By Susan Leclair, PhD, CLS (NCA)
Getting a laboratory test result really has no meaning without having something with which to compare it. Its meaning comes from comparing it to reference values, which are the values expected for a group of otherwise healthy persons. The process for developing reference values, or a reference range, is to collect results from a large, healthy population and to determine the average (mean) result and expected differences from that mean (standard deviation). The reference range is usually defined as the set of values 95 percent of the population falls within. Notice that 2.5% of the lower values and 2.5% of the higher values will not be included in the reference range, even though they were found in the “normal” population.
Is there a difference in values between children and adults?
Is there a difference in values based on age or gender or race?
What about people who are taking certain medications? Do they have an impact?
Is the specimen collected in the exact same manner all of the time? (fingerstick versus phlebotomy)
Is there more than one method for analyzing the specimen, and what is it that the test is really measuring? (for example, total thyroxine versus free throxine)
Is there a difference in values collected at different times of the day and, if so, is it big enough to matter?
Oxygen is present in different concentrations at different altitudes, so is that an issue for tests like hemoglobin or hematocrit?
And there are more factors that could influence the result!
Every laboratory uses its own instruments, reagents, techniques, etc., and that means that each laboratory will have its own unique sets of reference ranges. The ranges probably will not vary greatly among different laboratories, but values at the edges are those that cause patients the most concern. The list we present here is one set of reference values. Your laboratory may have a different set.
We should not call these “normal” values because folks who are ill can have them as well. We call them reference values to convey the idea that a lot of people will have these so you can use them as a broad guide. However, the most important test results are yours and how they vary over time.
Susan Leclair, PhD, CLS (NCA) is Chancellor Projessor Emerita at the University of Massachusetts Dartmouth; Senior Scientist, at Forensic DNA Associates; and Moderator and Speaker, PatientPower.info – an electronic resource for patients and health care providers.
Originally published in The CLL Society Tribune Q4 2019.