Dry blood is the solid residue that remains after the liquid components of blood evaporate or are absorbed into a surface. It is a familiar sight from minor cuts and represents a concentrated source of biological information used in scientific and medical investigations. The transition from a fluid to a dry state preserves cellular and molecular components, making them available for later analysis.
The Transformation from Wet to Dry Blood
When blood exits the body, it begins a two-part transformation. The first stage is coagulation, a natural response to injury designed to form a clot. This process is initiated by platelets, which activate a complex series of protein interactions known as the coagulation cascade. This cascade converts a soluble plasma protein, fibrinogen, into insoluble fibrin strands.
These newly formed fibrin strands create an intricate, web-like mesh structure. This mesh rapidly traps red blood cells, white blood cells, and platelets, forming a solid plug or clot that helps prevent further blood loss.
The second stage of the transformation is evaporation. The clot still contains a significant amount of water from the plasma. Over time, this water dissipates into the environment or is absorbed by the underlying material. As the water evaporates, the clot hardens and shrinks, resulting in a dry, solid stain.
Composition of a Dry Blood Stain
A dried bloodstain is a matrix of cellular elements held together by a fibrin protein mesh formed during coagulation. The most abundant components are red blood cells, which give the stain its color due to the protein hemoglobin. While numerous, red blood cells lack a nucleus and do not contain nuclear DNA.
Also trapped within the stain are white blood cells. Although far less numerous than red cells, they are important for genetic identification because they possess a nucleus. This nucleus houses the individual’s complete set of nuclear DNA.
The fibrin mesh acts as a biological glue, binding the cells to each other and to the surface. Also present are the platelets that initiated the clotting cascade, resulting in a stable, concentrated sample of biological material.
Forensic and Medical Analysis
Dried blood is a valuable resource in forensic science for identifying individuals. Scientists extract DNA from the nuclei of white blood cells preserved within a stain. The process involves rehydrating the stain and using chemicals to release the DNA. This genetic material is then analyzed to create a unique DNA profile for comparison to known samples or databases.
Beyond DNA, other tests can be performed on dry blood. Serological tests can determine the blood type (A, B, AB, or O) from antigens on the surface of red blood cells. While less specific than DNA profiling, blood typing can help include or exclude individuals during an investigation.
In medicine, Dried Blood Spot (DBS) testing is used for widespread health screening. This technique involves collecting a few drops of blood, often from a newborn’s heel, onto a specialized filter card. The dried card creates a stable sample that is easily transported to a lab. DBS testing screens infants for numerous genetic and metabolic disorders, allowing for early diagnosis and treatment.
Factors Affecting Preservation and Appearance
A bloodstain’s appearance changes considerably over time. Initially, a fresh stain is bright red due to oxygenated hemoglobin. As the stain dries and ages, the hemoglobin oxidizes, causing the color to darken from red to reddish-brown and eventually almost black.
The long-term preservation of a dried bloodstain and its DNA is influenced by environmental conditions. Exposure to ultraviolet (UV) light, particularly from direct sunlight, can rapidly degrade DNA molecules, breaking them into smaller, unusable fragments. High humidity and temperature also accelerate decomposition by promoting the growth of bacteria and fungi.
The surface on which the blood is deposited also affects preservation. Non-porous surfaces like glass or sealed metal preserve stains well, as the blood dries on the surface without being absorbed. Porous materials such as fabric or wood can draw the blood into their fibers, which can protect the DNA but may also make extraction more challenging.