Effects of age and gender
For many tests there is no single reference range that applies to everyone. Some tests may be affected by the age and sex of the person as well as many other considerations.
An example is alkaline phosphatase, which is an enzyme found in the cells that make bone, its levels change with bone growth. In a child or adolescent, a high alkaline phosphatase level is not only normal but also desirable because the child should be growing strong, healthy bones. The same levels 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 person’s sex is another important consideration for some tests, for example muscles contain an enzyme called creatine kinase. Men have more muscle and so they tend to have higher creatine kinase levels in their blood than do women.
Other factors affecting test results
Many additional factors can affect your test results: the time of day you have your blood sample taken, your intake of caffeine, tobacco, alcohol, and vitamin C; your diet (vegetarian versus meat eaters); stress or anxiety; or pregnancy. Even your posture when the sample is taken can affect some results, as can recent heavy exercise. For example, albumin and calcium levels can increase a little when you change from lying down to an upright position. Vigorous exercise can affect levels of creatine kinase (CK), aspartate aminotransferase (AST), and lactate dehydrogenase (LD).
All these considerations show the significance of taking blood or urine samples in a standardised fashion for carrying out, comparing and interpreting laboratory tests. It’s important to follow your doctor’s instructions when preparing for your tests, such as when you are told to come in first thing in the morning to have your blood taken before you eat anything.
The type of units used to report results can vary between laboratories. In Australasia and Europe most tests are reported in the units defined by the System Internationale such as moles and litres but in the US, the majority of tests are reported in mass units eg milligram (mg) in volumes of decilitres (dL). The misunderstanding of results and units can result in potentially hazardous mistakes.
What does it mean if my test result is out of the reference range?
First, there are a few reasons why a test result could fall outside the established reference range despite the fact that you are in good health:
- Statistical variability: Even when performing the same test on the same sample multiple times, 1 out of 20 (or 5%) determinations will fall outside an established range, based on the laws of probability. Sometimes, if the test is repeated on this same sample, the result will then be within range.
- Biological variability: If a doctor runs the same test on you on several different occasions, there’s a good chance that one result will fall outside a reference range even though you are in good health. For biological reasons, your values can vary from day to day. That’s why a doctor may repeat a test on you and why he or she may look at results from other times that you have the same test performed.
- 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 normal for them, do not always fall with in the expected range of the overall population.
Thus, a test value that falls outside the established reference range supplied by the laboratory may mean nothing significant. Generally, the test value may be only slightly higher or lower than the reference range and you may indeed be healthy.
Second, a result outside the range may indicate a problem and warrants further investigation. Your doctor will evaluate your test results in the context of your medical history, physical examination and other relevant factors to determine whether a result that falls outside the reference range means something significant for you.
The first thing your doctor is likely to do is to re-run the test on the same sample or he may request that you submit another sample for testing. Perhaps the analyte being measured happened to be high that day due to one of the reasons stated previously, or perhaps something went awry with the sample (the blood specimen was not refrigerated, or the serum was not separated from the red cells, or it was exposed to heat). Your doctor may also compare the latest test result to previous results if you have been tested for the same thing in the past to get a better idea of what is normal for you.
Laboratories will generally report the findings based on age and sex when appropriate, and leave it to the doctor to interpret the results based on factors such as diet, your level of activity, or medications you are taking. If you have a result that falls outside the reference range, talk to your doctor about what it means for you and what steps need to be taken next.
If you know of any special circumstances that could affect a test, mention them to your doctor; don’t assume your doctor has thought of every possible circumstance.
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 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. It may also be the case that the disease may not cause any abnormality in the particular tests that you had. For example early cervical cancer won’t cause any abnormality in common blood tests but may be obvious on a Pap smear.
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.