What is blood testing? |

Indications and Procedures

Most blood tests require a substantial sample of blood. A sample of several milliliters or more is withdrawn from a vein using a syringe. The sample is transported to a diagnostic laboratory for the appropriate analysis.

In blood typing, red blood cells are marked on their surface with the protein antigens designated A, B, AB (both A and B), or O (no antigen). Individuals possess antibodies, found in the liquid or serum portion of the blood, to the antigens that they themselves do not possess. In ABO typing, red blood cells are isolated from the blood sample and mixed with antiserum with a known antibody type. Cells marked with antigen A will clump when mixed with antibody A, cells with B antigen will clump when exposed to antibody B, AB antigens clump with either, and O clumps with neither. The type of positive clumping reaction determines blood type. Rh type is determined in the same manner, with Rh-positive blood exhibiting a clumping reaction with Rh antibodies. Histocompatibility leukocyte antigen (HLA) typing is performed similarly but uses white blood cells as the antigenic material. HLA typing is more complex and time consuming because more than forty different HLAs are known to exist.

Deoxyribonucleic acid (DNA) analysis requires isolating DNA from white blood cells and amplifying certain regions with a procedure called "polymerase chain reaction
(PCR)." The amplified DNA is cut with enzymes and run through an electric field in a gel matrix, thus separating the pieces of DNA by size. Specific patterns are visually analyzed. When this technique is repeated across many different regions of a person’s DNA, a pattern unique to that individual can be obtained. This information can be used to answer questions of paternity and relatedness or to help convict or exonerate suspects in murder and rape cases. A similar analysis is used to determine the presence of certain genetic diseases or the possibility of transmitting a genetic disease.

The use of blood tests to diagnose diseases covers a wide range, and the test is matched to the analysis requested. Abnormal levels of red blood cells can indicate anemia, while abnormal white cell counts may suggest a severe infection. Inappropriate enzyme levels could signal organ malfunction, and abnormal hormone levels indicate an endocrine disease. High levels of particular antibodies herald an infection with the corresponding antigenic agent.

Uses and Complications

Blood testing may be performed for a variety of reasons, including monitoring of organ function and blood clotting, evaluation of medication effectiveness, and diagnosis of and/or risk assessment for specific diseases or conditions. If an individual has suffered a large loss of blood and/or requires surgery that may necessitate a blood transfusion, ABO and Rh factor
blood typing is performed. Pregnant women need to know their own Rh status and that of their fetus in order to prevent gestational complications from Rh incompatibility. If an organ transplant is required, the HLAs present on all cells except red blood cells must be matched as closely as possible to prevent graft rejection. In the event of paternity determination or forensic analysis, the DNA sequences found in white blood cells can be analyzed for proper identification. Some genetic diseases in an affected individual, a fetus, or unaffected individuals who may transmit the genetic disease to their offspring can be detected with blood DNA analysis. Many types of disorders and diseases are diagnosed through a determination of the presence of abnormal levels of blood cell types, enzymes, hormones, and electrolytes (sodium, potassium, phosphate, magnesium, and calcium) or the presence of infectious agents in the blood, such as bacteria and viruses. Not all genetic diseases can be tested using DNA analysis. A partial list of those that can be detected includes hemophilia, Huntington disease, cystic fibrosis, and sickle cell disease. Others, such as Tay-Sachs disease, can be detected with an enzyme test of the blood.

Should proper ABO typing not be performed and an individual be given an incompatible blood transfusion, the antibodies in the individual’s blood would destroy the newly transfused cells, causing a life-threatening condition. Failure to diagnose and treat an Rh-incompatible pregnancy could result in death of the fetus or a baby born with severe hemolytic disease and jaundice because of the destruction of fetal blood cells by the mother’s immune system.

In modern hospitals with standard precautions, there are only minor side effects to blood withdrawal with a syringe. Some pain from puncturing the vein wall and localized bruising may occur. If the procedure is done under unsanitary conditions or if needles are used repeatedly, the risk of transmitting bacterial infections or viral infections, such as hepatitis and human immunodeficiency virus (HIV), increases dramatically.

Perspective and Prospects

DNA analysis is rapidly providing new and better diagnostic tools for the identification of genetic disease and its carriers as well as for the prediction of an individual’s tendency to develop heart disease and certain cancers, such as those of the breast and colon. Such predictive tests could save millions of lives through close monitoring and early treatment.

Such advances of technology, however, are not without consequences. Since genetic testing became routine in the 1980s, there has been great concern about the access of employers and insurance companies to such potentially damaging information. For many years, lack of regulation of DNA testing laboratories frequently resulted in incorrect or ambiguous test results. Movements toward regulation and licensing of these laboratories are alleviating these problems.

Bibliography

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