Forensic Blood Studies - Is It Blood or Not?

Whenever a crime occurs and police are called to investigate, crime scene investigators (CSI's) like the ones you see on TV look for clues in order to determine what happened. One of the clues they look for is biological evidence such as blood.

When a CSI technician comes across a suspected bloodstain, he must identify it. Identifying a bloodstain can be very difficult. Depending on how long the blood has been sitting there and the conditions to which it has been exposed, a bloodstain can resemble a purple paint spot or a brown grease stain. Even a veteran CSI technician may have a difficult time distinguishing blood from spaghetti sauce.

Whenever the medical examiner or criminal scientist comes face to face with a liquid sample or a stain that is suspected to be blood, he must answer the following three questions:

• Is it blood?


• Is the blood human?


• To whom does the blood belong?

Blood must be found in enough quantities and in good enough condition for it to be testable. In spite of the fact that modern methods make testing the smallest traces of blood a possibility, if a sample is highly degraded, the blood may be useless. Many chemicals can destroy blood to the point that DNA profiling and blood typing can not be executed. Putrefaction is another element that can degrade a sample of blood beyond the point of reparation. Putrefaction refers to the decay caused by bacterial activity inside cells. Because heat and moisture are conducive to bacterial growth, putrefaction occurs much more rapidly.

Most crime-scene blood samples that reach the forensics lab are dried and degraded to some point. However, dried blood samples have their pluses and minuses. Although liquid blood provides many more useful genetic markers, it is more likely to be destroyed because bacteria that cause blood and tissues to putrefy are prevalent in moist conditions. Dried blood samples on the flipside hinder bacterial growth and are less likely to degrade.

The next time you watch your favorite CSI TV program, you will have a better understanding of how blood is useful in the course of a criminal investigation.

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