A blood test is a procedure in which a sample of blood is taken from a patient and analyzed for abnormalities. Blood tests can help diagnose or monitor various conditions or diseases, including infectious diseases.

Samples can be obtained from a vein puncture, skin (capillary) puncture or artery puncture. Blood tests can be performed on different parts of the blood, including:

• Whole blood (blood that has not been separated into its many components)
  
  
• Blood plasma (the liquid part of the blood)
  
  
• Blood serum (plasma that has had the clotting agent removed)
  
  
• Blood cells (the individual red blood cells, white blood cells and platelets)

Many types of blood tests may be used to diagnose or rule out illnesses. Different types of tests may be performed depending on the nature of illness that is suspected. Tests that are often associated with infectious diseases include complete blood counts (CBCs), differential white blood cell counts, antibody testing and blood cultures.

Some blood testing requires little or no preparation to achieve an accurate result. However, in other cases, patients may need to fast (avoid food and beverage intake for a certain length of time prior to the test) or avoid certain substances or activities prior to testing to ensure that certain factors do not skew test results.

Blood tests may be performed in a physician’s office, hospital, healthcare facility or laboratory. The exact steps taken during each blood test may be different, depending on the nature of the test.  In some cases, a sterile, sharp lancet (sharp, pointed instrument) is used for pricking the patient’s skin. When a substantial amount of blood is required, it is usually drawn from a vein in a process called a venipuncture or venous phlebotomy.

Although blood tests may help detect many diseases, additional tests are usually necessary to verify the diagnosis. It is important to note that blood tests cannot detect all conditions, and that a normal blood test does not necessarily indicate that a person is healthy.

Results from blood tests are usually returned to the requesting physician within 24 hours, depending on the type of blood test performed. Some tests, such as a blood culture, may require up to several days or longer before test results are known.

There are few risks or side effects associated with blood tests. Sometimes, bruising may occur at the site of injection. In rare cases, the vein used to obtain the sample may become inflamed (superficial phlebitis), or infection (e.g., cellulitis) may result from breaking the skin.