Explaining reference ranges
This article was last reviewed on
This article waslast modified on December 20, 2017.

During a recent visit to your healthcare provider's office, you had blood drawn for lab tests and now you want to know if everything is "okay." You've received a copy of your lab report or an email telling you that your test results are available to view online. So you log on to the secure site and download your results. In scanning the page, you see a result that is highlighted as being outside the reference range and you wonder what that means for you.

Some lab tests provide a simple "yes" or "no" answer. For instance, was the test positive for the bacteria that cause strep throat? Many other tests, however, are reported as numbers or values. Laboratory test results reported as numbers are not meaningful by themselves. Their meaning comes from comparison to reference values. Reference values are the values expected for a healthy person. They are sometimes called "normal" values.

By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.

This website, Lab Tests Online, gives information about various tests, including the possible reasons test results may be "abnormal."

Three important things to know about reference ranges:

  • A normal result in one lab may be abnormal in another: You must use the range supplied by the laboratory that performed your test to evaluate whether your results are "within normal limits." While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents used, and analysis techniques. Consequently, for most lab tests, there is no universally applicable reference value. This is the reason why so few reference ranges are provided in the test information on this website, Lab Tests Online.
  • A normal result does not promise health: While having all test results within normal limits is certainly a good sign, it's not a guarantee. For many tests, there is a lot of overlap among results from healthy people and those with diseases, so there is still a chance that there could be an undetected problem. Lab test results in some people with disease fall within the reference range, especially in the early stages of a disease.
  • An abnormal result does not mean you are sick: A test result outside the reference range may or may not indicate a problem. Since many reference values are based on statistical ranges in healthy people, you may be one of the healthy people outside the statistical range, especially if your value is close to the expected reference range. However, the abnormal value does alert your healthcare provider to a possible problem, especially if your test result is far outside the expected values.
Accordion Title
About Reference Ranges
  • What is a reference range?

    A reference range is a set of values that includes upper and lower limits of a lab test based on a group of otherwise healthy people. The values in between those limits may depend on such factors as age, sex, and specimen type (blood, urine, spinal fluid, etc.) and can also be influenced by circumstantial situations such as fasting and exercise. These intervals are thought of as "normal ranges or limits."

    Though the term "reference interval" is usually the term preferred by laboratory and other health professionals, the more commonly-known term is "reference range," so that is the term used throughout this article.

    Reference ranges provide the values to which your healthcare provider compares your test results to and determines your current health status. However, the true meaning of a test result—whether it indicates that you are sick or well or at risk for a health condition—can only be known when all the other information your provider has gathered about your health, including the results of a physical exam, your health and family history, recent changes in your health, any medications you are taking, and other non-laboratory testing.

    Most people can now access their lab test results directly via the Internet, but very few lab reports have been designed to convey the meaning of those results in a way people who are not health professionals can understand or put in context. The information provided in this article will help you understand:

    • Why so few reference ranges are provided in the test information on this site: the accuracy of laboratory testing has significantly evolved over the past few decades, but some lab-to-lab variability can occur. This may be due to differences in lab testing equipment, chemical reagents, and analysis techniques. You must use the range supplied by the laboratory that performed your test to evaluate whether your results are "within normal limits."
    • A few tests do not have ranges, but limits at which decisions are made about whether you are healthy or should be treated. Through many years of research involving large, diverse populations, these limits have become standardized. An example is glucose testing for diabetes.
    • Each laboratory establishes or "validates" its own reference ranges, thus reflects differences that vary from lab to lab. The specific reference ranges that appear on your laboratory report are determined and provided by the laboratory that performed your test.
    • Reference ranges help describe what is typical for a particular group of people based on age, sex, and other characteristics. In the context of your personal information, you and your provider can use reference ranges as a guide to what your results mean and to help make decisions about managing your health.
    • There are some factors that can cause test results to be out of range when you are, in fact, in good health, and there are some common misconceptions about what lab results might mean.

    While this site can help you understand some of the implications of your test results, the best source of this information is your healthcare provider. You can use what you learn about your results from Lab Tests Online to talk to your provider, be prepared to ask the right questions during that conversation, and to take an active role in your healthcare decisions.

    View a sample report to see what references ranges look like and where they typically appear on lab reports.

