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Reference Ranges and What They Mean

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What does it mean if my test result is outside the reference range (low or high)?

Laboratories report patient test results along with their reference ranges. Results that are out of range are typically highlighted and may include a comment when out-of-range results have clinical significance. Some reports include "critical values" that represent potentially life-threatening abnormalities. Every laboratory identifies certain key tests that have been associated with these life-threatening events whereby critical values are required to be immediately reported to the health practitioner.

When a lab report indicates that one or more of your test results are out of range, your healthcare provider will evaluate those results in the context of your medical history, physical exam, and family history, among other factors.

You and your healthcare provider may consider the following questions in order to determine what should happen next:

How far out of range are your results?
Results that are far above or far below the reference range are an obvious indication that further investigation is needed. But what about results that are only slightly above or below the range? As we will discuss below, healthy people sometimes have test results outside the range. On the other hand, for some analytes, such as tumor markers or creatinine, test results that are even slightly out of range can be significant. Your provider may recommend follow-up testing to find out if the result returns to within range or persists in being outside the range.

How do the results fit with the rest of your clinical picture?
Depending on the information your provider has already gathered about your health status, a test result outside the range could help confirm a diagnosis, indicate the severity of a health problem, or point to the need for additional tests to be run. If a result does not seem to fit with the rest of your clinical picture, your healthcare provider may:

  • Reorder the test.
    • It is possible that the analyte being measured happened to be high on the day your sample was drawn because of something you ate, recent physical exertion, or other circumstantial situation.
    • Technical errors due to improper processing or transportation of the specimen (refrigeration issues, exposure to heat, timely separation of blood red cells from plasma/serum) may have occurred.
    • Perhaps you did not fully comply with the test preparation instructions. Did you fast or avoid certain foods for the recommended amount of time? Did you stop taking prescription medications, over-the-counter medications, or supplements as instructed by your healthcare provider before the test? Did you avoid cigarettes or alcohol before the test as instructed? Compliance with test preparation instructions makes your sample as close as possible to others; it keeps you within the parameters of your reference group.

Are the present results different from those you have had in the past?
Your provider will evaluate whether an out-of-range test result is a new change for you or represents the progression or recurrence of a condition for which you may be receiving treatment.

Factors affecting lab test results
There are a few reasons why a test result could fall outside of the established reference range despite the fact that you are in good health. Generally, these factors only come into play when the test value is slightly higher or lower than the reference range.

  • Statistical variability: It is common practice for reference ranges to cover 95% of results for a healthy population. Statistically speaking, that means 5% of people in that same population will have results that fall outside the limits.
  • Biological variability: If your provider runs the same test on you on several different occasions, there's a good chance that at least one of those times the result will fall outside the reference range even though you are in good health. Your body is always changing. Your age, diet, hormonal cycles, physical activity level, alcohol intake, even a change of season can cause alterations in your body chemistry that will show up on a test result.
  • Individual variability: References ranges are usually established by collecting results from a large population and determining from the data an expected average (mean) result and expected differences from that average (standard deviation). There are individuals who are healthy but whose tests results, which are typical for them, do not always fall within the expected range of the overall population.

If you know of any special circumstances that could affect a test, mention them to your healthcare provider; don't assume your provider has thought of every possible circumstance.

The differences between reference ranges from different labs typically are generally not significant, but it is possible that one lab will report a result as being within range while another could report that same result as being out of range. It should be noted that all clinical laboratories are periodically inspected as directed by federal guidelines (Clinical Laboratory Improvement Act of 1988, or CLIA '88) and are subjected to extensive review of quality control procedures. If you have a health condition that is being monitored with lab tests, it may be recommended to have the same lab perform the tests for consistency. This is something to keep in mind in the following circumstances:

  • You change healthcare providers and the new practitioner uses a different lab than the one your previous practitioner used.
  • Your provider starts having tests performed by a new lab.
  • You have tests performed by a hospital laboratory (if it is different from the lab that usually performs your tests).

It is your provider's job to not only consider that an out-of-range result could be due to the lab change but also to consider how great a change is reflected in the new result and the whether it might actually represent a significant change in your health.

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