My patients often have questions about the results of their laboratory tests, or bloodwork, especially in this era of “Doctor Google.” I thought it would be good to point out some things that will help you understand the results you receive. It’s important to remember that the conclusions we can make from a collection of lab results are often much more complicated than people realize.
Just because a value is reported as within the normal range does not always mean it is truly normal, and the opposite is frequently true as well—a value outside the normal range is often not a sign of any disease state. To complicate matters, there are some ethnic and racial differences in normal ranges as well.
Here’s what you need to know about normal and abnormal lab results. Be sure and discuss any specific concerns with your primary care physician.
What is the “normal” range?
The standard method for reporting test results is based upon what is called the 95% confidence interval, which is widely accepted in the field of statistics. In essence, the “normal” range is that demonstrated by 95% of the healthy population. The bell curve for the range of a given lab value cuts off the 2.5% at the upper end of normal and the 2.5% at the lower end of normal. Results are reported this way because to widen it to include 100% would allow for extreme values that in many cases might be a sign of disease.
Think of the example of the famous jockey, Willie Shoemaker, who stood all of 4 feet 11 inches, and the basketball player Wilt Chamberlain, who was 7 feet 1 inch. While both of these men were considered healthy, there are instances in which heights at those extremes are due to such things as bone or pituitary disease, so to include such extremes in the range of normal might miss some cases of height extremes caused by medical abnormalities.
The 95% confidence interval means that 5% of healthy people might have a lab value outside the range reported as normal. When we run 40 or 50 lines of lab data, each with the 5% chance of a value outside the typical normal range, there is a pretty good chance that several of the values will fall out of that range in someone who is perfectly healthy.
How your doctor interprets your lab values
Here is where medical education and experience are invaluable to interpret whether a lab value is a concern worthy of additional investigation. I will often tell patients that their “abnormal” values are insignificant, not a sign of any disease state, and normal for them. The pattern of lab values often gives us more information than focusing on a single number.
If someone has two or three slightly elevated liver enzymes, it is much more likely an indicator of true disease than if only one is elevated. If we have prior values for comparison, it is useful to look for a trend that clues us into a potential problem. Someone with a platelet count that has been slowly falling over a series of lab reports might still have a value within the normal range, but the trend would suggest an evolving medical problem to their physician.
The history and exam are invaluable in discerning the significance of abnormal lab values. An elevated white blood count might be expected with an infection like pneumonia but could be a sign of a serious blood disorder like leukemia if no infection is suspected. An elevation of creatinine (a measure of kidney function) would be worrisome for kidney disease in a frail or thin person, but it might be normal for someone with heavy muscle mass since muscle is one source of blood creatinine.
So, it is wise to look over your lab results, but at the same time, to appreciate the complexity of the process of interpreting them. This is one area where your primary care physician can spend some time analyzing the numbers to advise and guide you to continued good health.
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About the author
Peter Stack, MD, FACP
Peter Stack, MD, FACP, is an internal medicine physician on the medical staff at Baylor Scott & White Medical Center – Plano. He enjoys gardening and birding in his spare time. Connect with Dr. Stack today.