What is Blood Testing?

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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 03 October 2019
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Blood testing is a form of medical testing in which the blood is screened to provide information about a patient's condition. Blood contains a tremendous amount of highly useful information about what is going on inside the body. There are a wide range of reasons to order blood testing, ranging from a blood workup in a hospital used as part of the diagnostic process to a routine STI screening requested by a patient who is concerned that he or she may have been exposed to sexually transmitted infections.

In blood testing, a sample of blood drawn with a needle from a vein or with a bloodstick on a finger is subjected to testing in the laboratory. Blood samples taken from a vein can be spun in a centrifuge to separate out the different blood components for a complete blood count (CBC), in which the levels of various blood components are determined. They can also be subjected to an array of tests; sometimes multiple vials are drawn to provide an ample supply of blood for testing.


Blood can be subjected to tests which reveal the presence of viral infection and provide a viral load count, indicating how many copies of the virus are present in the blood. Testing can also be used to check hormone levels, which may be used to diagnose various hormone imbalances, to monitor the progress of hormone therapy, or to determine whether or not a woman is pregnant. A blood test can also be used to check glucose levels, as in the fingersticks which some people with diabetes perform to check their blood sugar, and to check cholesterol levels, the amount of medication in the blood, levels of various minerals in the blood, and to assess enzyme levels in liver function tests.

Blood testing is also used in blood matching, a process which is used to determine blood types. It is important to know a patient's blood type when he or she donates or receives blood, and more extensive matching is conducted for organ transplants to find the best match. If someone's blood is not a match, severe complications can occur during transfusion or organ donation.

Some medical facilities have an in house lab which can do blood testing on the spot and return results very quickly. In other cases, it may be necessary to ship blood samples to a laboratory off site. Blood test results are presented in the form of a written report which provides numerical values and any comments from the laboratory technician, and they are kept in a patient's file. As a general rule, if blood results are unremarkable, a doctor will not call the patient. Patients who are curious can ask to have their results explained.


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