Health Encyclopedia

Blood Test

A blood test is a lab analysis of the elements in your blood. Regular blood tests might be ordered to keep track of how well you and your doctor are managing a condition such as diabetes or high cholesterol. They are also ordered during routine checkups and sick visits.

Blood test are ordered by healthcare providers to help:

  • Find out how well important organs such as your kidneys, liver, heart, or thyroid are working.

  • Help diagnose diseases such as cancer, diabetes, coronary heart disease, and HIV/AIDS.

  • Find out if your medicine is working to make you better.

  • Diagnose bleeding or clotting disorders.

  • Find out if your immune system is having problems fighting infections.

  • Diagnose anemias. These are iron-deficiency anemia, pernicious anemia, aplastic anemia, and hemolytic anemia.

  • Find variations in hemoglobin such as hemoglobin S, C, or E. These are common in people of African, Mediterranean, or Southeast Asian background.

  • Monitor chronic health conditions and diseases.

  • Find health problems in their early stages. 

It is your right to know why a certain test has been ordered, so ask your doctor if you are not sure why he or she wants you to have the test.

Types of blood tests

These are common blood tests:

  • Complete blood count, also called a CBC

  • Blood chemistry tests

  • Blood enzyme tests

  • Blood tests for heart disease risk

Blood tests can give your doctor a lot of information about whether certain elements in your blood fall within a normal range. But in many cases they are only part of the information your doctor needs to make a diagnosis of a health condition. You might need to have some other types of tests as well.

Preparing for a blood test

Many blood tests don't require you to do anything in advance. These are to see what your blood is like under normal conditions.

For others, however, you will have to fast for a certain amount of time before the blood test. Fasting usually means no eating or drinking anything after midnight on the night before the test. These tests are often scheduled for early in the morning.

Your doctor will let you know if you need to fast before a blood test.

The procedure

In order to test your blood, a technician called a phlebotomist will use a needle to take a sample of blood, in most cases from a vein in your arm. You will be seated or lying down during the procedure. You will probably be asked to make a fist. The technician will use a band to constrict your arm. Once he or she finds a usable vein, the technician will clean the area and then insert the needle. You might feel a small pricking sensation. Once the technician has drawn enough blood, he or she will take the needle out and put an adhesive bandage over the site. You may be asked to press firmly on the site to stop any bleeding.

If the sight of needles makes you nervous, you can look away during the procedure or bring a friend to help distract you.

After the procedure

Your blood sample will be sent to a lab. There trained technicians look for the information the doctor has ordered. This may take as little as a day or up to a week. Check back with your doctor's office to find out about the results. 



Medical Reviewers:

  • Godsey, Cynthia, MSN, APRN, MSHE, FNP-BC
  • Pierce-Smith, Daphne, RN, MSN, CCRC