MedLine - Medical TestsLearn about medical tests, including what the tests are used for, why a doctor may order a test, how a test will feel, and what the results may mean. Medical tests can help detect a condition, determine a diagnosis, plan treatment, check to see if treatment is working, or monitor the condition over time. A doctor may order these tests as part of a routine checkup, to check for certain diseases and disorders, or to monitor your health. MedlinePlus is a service of the National Library of Medicine (NLM), the world's largest medical library, which is part of the National Institutes of Health (NIH).