  • Where are the reference ranges on this web site?

    Very few tests will have reference ranges that are exactly the same for most laboratories. That's why very few of the test articles on this site include reference ranges. (Those that do list them in the section titled "What does the test result mean?" For some, you may need to click on the button on the right hand side of the page that says "Looking for Reference Range?" or "See Reference Range".)

    To evaluate whether your numbers are within normal limits, it is best to refer to the reference range printed on the report you receive from the laboratory that performed your tests. A report will typically display your results followed by that lab's reference intervals for each test performed. Your report may look something like this, indicating that this test result falls within the established normal reference range:

    Reference Range Example
    Test Name  Result Reference Range
    Potassiium 4.4 mmol/L 3.5-5.1 mmol/L

     

    (To see an example of what a full report with several test results looks like, see this sample cumulative report.)

    Only a few of the test descriptions on this website include reference ranges. There are several reasons for this:

    1. In general, reference ranges for most analytes are specific to the laboratory that performs the test. Different laboratories use different kinds of equipment and different kinds of testing methods for analysis. This means each laboratory must determine its own reference ranges—whether by using data from its own equipment and methods, citing reference ranges from test manufacturers or other laboratories, or by testing a pool of perceived normal and healthy individuals. Consequently, there are no universally standardized reference ranges. Of course, each test does have a theoretical reference range that we could include on this site, which can be found in many books and other online sources, but it may have little meaning for you.
    2. You may notice that the few, select reference ranges listed on this site apply only to adults, and there are no ranges included for children or adolescents. This is because from birth through adolescence, a child's body goes through many changes, often quite rapidly. Several things that are tested in a laboratory, such as chemical levels, hormones, etc, vary greatly as a child goes through the different stages of growth and development. The laboratory in which your child's sample is tested has established reference ranges for the different stages of child development. The best source of information regarding your child's lab test results is your child's healthcare provider.
    3. We want you to be informed, but we don't pretend to take the place of communication between you and your healthcare provider. We want you to understand what each test on this site is for, but because we can't be aware of all the factors that could affect your test results, we can’t interpret the results without more information. If you need further explanation of your results, you should talk to your healthcare provider. This remains true even for those tests, such as the components of the basic metabolic panel (BMP), for which we have included reference ranges. Remember, a reference range is merely a guide for your healthcare provider. He or she will interpret the result in the context of your medical history and current presentation – something that no website is yet able to do.
  • What are decision limits?

    For a small number of tests, long-term studies of certain disease processes have led to the establishment of decision limits that are more useful than reference ranges in determining clinical outcomes and guiding treatment decisions. Decision limits are values that represent either the upper or lower quantity of an analyte that are consistent with a disease state or indicate a need for treatment.

    Blood glucose is an example of an analyte for which decision limits have been established and are widely used by healthcare providers. For adults in a routine setting in which fasting blood glucose testing is done to detect type 2 diabetes, a fasting glucose level of 126 mg/dL (7.0 mmol/L) or above, obtained on more than one testing occasion, indicates diabetes. Treatment is required to reduce the risk for cardiovascular disease, kidney disease, and other long-term complications of diabetes.

    In this situation, it is a value above a particular limit that provides information rather than a value that falls within or outside a set range of numbers.

  • How are reference ranges determined?

    The short answer to this question is by testing a large number of people who have key similarities and observing what appears to be "typical" for them.

    To determine ranges, labs may conduct their own studies for the tests they perform, they may adopt reference ranges from test manufacturers or other labs, or they may derive reference ranges from existing patient data.

    • The most important step in determining a reference range for any test is to define the reference population – the group of people who will be represented in the reference range. Depending on the test and factors that may influence its results, reference populations may be chosen based on age, sex, race, general health, and/or medical history.
    • Next, a large number (minimum of 120) of people who fit the profile of the reference population are tested under nearly identical conditions, and the results are analyzed.
    • For many tests, reference ranges include the values that are statistically analyzed and reported for the middle 95% of the reference population.

    Why might there be more than one reference range for a test?
    For some tests, there is no single reference range that applies to everyone. By far, the most common factors that cause variations in reference values are age and/or sex.

    For example, the range used to evaluate a creatinine test result depends on a person's age and sex as shown in the table below.

