Understanding Lab Test Results

Results

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 are not 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 is not always clear if the level of antibodies is high enough to indicate an infection.

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

A false-positive test result is one that indicates a disease or condition is present when it is not present. A false-positive test result may suggest that a person has the disease or condition when he or she does not have it. For example, a false-positive pregnancy test result would appear to detect the substance that confirms pregnancy, when in reality the woman is not 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 does not have a disease or condition being tested for when he or she does have it. For example, a false-negative pregnancy test result would be one that does not detect the substance that confirms pregnancy, when the woman really is pregnant.

Some lab tests can give you specific information. For example, your doctor may suspect you have strep throat and order a throat culture to see if streptococcus bacteria are present. A positive lab test confirms that you have strep throat and helps your doctor choose the right treatment for you.

But some tests give only a clue that must be considered with other information to support a diagnosis, identify a risk, or help choose a treatment. For example, if your cholesterol test results show you have high LDL ("bad") cholesterol, your doctor will weigh your other risk factors for heart disease before deciding on treatment.

What do the units mean?

Lab test results usually contain a number followed by a unit of measurement, such as 37 milligrams per deciliter (mg/dL). The units provide a way to report results so that they can be compared. Usually, but not always, the same test is reported in the same units no matter which lab did the test.

What is a reference range?

Many lab test results are expressed as a number that falls within a reference range. A reference range is determined 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 men would be given a specific test and the results averaged in order to create the reference range for that group.

Each reference range is different because it is created from information from a specific group. For example, the following table shows reference ranges for a sedimentation rate test. This test helps determine whether inflammation, infection, or an autoimmune disease may be present.

Sedimentation rate

Men:	0–15 millimeters per hour (mm/hr)
Women:	0–20 mm/hr
Children:	0–10 mm/hr
Newborns:	0–2 mm/hr

What if your results are different than the reference range?

It is possible to have a result that is different than the reference range even though nothing is wrong with you. Sometimes certain factors can affect your test results, such as pregnancy, a medicine you are taking, 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, further testing may be needed. 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. Do not compare results from different labs.

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

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Author:	Maria Essig	Last Updated: July 8, 2009
Medical Review:	E. Gregory Thompson, MD - Internal Medicine Anne C. Poinier, MD - Internal Medicine
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