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This article waslast modified on 15 June 2021.

For basic rules that can help you to make sense of your results, see Reading your results.

The interpretation of any clinical laboratory test involves comparing the patient's results with the test's "reference range".

The first step in determining a reference range is to define the population to which the range will apply. A large number of individuals from a group who are thought to represent a "normal" population, will be tested for a particular laboratory test. The reference range is then derived mathematically by taking the average value for the group and allowing for natural variation around that value (plus or minus 2 standard deviations from the average). In this way, ranges quoted by labs will represent the values found in 95% of individuals in the chosen ‘reference’ group. In other words, even in a "normal" population, a test result will lie outside the reference range in 5% of cases (1 in 20). This is why the term "reference range" is preferred over "normal range".

When you examine test results from different populations, you quickly discover that what is "normal" for one group is not necessarily normal for another group. Indeed for tests such as cholesterol the idea of a normal range has been replaced to a large extent by use of target values, achieved either by lifestyle changes or active treatment.

Whether or not your test result is within the laboratory reference range, the result must be considered within the context of your personal circumstances, and with the benefit of your doctor's knowledge of your past medical history, current medication and the results of any other investigations.

Some typical reference ranges for a small number of commonly requested tests are listed below. Users of Lab Tests Online UK should however note that these ranges may differ between laboratories for the reasons described on these pages.

Serum Sodium 133 – 146 mmol/L
Serum Potassium 3.5 – 5.3 mmol/L
Serum Urea 2.5 – 7.8 mmol/L
Serum Chloride 95 – 108 mmol/L
Serum Bicarbonate 22 – 29 mmol/L
Serum Phosphate 0.8 – 1.5 mmol/L
Serum Magnesium 0.7 – 1.0 mmol/L
Serum Albumin 35 – 50 g/L
Serum Total Protein 60 – 80 g/L
Serum Osmolality 275 – 295 mmol/kg

About Reference Ranges
  • Age & Sex

    Effects of Age and Sex

    For many tests, there is no single reference range that applies to everyone because the tests performed may be affected by the age and sex of the patient, as well as many other considerations. For example alkaline phosphatase is an enzyme found in the cells that make bone, so its concentration in the body rises in proportion to new bone cell production. In a child or adolescent, a high alkaline phosphatase is not only normal but also desirable because the child should be growing healthy bones. But these same concentrations found in an adult are a sign of trouble. So experience from testing large numbers of people has led to different reference ranges for different age groups.

    The patient’s sex is another important consideration for some tests for example muscles contain an enzyme called creatine kinase (CK). Because of their larger muscle mass, men tend to have higher CK concentrations in their bloodstream than women.  

  • Other factors

    Other Factors Affecting Test Results

    A plethora of additional factors can affect your test results: the time of day you have your blood sample taken, your intake of caffeine, tobacco, alcohol, biotin and vitamin C; your diet (vegetarian vs. meat eaters); stress or anxiety; or a pregnancy. Even your posture when the sample is taken can affect some results, as can recent heavy exertion. For example, albumin and calcium concentrations can increase a little when moving from lying down to an upright position. Vigorous exercise can affect concentrations of creatine kinase (CK), cholesterol, aspartate aminotransferase (AST), and lactate dehydrogenase (LD).

    All these considerations show the significance of taking blood or urine samples in a standardised fashion for performing and interpreting laboratory tests. It’s important to follow your doctor’s instructions when preparing for your tests, such as coming in first thing in the morning and getting your blood taken before you eat anything.

  • Few on this site

    Why are there few reference ranges included on this website?

    With all this talk of reference ranges, you may notice that very few are included on this website. There are several reasons we chose not to include them:

    1.  For most tests, the reference ranges are specific to the laboratory that produces the test results. Different laboratories use different kinds of equipment and different kinds of testing methods. That means they have to establish their own ranges, and will provide your test result with an accompanying reference range appropriate to the laboratory. The doctor should therefore apply the reference range supplied by the laboratory which performed the test rather than that used by another laboratory or one given in a book.

    2.  We want you to be informed, but we can not pretend to take the place of communication with your doctor or healthcare professional. We want you to understand what the test is, and how it can be used but because we can't be aware of all the factors that could affect your test results, we can't interpret the results. If you need further explanation of your results, you should talk to your doctor or health professional.

    3.  The type of units used to report results can vary between laboratories. In Europe most tests are reported in the units defined by the System Internationale (SI) such as moles and litres but in the US, the majority of tests are reported in mass units e.g. milligram (mg) in volumes of decilitres (dL). The misuse of results and units can result in potentially hazardous mistakes.

  • Common misconceptions

    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—the only sure signal it sends is that your doctor or healthprofessional should investigate it further. You can have an abnormal value and have nothing wrong—but your doctor should try to determine the cause.

    It’s possible that you fall in that 5% of healthy people who fall outside the reference range. In addition, there are many things that can give a misleading result without indicating a major problem. A 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. If your doctor is unsure about the test result they may wish to repeat it. Some abnormal results may disappear on their own, especially if they are on the border of the reference range.

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

    Truth: It is certainly a good sign, but it’s only one set of tests, not a guarantee. There is a large overlap amongst the results from healthy people and those from people with diseases, so there is still a chance of 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.

    If you’re trying to follow a healthy lifestyle, take it as a good sign, and keep it up. But if you’re engaging in high-risk behaviour, 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 licence for an unhealthy lifestyle.

    If you previously had abnormal results, normal results certainly provide good news. But your doctor may want to conduct follow-up tests some months later to make sure you’re still on track and to follow any trends.