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.