If you've ever had to give a tube of blood or a little cup of urine in your doctor's office, you've had a laboratory test. Laboratory tests check a sample of your blood, urine or body tissues. A technician or your doctor analyzes the test samples to see if your results fall within the normal range. The tests use a range because what is normal differs from person to person. Many factors affect test results. These include Your sex, age and race What you eat and drink Medicines you take How well you followed pre-test instructions Your doctor may also compare your results to results from previous tests. Laboratory tests are often part of a routine checkup to look for changes in your health. They also help doctors diagnose medical conditions, plan or evaluate treatments, and monitor diseases.