Blood Volume Test

The forensic investigation of a crime or accident scene involves the collection and analysis of evidence , including any blood present at the crime scene. One facet of a forensic examination that concerns blood is known as the blood volume test.

A blood volume test is designed to determine the quantity of blood that has been shed in a particular area. As well, the test seeks to relate the shed blood to a blood stain on a surface such as a rug or floor, or a blood spatter pattern on a surface such as a wall. Finally, the blood volume test can provide information concerning the length of time that was needed to create the stain or spatter pattern.

The volume of blood that is present at a crime or accident scene can tell a forensic examiner a great deal about the origin of the blood. For example, a small quantity of blood is typically produced by a relatively insubstantial cut, whereas a copious quantity of blood is more characteristic of a severe cut such as a deep stab wound, bullet impact or the severing of an artery.

Blood is sometimes collected directly from a person. As in a hospital, this collection involves drawing the blood from the person into a sterile container (Vacutainer) that is specifically designed to hold blood.

Blood that has collected on surfaces can be collected in a clean unused plastic container. It is likely that not all the blood will be collected. But, enough can be retrieved to discriminate between a small or copious blood spill. Once collected, the blood volume test should be refrigerated, transported to the laboratory, and analyzed as soon as possible, to prevent contamination of the specimen or destruction of blood cells.

The retrieval of dried blood is more complicated. The area containing the dried blood can be scrapped or cut free of the surrounding fabric. Sometimes, dried blood can be lifted off a surface by applying sticky tape (fingerprint tape is typically used), or absorbing the blood onto a pre-moistened thread or cotton square.

The collected dried blood can subsequently be reconstituted into a known volume of liquid for the determination of the blood volume.

Determination of the volume of the sample occupied by blood cells is not complicated. Each sample is spun in a centrifuge, which causes the heavier blood cells to move to the bottom of the tube. The tubes have gradations that allow the packed volume of the blood cells to be measured, relative to the total volume of the cells and liquid (plasma).

see also Blood; Blood, presumptive test; Blood spatter; Crime scene investigation.