1. Serological Tests for Blood
1.1. A serological test is a test that involves the use of antibodies to detect the presence of specific antigens. If presumptive testing indicates that a certain stain is in fact blood then the next step is to ascertain whether this is human blood.
1.1.1. This can be done by
1.1.2. Using Precipitin serological test to identify the presence of proteins specific to humans
1.1.2.1. The precipitin test for species of origin is based on antigen-antibody complex formation, which produces a clearly visible cloudy precipitate.
1.1.2.1.1. The precipitin test may be applied to blood stains in a number of different ways. For example, it may be conducted in a capillary tube, with a layer of human antiserum (serum containing antibodies specific for human antigens) overlain by a layer containing an extract of the bloodstain under investigation.
1.1.2.2. Another method known as cross-over electrophoresis is where a gel-coated slide containing twin wells is used. A liquid extract of the bloodstain is placed in one depression, while human antiserum is placed in another.
1.1.2.2.1. The application of an electric current to the slide induces the antibodies (from the antiserum) and the antigens (from the blood) to move towards each other.
1.1.3. And/or analysing for DNA sequences specific to humans.
2. Bloodstain Pattern Analysis
2.1. Bloodstain pattern analysis can provide information on what may have happened during the course of the crime and the order in which these events may have took place.The evidence provided by the bloodstain pattern analysis may be used to refute or corroborate a particular version of events as given by a suspect or witness.
2.1.1. When it is considered that adult human males contain about 5-6 litres of blood, and adult females contain 4-5 litres of blood, it is not surprising that instances of violent crime, copious amounts of blood are found at the scene. As with all physical evidence, it is essential that the bloodstains present at the crime scene are recorded by an appropriate combination of notes, sketches, photographs and/or video footage.
2.1.1.1. Active Bloodstains
2.1.1.1.1. Active bloodstains are defined as those caused by blood that may have been made to travel by a force other than gravity.
2.1.1.1.2. It may be possible to ascertain the direction in which droplets of blood were travelling when the hit a target surface such as a wall. If on impact, individual drops create tear-shaped stains, the direction of travel may be discerned from the direction in which the tails of the individual stains point.
2.1.1.2. Passive bloodstains are those that are formed solely under the influence of gravity. They include such features as blood flows, pools and drops. The interpretation of patterns created by blood flows may provide information about whether a body has been moved since death.
2.1.1.2.1. An example of other types of passive bloodstains may reveal information about the length of time that has passed since the bloodshed occurred. For example, in the case of drops and pools of blood, drying times may be estimated by comparison with the results from the crime scene.
2.1.1.2.2. Another important aspect of bloodstains drying is the initial formation of an outer ring of dried blood within a very short time period. this has been demonstrated to occur within 50 seconds of the blood being shed. Attempts to remove such drying bloodstains after this time usually fail to eradicate the encrusted outer ring.
2.1.1.3. Passive Bloodstains
2.1.1.4. Transfer Bloodstains
2.1.1.4.1. Transfer bloodstains are those that have been deposited on surfaces as a result of direct contact with objects contaminated with wet blood. It can help to establish the movement of the individuals involved.
3. Three Main Cellular Components of Blood
3.1. Erythrocytes (Red blood cells)
3.1.1. Most common type of blood cell which accounts for over 44% of the total blood volume.
3.1.2. They contain haemoglobin, an iron-containing protein responsible for carriage of oxygen (and carbon dioxide) in the blood
3.1.3. In contrast to other mammalian cells, they lack nuclei.
3.2. Leucocytes (White blood cells)
3.2.1. These cells together with thrombocytes constitute to less than 1% of the total blood volume.
3.2.2. White blood cells are involved in protecting the body from infection. They may be further subsided into phagocytes and lymphocytes, which are responsible for the capture and ingestion of foreign substances (such as bacteria) and the production of antibodies respectively.
3.3. Thrombocytes (Platelets)
3.3.1. These are non-nucleated cell fragments, which are formed from the fragmentation of very large cells called megakaryocytic in the bone marrow.
3.3.2. Thrombocytes are involved in the process of blood clotting.
4. Presumptive Tests For Blood
4.1. At a crime scene presumptive tests may be used to detect the presence of blood that might be otherwise overlooked, either because it is in minute amounts or because it merges well with its background.
4.1.1. They may also be used to indicate whether a stain is actually composed of blood and not some other substance such as ink, rust or chocolate.
4.1.1.1. With the exception of luminol, presumptive tests are not usually carried out directly on the object's bearing, instead they are performed on filter paper.
4.2. Presumptive tests for blood are based on the ability of the haemoglobin present in the red blood cells to catalyse the oxidation of certain reagents. In most cases, hydrogen peroxide is used as the oxidising agent (H202(aq))
4.2.1. One example that is widely used is phenolphthalein which is colourless in its reduced form, but bright pink when oxidised.
4.2.1.1. A small circular piece of absorbent card or paper is folded in half and then in half again to form a point where a small amount of the stain is then scraped onto.
4.2.1.1.1. In the phenolphthalein test, a drop of the dye in its reduced form is added to the test material . The presence of blood is indicated by the development of pink colouration when a drop of hydrogen peroxide aqueous is added.
4.2.1.1.2. Another reagent used for this purpose is called leach-malachite green (LMG) which is also colourless in its reduced form but blue-green when oxidised.
4.3. However, caution should be applied in the interpretation of the results as some vegatable materials such as horseradish and potatoes contain the enzyme peroxidase which may give positive results. these positives are known as false positives.
4.3.1. Moreover, it should be noted that as colour change tests give positive results in the presence of haemoglobin , they do not distinguish between human blood and that of other animals.
4.3.1.1. Tests such as precipitin serological test or the analysis of blood DNA would therefore have to be used
4.4. In some circumstances, the application of the luminol test for blood may be more appropriate than the test described above. The luminol test is particularly useful when, for example, the surrounding surfaced have been washed down in order to eradicate any obvious bloodstains.
4.4.1. An advantage of luminol test would be that it does not affect any subsequent ABO or DNA profiling.
4.4.1.1. To perform the test, an alkaline solution containing both luminol and an appropriate oxidising agent such as H202 is prepared and sprayed onto the search area. Where blood is present, the luminol is catalytically oxidised and a distinct glow is produced.
4.4.1.1.1. Again there can be false positives produced by substances such as household bleaches, metals and vegetable peroxidases.