Reference ranges - What do they mean?

Dr Th Dhabali Singh *

Babina Diagnostics at Soibam Leikai, Imphal East (Pix in June 2012) :: Pix - TSE

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 signal it sends is that it should be investigated further. One can have an abnormal value and have nothing wrong. The doctor should try to determine the cause.

Myth: "If all the results are normal, one has 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 large overlap among results from healthy people and those with diseases, so there is still a small chance that there is 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.

THE "NORMAL" OR REFERENCE RANGE

The term "normal range" is not used very much today because it is considered to be misleading. If a patient's results are outside the range for the test, it does not automatically mean that the result is abnormal. Therefore, today "reference range", "reference values" or "reference intervals" are considered more appropriate.

Test results - all medical data - can only be understood once all the pieces are together.

WHAT IS A REFERENCE RANGE?

Some tests provide a simple "yes" or "no" answer. Was the culture for acid fast bacilli positive? Did the test find antibodies to a virus that indicates an infection?

For many tests, the meaning of the results depends on their context. A typical lab report will provide the results followed by a reference range. For example, the results for thyroid stimulating hormone (TSH) might look something like: 2.0 mIU/L, reference range 0.5 - 5.0 mIU/L.The test results indicate that it falls within the reference range.

The "reference range" is established by testing a large number of healthy people and observing what appears to be "normal" for them.

The first step in determining a given reference range is to define the population to which the reference range will apply, for example, healthy females between 20 and 30 years of age. A large number of individuals from this category would be tested for a specific laboratory test. The results would be averaged and a range (plus or minus 2 standard deviations of the average) of normal values would be established.

The term "reference range" is preferred over "normal range" because the reference population can be clearly defined.

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.

OTHER FACTORS AFFECTING TEST RESULTS

Laboratories will generally report test results accompanied by a reference range keyed to the age and sex,if appropriate. The physician then willstill need to interpret the results based on personal knowledge of one's health status,including, any medications he or she may be taking. Numerous additional factors can affect the results: one's intakeof caffeine, tobacco, alcohol, and vitamin C; the diet (vegetarian or non-vegetarian); stress or anxiety; physical exhaustion; pregnancy, etc.

Reference ranges also vary with the racial stock to which the patient belongs. Factors such as occupation and altitudehave been known to affect results. Regular exercise can also affect values of certain tests. Results of some tests can vary in people who participate for months and years of strenuous exercise such as distance running and weightlifting.

All these considerations underscore the significance of taking blood or urine samples in standardised fashion for performing and interpreting laboratory tests (and home tests as well). It is important to comply with the doctor's instructions in preparing for the test, such as coming with overnight fasting to get blood sampledrawn. That compliance makes the sample as close as possible to others; it keeps one within the parameters of his or her reference group.

WHEN "NORMAL" DOES NOT MATTER

For some tests, such as cholesterol, rather than worry about the reference range, the vast majority of people need only be concerned if their test results fall above or below a cut-off value that is sometimes referred to as a "decision point".

There are many other tests for which "normal" range is irrelevant. In addition, clinically significant, dramatic changes in a person's test values, even if those values remain within the reference range for that test, should be brought to the doctor's attention.

CONCLUSION

In general, reference ranges are specific to the laboratory that produces the test results. For many analytes, different laboratories use different kinds of equipment and testing methods. Hence, in reality, there is no such thing as a standard reference range. Of course, each test does have a theoretical reference range which can be found in many text books. However, for the result to have a greater meaning, it should be interpreted from the reference range provided by the laboratory.