemoglobin molecule. All blood has heme in it and even when blood evaporates on a surface or is cleaned up, the hemoglobin molecules may remain. Sometimes referred to as the Kastle-Meyer test, this blood test has been described as a viable forensic process since 1903.
How Is the Phenolphthalein Blood Test Conducted?
The Phenolphthalein blood test is a common task in forensic science today, especially from a law enforcement perspective. By locating hemoglobin molecules, it becomes possible to collect evidence against a possible suspect. To start this blood test, the technician will take a sample of what is believed to be blood. Then the Phenolphthalein agent is added to the presumed blood sample.
After a few seconds, the technician will then use a drop of hydrogen peroxide on the swab as well. If the Phenolphthalein blood test is positive, then the swab will turn a pink color very rapidly. This will indicate the presence of hemoglobin and indicate to the technician that they have collected blood. If no color occurs rapidly, then the sample is not blood.
The color of the swab will naturally turn pink due to oxidization in the air in about 30 seconds, so if findings aren’t recorded immediately, a false positive might be inadvertently registered. Sometimes the swab used for collection may be treated with ethanol to enhance the sensitivity of the hemoglobin to the Phenolphthalein.
Are There Any Issues With the Phenolphthalein Blood Test?
The primary issue with this particular blood test is that there are some specific substances that may register a false positive after being collected. Nickel and copper salts are known to turn the swab pink immediately after the Phenolphthalein has been introduced to the sample. This is why it is important to wait a few seconds between the Phenolphthalein and the hydrogen peroxide.
This blood test has the same reaction to human blood as it does to any other blood that is based on hemoglobin. Therefore it is only one stage of the screening process of evidence that has been collected. After the Phenolphthalein blood test is positive, the technician will typically run what is known as the Ouchterlony test to determine what type of blood has been collected.
There may also be issues during the test procedure itself if the technician makes an error. Adding too much Phenolphthalein may cause a false negative result. A positive pink color should also not be considered as evidence by itself that was has been collected is actually blood. It is a sensitive blood test that can lead a technician to a correct result, but not on its own.
How Sensitive Is the Phenolphthalein Blood Test?
The Phenolphthalein blood test is believed to be capable of detecting a blood dilution that is as small as 1:107. Because of this, most labs will consider a negative result to be a lack of hemoglobin molecules. In some specific instances, however, even smaller dilutions of blood may be present and the blood test was just not able to detect them.
This sensitivity can also be problematic because any food samples that contain blood will actually provide a positive Phenolphthalein blood test. If someone dropped a steak on the floor that was rare, it could leave enough hemoglobin to generate a positive test and the follow-up species test would be required to determine what kind of blood it was.
Why Is It Used Today?
The Phenolphthalein blood test is primarily used by law enforcement officials as a method of evidence gathering at a crime scene. It is one of the most commonly used tests for field work and is easy enough to use that some agencies even allow police officers not trained in forensic science to collect samples for detection.
The modern Phenolphthalein blood test is small enough that it can be housed in a small plastic cup. It’s a 4 step process with instructions that will immediately give a flash of pink color so that officials can determine if the stain is blood or something like red wine.
The Phenolphthalein blood test is not something that a doctor will order for their patient. It is instead used to collect dried or liquid samples from any materials to determine if the stain is actually blood. This can help officials determine if something that have found should be considered as evidence. When used with other laboratory tools, the Phenolphthalein blood test is an effective way to screen substances and stains. As long as the timing is performed appropriately, this test can accurately determine if hemoglobin is present.