A blood test is a laboratory analysis conducted on a sample of blood drawn either from a vein in the arm using a hypodermic needle or collected through a fingerprick. Multiple individual tests targeting specific components, such as glucose or cholesterol, are frequently combined into a single test panel referred to as a blood panel or blood work. Blood tests are widely used in healthcare to assess physiological and biochemical conditions, including the presence of disease, mineral levels, the effectiveness of medications, and how well organs are functioning. Standard clinical panels include the basic metabolic panel and the complete blood count. Blood tests are also used in forensic and legal contexts to detect drug use, typically in criminal investigations rather than employment screening.

Before a blood test, you should avoid consuming anything other than plain water if you have been asked to fast, refrain from strenuous exercise, and avoid smoking or chewing gum, all of which can skew your results.

Venipuncture is a useful and minimally invasive method for obtaining blood cells and the fluid surrounding them, known as plasma, for laboratory analysis. Blood circulates continuously through the body, delivering oxygen and nutrients to tissues while carrying waste products back to the systems responsible for eliminating them. Because of this, the condition of the bloodstream both influences and reflects a wide range of medical conditions, which is why blood tests are the most frequently performed medical tests overall. When only a small amount of blood is needed, a fingerprick is used rather than drawing from a vein.

Determining your blood type requires a specific procedure called blood typing, which tests for the presence of A and B antigens on your red blood cells along with the Rh factor to establish whether you are positive or negative. This is typically performed using a technique called hemagglutination, in which your blood is combined with specific antibodies to see whether clumping occurs. Routine blood tests such as the complete blood count, basic metabolic panel, or tests for cholesterol, glucose, or infection cannot determine your blood type because they are measuring entirely different things, such as cell counts, organ function, or the presence of pathogens, not the antigens that define your type.

In the United States, blood tests can cost anywhere from roughly one hundred to five hundred dollars depending on the test. By comparison, the Canadian government typically covers between five and fifty-three dollars per test when a Canadian citizen requires one depending on the area and the type of test.

A test used to establish biological parentage is technically a DNA test rather than a standard blood type test. It works by extracting DNA from white blood cells and comparing specific genetic markers known as short tandem repeats between the child and the person in question. Because you inherit one copy of each marker from each biological parent, the laboratory checks whether the child’s markers align with those of the alleged parent across every location tested, typically fifteen to twenty markers in total. A match across all markers produces a probability of biological relationship of 99.99 percent or higher. If two or more markers fail to match, the person is conclusively excluded as a biological parent. A conventional ABO blood type test can only eliminate possibilities, for instance confirming that two type O parents cannot have a type AB child, but it can never identify who the actual parent is. Only DNA analysis can establish that.

During the mid-twentieth century, many American states required couples to undergo blood tests before marriage, though these had nothing to do with paternity or DNA. They were designed specifically to screen for syphilis and sometimes rubella or HIV, with the goal of preventing the spread of infectious disease and protecting future children. These requirements began to disappear in the 1980s as the cost-to-benefit ratio shifted, and by the 2000s nearly every state had eliminated them. A small number of jurisdictions, including Montana and Washington D.C., were the last to require them before phasing it out.

In 2008, researchers announced that saliva testing could eventually offer a more cost-effective alternative for certain blood tests, given that saliva contains roughly twenty percent of the proteins found in blood. However, saliva testing is not suitable for every type of measurement. Lipid levels, for example, cannot be assessed through saliva.