Many people get a lab order from their doctor and are not sure what the test is for or what happens next. This is true for patients, and it is also true for the family members who help them.
This article explains what a lab test is, what kinds of samples are used, why doctors order tests, and how results are read. It is written in plain words so you can go into your next test knowing what to expect.
What a lab test is
A laboratory test, or lab test, looks at a small sample taken from your body. The sample may be blood, urine or another body fluid, or a small piece of tissue.
The sample goes to a laboratory. There, trained staff test it to see whether certain substances are in it, and if so, how much.
The kinds of samples labs use
Blood testing is one of the most common kinds of lab test. Blood tests look at cells, chemicals, proteins and other substances in the blood. The most common way to take a blood sample is from a vein in the arm with a small needle. This is called a venipuncture, and the person who does it is often called a phlebotomist.
MedlinePlus says you may feel a little sting when the needle goes in or out, and the draw usually takes less than five minutes. It also says that most blood tests need no special preparation, though some do.
Labs also test urine, other body fluids and small tissue samples. Your doctor's order says which sample is needed.
Why doctors order lab tests
Doctors use lab tests for several reasons. MedlinePlus lists these:
- To find out whether you have a certain disease or condition, or to rule one out.
- To keep track of a condition over time.
- To see whether a treatment is working.
- To help choose the best treatment.
Lab tests are also a common part of a routine checkup. At a checkup, tests can help your provider look for changes in your health, even when you feel fine.
A lab test is only one piece of information. MedlinePlus notes that lab tests do not give a complete picture of your health. Your provider also looks at your physical exam, your health history, your family history and any imaging tests.
What happens to your sample
After the sample is collected, it is labeled and sent to a laboratory. The U.S. Food and Drug Administration (FDA) calls these tests in vitro diagnostics, which means tests done on samples taken from the body.
In the United States, clinical laboratories are overseen under a federal law called the Clinical Laboratory Improvement Amendments, or CLIA. A laboratory must have the right certificate under CLIA before it can accept human samples for testing. Three federal agencies share the work: the Centers for Medicare and Medicaid Services, the FDA, and the Centers for Disease Control and Prevention.
When the testing is done, the results go back to the doctor or other provider who ordered the test.
How results are read
Many results are given as a number. The report often shows that number next to a reference range. A reference range is the span of results that is considered normal. It is based on test results from large groups of healthy people.
MedlinePlus explains a few points that are good to know:
- Different labs may use different reference ranges, so results from two labs cannot always be compared side by side.
- Because ranges vary, the word "normal" can be misleading.
- Some results are simply "positive" or "negative." Positive means the test found what it was looking for. Negative means it did not. A result that is neither is called inconclusive.
Tests are not perfect. A false positive shows a condition that is not there. A false negative misses a condition that is there. This is one reason your provider looks at your results together with everything else they know about you.
What can change a result
Illness is not the only thing that affects a lab result. MedlinePlus lists several other things that can change results:
- Certain foods and drinks.
- Certain medicines or supplements.
- Hard exercise before the test.
- Whether the preparation instructions were followed, including fasting.
Age and sex can also affect what is considered normal. For these reasons, it helps to read the instructions on the lab order before the test and to ask your provider if anything is unclear.
Talking with your provider
Your provider is the right person to explain what your results mean. MedlinePlus says it is important to talk with your provider to find out what your results mean for you.
Some questions you may want to ask are:
- What is this test checking for?
- Do I need to do anything to prepare?
- When will the results be ready, and who will contact me?
- What do my results mean, and is any follow up needed?
If you help a parent or another family member with their care, you can ask these same questions with them.
The short version
A lab test looks at a small sample of blood, urine or other fluid or tissue. The sample goes to a certified laboratory, and the results go back to the provider who ordered the test. Doctors use lab tests to find, rule out and track health conditions, and to see how treatment is going. Many things besides illness can change a result, and your provider is the right person to explain what yours mean.
This article is general information and not medical advice. Talk with your doctor about your own care.