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A laboratory test examines a sample of blood, urine, or other tissue for certain features.

Why are lab tests performed?

Lab tests may be ordered by your physician or health care provider for a variety of reasons, including:

Variability of Lab Test Results

Laboratory test samples are analyzed to determine whether the results fall within normal ranges. Results also may be compared to previous test results.

Normal test values are usually given as a range, rather than as a specific number, because normal values vary from person to person. What is normal for one person may not be normal for another person. Many factors (including the patient’s sex, age, race, medical history, and general health) can affect test results.

Sometimes, test results are affected by specific foods, drugs the patient is taking, and how closely the patient follows pre-test instructions. That is why a patient may be asked not to eat or drink for several hours before a test.

It is also common for normal ranges to vary somewhat from laboratory to laboratory.

Evaluation of Test Results

Some laboratory tests are precise, reliable indicators of specific health problems. Others provide more general information that simply gives doctors clues to possible health problems.

Information obtained from laboratory tests may help doctors decide whether other tests or procedures are needed to make a diagnosis.

The information may also help the doctor develop or revise a patient’s treatment plan.

All laboratory test results must be interpreted in the context of the overall health of the patient and are generally used along with other exams or tests. The doctor who is familiar with the patient’s medical history and current condition is in the best position to explain test results and their implications.

Patients are encouraged to discuss questions or concerns about laboratory test results with the doctor.


Last updated April 13, 2013.

Reference: NIH