By Dr. Brian McDonough, Medical Editor

PHILADELPHIA (CBS) - Patients are always being sent for tests and there are hundreds of potential studies that doctors can order.

There are two types of tests: screening tests and diagnostic tests.

Screening tests are for conditions such as cancer or high blood pressure and help us detect them early, when they are more treatable. Diagnostic tests, on the other hand find the root of your problems.

If you have chest pain or a stomachache, your doctor may order a test to see why. This could be as simple as a urine test. More complex tests include CT scans or biopsies, in which your doctor takes a tissue sample and looks at it under a microscope.

If you have a condition like diabetes, tests can also help you and your doctor choose between therapies or check how well your treatment is working.