Blood Test - What to Know

A blood test is a laboratory analysis performed on a blood sample that is usually extracted from a vein in the arm using a needle, or via fingerprick. Blood tests are used to determine physiological and biochemical states, such as disease, mineral content, drug effectiveness, and organ function. They are also used in drug tests.

Venipuncture is useful as it is a relatively non-invasive way to obtain cells and extracellular fluid (plasma) from the body for analysis. Since blood flows throughout the body, acting as a medium for providing oxygen and nutrients, and drawing waste products back to the excretory systems for disposal, the state of the bloodstream affects, or is affected by, many medical conditions. For these reasons, blood tests are the most commonly performed medical tests.

If only a few drops of blood are needed, a fingerstick is performed instead of drawing blood from a vein.[2]
Phlebotomists, laboratory practitioners and nurses are those charged with patient blood extraction. However, in special circumstances, and emergency situations, paramedics and physicians sometimes extract blood. Also, respiratory therapists are trained to extract arterial blood for arterial blood gases.

A basic metabolic panel measures sodium, potassium, chloride, bicarbonate, blood urea nitrogen (BUN), magnesium, creatinine, glucose, and sometimes includes calcium. Blood tests focusing on cholesterol levels can determine LDL and HDL cholesterol levels, as well as triglyceride levels.

Some blood tests, such as those that measure glucose, cholesterol, or for determining the existence or lack of STD, require fasting (or no food consumption) eight to twelve hours prior to the drawing of the blood sample.[6]
For the majority of blood tests, blood is usually obtained from the patient's vein. However, other specialized blood tests, such as the arterial blood gas, require blood extracted from an artery. Blood gas analysis of arterial blood is primarily used to monitor carbon dioxide and oxygen levels related to pulmonary function, but it is also used to measure blood pH and bicarbonate levels for certain metabolic conditions.

While the regular glucose test is taken at a certain point in time, the glucose tolerance test involves repeated testing to determine the rate at which glucose is processed by the body