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Understanding Lab Test Results

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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 rate1


0-15 millimeters per hour (mm/hr)


0-20 mm/hr


0-10 mm/hr


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 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.


WebMD Medical Reference from Healthwise

Last Updated: September 09, 2014
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.
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