Laboratory tests are among the most common and important tools of modern medicine. Results from clinical laboratory tests contribute to the majority of health care decisions. They provide information about your health status and risk factors and can be used to determine a diagnosis, guide therapy, and estimate outlook (prognosis). Many of the decisions you and your health care provider make about your care are based on results of laboratory tests.

No lab test is 100% accurate. However, medical testing that takes place in certified laboratories in the U.S. is performed by trained professionals and follows rigorous quality assurance standards with extensive oversight. You can trust the data generated by today’s exacting medical tests. However, test results are most reliable when used in conjunction with other meaningful information, such as your medical and family histories and a physical exam. Additionally, there should be open communication between you and your doctor or other members of your health care team. To learn more, read How Reliable is Laboratory Testing?

Typically, a blood sample is taken by inserting a needle into a vein, usually in your arm. Before your blood is drawn, an elastic band is tied around your upper arm to increase blood in the veins, and the puncture location is wiped clean with an antiseptic. A blood draw using a needle may cause a temporary sting. The blood draw normally lasts for less than a minute. Sometimes a blood sample is collected by puncturing the tip of a finger and drops of blood are collected onto a test pad, filter paper, or into a small tube.

Blood tests cannot definitively tell whether or not a person has cancer. However, some blood cancers may be detected if there are abnormal findings on a complete blood count (CBC). Additional tests, such as a bone marrow biopsy, are performed to confirm the diagnosis. For other types of cancers, your health care provider may order testing if it is suspected that you have cancer. Typically, other procedures, such as a physical exam, imaging tests, and a biopsy, are necessary to help make a diagnosis. For more details, read the guide on Tumor Markers.

There are many factors that determine the cost of a lab test, including the type of test, where the test is performed, and if you have insurance coverage. When prescribed by a doctor, testing is often covered if you have insurance, but you may still have a copay or deductible. There can also be fees charged by the technicians who draw your blood. Point-of-care testing at health fairs is usually done at no charge or very low-cost. Check with your doctor and insurance plan about the cost of your test.