Why the Doctor Takes a Blood Sample

Why Your Health Care Provider Takes a Blood Sample

You probably don't enjoy giving a blood sample, but it's an important part of a physical exam. From a small sample of your blood, your health care provider can find out a lot about your overall health.

Here are four common tests, according to the American Association of Clinical Chemistry: blood lipid level, complete blood cell count (CBC), blood sugar level, and thyroid function tests.

Blood lipid level

The levels of total cholesterol, LDL (low density lipoprotein cholesterol), and HDL (high density lipoprotein cholesterol) in your blood may indicate your chances of someday developing heart disease. Cholesterol, a soft, fat-like substance found in the cell walls of all animals, can stick to the walls of arteries and form plaques. These plaques can hamper and even block the flow of blood to the heart. A high total cholesterol and/or a high LDL level means you probably need to exercise more and cut back on foods high in saturated fat and cholesterol. Your health care provider may also recommend medication.

Complete blood cell count

The red cells in your blood carry the oxygen that body tissues need to function. The CBC includes two measurements of the red cells called hemoglobin and hematocrit. If these are low, anemia is present. Anemia can account for fatigue, paleness, shortness of breath, and even drowsiness. Women are more susceptible to anemia. Childbirth can bring on anemia because bleeding during the birth process may deplete the body's supply of iron, which is essential for red blood cells. Menstruation can also reduce a woman's red blood cell count. Anemia can be treated. The CBC will also provide a count of white cells, which are responsible for fighting infection. Platelets are counted as well. Platelets help the blood form a clot when you are cut or injured.

Blood sugar

A fasting high blood sugar level may indicate that you need further testing to look for diabetes, a disease with sometimes subtle symptoms. Diabetes, if untreated, can lead to blindness and other medical complications. Symptoms of diabetes include excessive thirst, more frequent urination, and fatigue. If detected early, diabetes can be treated to lessen its impact on health.


This gland at the base of your neck produces a hormone that controls your metabolism. With too little of the hormone, you feel tired and gain weight easily. With too much of the hormone, you feel jittery, your heart may beat rapidly, and you lose weight.

For these four tests, your health care provider only needs about two small vials of blood. In most cases, a nurse or a medical specialist called a phlebotomist draws the blood sample with a needle inserted in a vein in your arm. The procedure takes only a few minutes.

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