    Age Sex Creatinine Reference Ranges
    18-20 Male 0.9-1.3 mg/dL
      Female 0.6-1.1 mg/dL
    60-90 Male 0.8-1.3 mg/dL
      Female 0.6-1.2 mg/dL

    Creatinine is produced as a natural byproduct of muscle activity and is removed from the blood by the kidneys. Creatinine is often measured as a gauge of how well the kidneys are functioning. Creatinine levels are affected by a person's muscle mass. Because men have greater muscle mass than women, the reference values for men are higher than for women.

    Reference ranges for children who are still growing and developing are often very different than those for adults and may change significantly as they age. Alkaline phosphatase (ALP), for instance, is an enzyme found in the cells that make bone, so its concentration in the body rises in proportion to new bone cell production. The reference values for ALP in children and adolescents, who should be growing healthy bones, are higher than they are for adults in whom high ALP levels may signify bone or liver disease.

    Some reference ranges apply only to certain subsections of a population. For example, pregnancy changes many aspects of a woman's body chemistry, so what is typical for pregnant women is not typical for those who are not pregnant. Thus, pregnancy-specific reference ranges are established for a variety of tests.

    A very short list of examples of additional factors that might cause a lab to establish more than one reference range for a test includes:

    • The type of sample used (for example, whole blood, plasma, serum, urine, saliva, or other body fluid)
    • The time of day when a sample is collected
    • Whether the test is performed on a patient who has been fasting
    • The phase of a female patient's menstrual cycle at the time of testing or whether she is in perimenopause or menopause

    The reason a test is performed sometimes guides which reference range is used to interpret results and guide treatment decisions. For example, the hemoglobin A1c (HbA1c) test can be used to screen for diabetes risk, aid in confirming a diagnosis of diabetes, or monitor how well a person with diabetes has controlled his or her blood sugar over a three-month period. The table below shows how a provider might use the results of the hemoglobin HbA1c test to guide treatment decisions.

    Diabetic Status HbA1c Levels
    Non-Diabetic Less than 5.7% (39 mmol/mol)
    Prediabetic 5.7% - 6.4% (39 - 46 mmol/mol)
    Diabetic 6.5% (47 mmol/mol) or greater
    Well-controlled Diabetes Less than 7.0% (52 mmol/mol)

    Let's look at two people who have the same HbA1c result of 6.6%, one who is being screened for diabetes and another who is already known to have diabetes.

    • For the person who is being screened, a result of 6.6% would lead a healthcare provider to suspect that the person has diabetes. The HbA1c test must be repeated at a later date and if the second result is again greater than 6.5%, a diagnosis of diabetes is confirmed.
    • For the person who has already been diagnosed with diabetes, an HbA1c result of 6.6% is good news. It means the condition is being well controlled by the current treatment (whether diet and exercise and/or medication).
  • 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.

  • Common Misconceptions

    There are two main misconceptions about test results and reference ranges:

    Myth: "An abnormal test result is a sign of a real problem."

    Truth: A test result outside the reference range may or may not indicate a problem, but it signals your healthcare provider to further investigate your condition. You can have a value outside the range and have nothing wrong—but your provider should try to determine the cause.

    It's possible that your result is within that 5% of healthy people who fall outside the statistical reference range. In addition, there are many things that could throw off a test without indicating a major problem. High blood sugar could be diet-related rather than caused by diabetes. A lipid result could be high because you didn't fast before the test. High liver enzymes can be the temporary result of a recent drinking binge rather than a sign of cirrhosis. Prescription drugs might interfere with test results. While the FDA requires extensive examination of side effects, including changes in laboratory test results, it may not be uncommon for many of these drugs to interfere with certain laboratory tests, resulting in falsely high or low values.

    Most likely, your provider will want to rerun the test. Some results outside the range may resolve on their own, especially if they are on the border of the reference range. Your provider will also seek explanations for an abnormal result, such as those above. A key point your provider will address is how far out of the reference range is the result and if the results are repeatable.

    Myth: "If all my test results are within range, I have nothing to worry about."

    Truth: It's certainly a good sign, but it's only one set of tests, not a guarantee. There is a lot of overlap among results from healthy people and those with diseases, so there is still a chance that there could be an undetected problem. Just as some healthy people's results fall outside the reference range, lab test results in some people with disease fall within the reference range, especially in the early stages of a disease.

