To Your Health

Laboratory testing is one of the most important aspects of modern medicine

Q. I just talked to my doctor about some recent blood tests for anemia, but I'm still not sure what the results really mean. Everything was within the normal range except for a couple of things called "RDW" and "MCV," which were slightly high. There were a lot of other tests with capital-letter abbreviations that my doctor didn't discuss because these were normal. What do all these letters stand for anyway?

A. Laboratory testing is one of the most important aspects of modern medicine, estimated to contribute 60-70% of the information your physician needs for a clinical diagnosis. For many diseases, abnormal blood-test results appear long before any warning signs or symptoms become apparent. Doctors rely on laboratory tests to help in diagnosing a wide range of conditions, to rule out the possibility that certain diseases are present that explain your symptoms, and to help monitor your response to treatment.

Evidently your doctor ordered what is known as a "complete blood chemistry panel," or CBC (here we go with more of those capital letters). There is a lot of information in a CBC, everything from how many red cells are in your blood to what your immune system is working on today. Entire books are written on the interpretation of the 15 or so numbers that come out of a routine CBC, and when something unusual is found there are several additional follow-up tests that will give your physician even more good information. From the pattern that emerges out of all those numbers, an astute physician can spot the typical problems, including a deficiency in certain nutrients if deficiencies exist.

In the past, a CBC was done by an experienced technician looking at blood through a microscope and laboriously counting and describing what he or she saw. This was a costly and time-consuming task. Today it is done by a machine called a Coulter counter, an ingenious device invented in 1953 by Wallace Coulter that can count and calculate the size of cells in solution. The Coulter counter lowered the price of a CBC to less than $20, making it affordable for common use.

The two tests you mention, RDW and MCV, relate to the size of your red blood cells. RDW stands for "red cell distribution width" and MCV for "mean corpuscular volume." Ideally, your red blood cells are neither too small nor too large (about 95 cubic microns as indicated by MCV) and all about the same size (according to RDW). Size is important because the ability of a red blood cell to transport oxygen from your lungs to your body tissues is affected by its size.

RDW really cannot go below 10 or so but it can go rather high, 25 or higher, indicating wide variation in the size and shape of red cells. An elevated RDW may be the earliest indication of anemia. To properly diagnose the cause of the anemia your physician then uses other numbers, such as MCV. For instance, newly made red cells, called reticulocytes, are slightly larger than mature red cells. If you are in the process of recovering from anemia, MCV is likely to be high temporarily because you are making a lot of reticulocytes that have not yet matured. An elevated RDW with low MCV would have an entirely different interpretation.

Laboratory tests may be an almost invisible side of medical care to patients, but they help your doctor make decisions on what is happening inside you with greater confidence.

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