Overview

Lab tests play an important role in your health care. They help your doctor make a diagnosis or treatment decisions. But they may not provide all of the information that your doctor needs.

Your doctor will not usually make a decision or diagnosis based only on the results of a lab test, unless the test results are clear. Instead, the doctor will use test results along with information about your health, age, and other factors.

Making sense of your lab test involves more than just knowing why the test is done. It's also important to understand what the results mean and what can affect the results. Sometimes when you last ate or exercised can affect results. Medicines or herbal supplements and your age also can affect them.

Why Tests May Be Done

There are many reasons for lab tests. You may feel fine and still have a test, such as when you have an annual physical exam.

You may have a test to:

• Find the cause of symptoms.
• Confirm a diagnosis.
• Screen for a disease.
• Find out how serious a disease is.
• Find out if a treatment is working.
• Make sure medicines are not causing a problem.

What the Results Mean

You can learn more about your lab results in several ways. Ask your doctor to explain what the results mean. Prepare questions before you visit your doctor so you don't forget them. Don't be afraid to ask questions. If you don't understand the answer, ask again.

The person you talk to about your results may not be a doctor. You might also get information from a nurse or a physician's assistant.

Your insurance company or doctor's office may have a website where you can get help with lab test results. You can also look on a trusted online medical information site.

What does a positive or negative result mean?

Lab test results may be positive, negative, or inconclusive. Your doctor will discuss what your test results mean for you and your health.

• A positive test result means that the substance or condition being tested for was found. Positive test results also can mean that the amount of a substance being tested for is higher or lower than normal.
• A negative test result means that the substance or condition being tested for was not found. Negative results can also mean that the substance being tested for was present in a normal amount.
• Inconclusive test results are those that aren't clearly positive or negative. For example, some tests measure the level of antibodies to some bacteria or viruses in blood or other bodily fluid to look for an infection. It's not always clear if the level of antibodies is high enough to be a sign of an infection.

What are false-positive and false-negative test results?

A false-positive test result is one that shows that a disease or condition is present when it isn't present. A false-positive test result may suggest that a person has the disease or condition when they don't have it. For example, a false-positive pregnancy test result would appear to detect the substance that confirms pregnancy, when in reality the person isn't pregnant.

A false-negative test result is one that does not detect what is being tested for even though it is present. A false-negative test result may suggest that a person doesn't have a disease or condition being tested for when they do have it. For example, a false-negative pregnancy test result would be one that does not detect the substance that confirms pregnancy, when the person really is pregnant.

What do the numbers mean?

Many lab test results are reported as a number that falls within a reference range. A reference range is found by testing large groups of healthy people to find what is normal for that group. For example, a group of 30- to 40-year-old adults would be given a specific test. Then the results would be averaged to create the reference range for that group.

Each reference range is different. That's because it's created from information from a specific group. For example, the table below shows reference ranges for a sedimentation rate test.^{footnote 1} This test helps find out if a person has inflammation, an infection, or an autoimmune disease.

Sedimentation rate

Group	Range
Adults:	0–20 millimeters per hour (mm/hr)
Children:	0–10 mm/hr
Newborns:	0–2 mm/hr

What if your results are different than the reference range?

It's possible to have a result that's different than the reference range even though nothing is wrong with you. Sometimes certain things can affect your test results, such as pregnancy, a medicine you take, eating right before a test, smoking, or being under stress.

When your lab numbers are lower or higher than the numbers in the reference range, you may need more tests. Your doctor may want to repeat the test or order another test to confirm the results.

Why do values or reference ranges vary from lab to lab?

Labs may use different types of equipment and tests. And sometimes they set their own reference ranges. Your lab report will contain the reference ranges your lab uses. Don't compare results from different labs.

Only a handful of tests, such as blood sugar, have standardized reference ranges that all labs use. This means that no matter where those tests are done, the results are compared to the same reference ranges.

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Home Tests

You can do some types of tests at home, such as testing for blood sugar, pregnancy, urinary tract infections, and HIV infection. Some home tests give you results right away, such as a pregnancy test. Others provide a way for you to send a sample to a lab for testing. The lab then reports results back to you.

The quality and reliability of home tests vary greatly. Ask your doctor or pharmacist to recommend a reliable brand. Follow the instructions, and check with your doctor if you are concerned about the results. Your doctor will usually do further testing to confirm your results.

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