    If you're trying to follow a healthy lifestyle, take test results that are within range as a good sign, and keep it up. But if you're engaging in high-risk behavior, such as drug and alcohol abuse or a poor diet, it only means "so far so good," and the potential consequences haven't caught up with you yet. A good test result is not a license for an unhealthy lifestyle.

    If you had results outside a range previously, results within the range certainly provide good news. However, your healthcare provider may want to conduct follow-up tests some months later to make sure you're still on track and to document any trends. A rise or drop in the level of a critical analyte, even if it is still within normal limits, could mean something significant.

View Sources

NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used. To access online sources, copy and paste the URL into your browser.

Sources Used in Current Review

Barth JH. Editorial: Reference Ranges Still Need Further Clarity. Annals of Clinical Biochemistry. 2990;46:1-2.

Boyd JC. Defining Laboratory Reference Values and Decision Limits: Populations, Intervals, and Interpretations. Asian Journal of Andrology. 2010;12: 83–90.

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Ceriotti F, Henny J. Are my laboratory results normal? Considerations to be Made Concerning Reference Intervals and Decision Limits. Pediatric Reference Intervals. 2008;19:1-9.

Jones G, Barker A. Reference Intervals. Clinical Biochemistry Review. 2008;29 (Suppl i):S93-S97.

Phillips P. Pitfalls In Interpreting Laboratory Results. Australian Prescriber. 2009;32:43-46.

(2009) Determining Laboratory Reference Intervals: CLSI Guideline Makes the Task Manageable. Available online at http://labmed.ascpjournals.org/content/40/2/75.full#content-block. Accessed August 2015.

(December 11, 2013) National Cancer Institute. Understanding Laboratory Tests. Available online at http://www.cancer.gov/cancertopics/factsheet/detection/laboratory-tests. Accessed August 2015.

Graham Jones, Antony Barker. Reference Intervals. Clin Biochem Rev. 2008 Aug; 29(Suppl 1): S93–S97. Available online at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2556592/. Accessed August 2015.

(July 2012) Al-Borai A. Frequently-Asked-Questions on Reference Intervals and Biological Variation. Westgard QC. Available online at https://www.westgard.com/faq-ri-bv.htm. Accessed August 2015.

Sources Used in Previous Reviews

Books:

Clinical Diagnosis and Management by Laboratory Methods. 20th ed. Henry JB, ed. New York: Saunders: 2001.

Laboratory Medicine: Test Selection and Interpretation. Howanitz JH and Howanitz PJ, eds. New York: Churchill Livingstone; 1991:6-8.

National Committee for Clinical Laboratory Standards. How to Define and Determine Reference Intervals in the Clinical Laboratory: Approved Guideline. 2nd ed. Wayne, PA: 2000.

Sacher RA, McPherson RA, Campos J. Widmann's Clinical Interpretation of Laboratory Tests. 11th ed. Philadelphia: F.A. Davis Company; 2000:10-17.

The Science of Laboratory Diagnosis. Crocker J and Burnett D, eds. Oxford: Isis Medical Media; 1998: 391-4.

Tietz Textbook of Clinical Chemistry. Burtis CA and Ashwood ER, eds. Philadelphia: W. B. Saunders Company; 1994: 454-464.

Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Burtis CA, Ashwood ER, Bruns DE, eds. St. Louis: Elsevier Saunders; 2006. Pp. 425-437.

McClatchey, et al. Clinical Laboratory Medicine. Second Edition.

Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, Fourth Edition.

Interviews:

Roberta Reed, PhD, Mary Imogene Bassett Hosp., Cooperstown, NY, (by Eric Seaborg), 6/7/01.

David Sundwall, MD, President of the American Clinical Laboratory Association, Washington, DC (by Eric Seaborg), 6/7/01.

Pennell Painter, PhD, Professor of Pathology, Technical Director of the Dynacare Tennessee Medical Laboratories at the University of Tennessee Medical Center at Knoxville (by Jason Kahn).

Internet:

National Cholesterol Education Program website, available online through http://www.nhlbi.nih.gov

Cornell University Veterinary School website, available online through http://web.vet.cornell.edu