What do changes in laboratory test results mean?
Susan J. Leclair,Ph.D., CLS(NCA), Chancellor Professor, University of
Massachusetts Dartmouth, Dartmouth, Massachusetts USA
Because CLL is found in the blood stream, its progression is followed in the blood stream. The section “Understanding Your Blood Results” gave a list of the tests that are most often used. But, testing by its very nature is not perfect. Changes in values can happen throughout the day in response to walking, eating, laughing, sleeping, etc. Variation in testing methods is also an issue. Finally, other situations such as taking medication can alter results. Since there are a lot of tests listed there, this section will deal with the hemoglobin and the differential tests only.
Erythrocyte Physiologic Variation
Hemoglobin and hematocrit values will vary during the day, in part due to changes in hydration and in part due to hormonal control called the circadian rhythm. Values are highest in the morning as one would expect since you haven’t been drinking fluids during the night.Red blood cells are relased into the blood stream around 10 o’clock every day so that release might also increase these values. Throughout the day, the increase in fluids and the change in hormonal levels will cause the values to gradually decrease to a low in the later afternoon. How much of a variation? About 1 gram of hemoglobin or three percentage points of the hematocrit. So rule number 1 – always have your blood specimen collected at the same time of the day. It really doesn’t matter when just as long as it is consistent.
Erythrocyte Testing Variation
In the 20th century, the hematocrit or packed cell volume (PVC) was considered to be the most accurate test in use. With the advent of the multi-channel instrument, that accuracy was diminished as the hematocrit is now not tested but calculated. As a result, most physicians rely on the hemoglobin as a measure of anemia.
Hemoglobin can be measured in many different ways. The most common is that used in almost all multi-channel instruments. In this method every form of hemoglobin (hemoglobin with oxygen, hemoglobin with carbon monoxide, hemoglobin with carbon dioxide, etc.) but one are chemically altered to one form and that one form is measured. The amount of blood used for this test is exceptionally small – 20 microliters. But because it is so small, there can be subtle alterations in the amount of cells present. As a result, if I were to perform a hemoglobin analysis ten times using blood from the same test tube, I would get a range of results. The range is rather small – plus or minor 0.5 but that would be enough to change a 9.0g/dL value to an 8.5g/dL. I chose that value for, in many countries, transfusions are considered to be justified if the hemoglobin is below 9.0. It also means that a hemoglobin value of 8.5g/dL could suggest a more severe anemia than is present because the “real” value is 9.0/dL.
When you are using your electronic spreadsheet to follow your own values, you need to know these variations in order not to consider a jump of 1.0g or less to be important. You also need to understand that this change is not going to be noticed by your physician. He or she will simply note it and move on.
Neutrophil Physiologic Variation
Neutrophils are the white blood cell most commonly involved in infection fighting and healing. They work more effectively in combination with lymphocyte communication but they can function on their own. They came be found in two different locations or pools in the peripheral bloodstream: in the center of the stream allowing them to move rapidly from one place to another and lined up on the walls of the blood vessels allowing them to move out into the tissues when there is damage (ex., a paper cut).
During a routine collection of blood, the only cells that are transferred into the evacuated tube are the cells in the circulating pool. Studies have proved that at least the same number of cells in the circulating pool are found marginating along the vessel walls. If they would just stay, there would not be an issue but they don’t. You can cause the marginating cells to drop off into the circulation fairly easily. Exercise such as rapidly climbing a flight of stairs or running will do it. A flush of epinephrine (adrenaline) as occurs when you know you are late for your appointment will do it.
Fevers, insomnia, emotional stress will do it. A good example of this occurs with every delivery of a newborn. As all mothers can attest, labor is significant exercise and the typical 24-hour post delivery white blood cell count is typically greater than 20.0x109/L or nearly twice the upper limit of acceptability.
Medication such as corticosteroids or cardiovascular medications such as ACE inhibitors (Lisinopril is one example) will suppress the making of neutrophils, thus lowering their number. Chronic inflammation such as arthritis causes these cells to migrate out into the tissues, thus lowering their presence in the peripheral blood count. These variations are important for physicians worry about neutrophil counts that are lower than 2.0x109/L. This is the point at which a person becomes at risk for infections that are more easily acquired and healing that takes longer than warranted.
Neutrophil Testing Variation
In the differential, a number of cells are counted and identified. The traditional manual differential is performed by randomly finding 100 cells and evaluating them. This method works very well when the total white cell count is within reference range and the ratio among the various cell types is normal. However, in CLL, it is obvious that this is not the case. There might be such an increase in lymphocytes that the neutrophils, while present in the correct numbers, will be overwhelmed. This would be something similar to having a soccer team that always has 1,000 of its fans at matches appear to have no fans if the opposing team showed up with 100,000 fans. One method to eliminate this increase bias would be to count more cells and, indeed, there was a process by which one counted 200 or 500 cells if the total white cell count was increased. In this current setting of rapid turn around times and /or fewer laboratory scientists, this is not performed. However, this still brings up the situation of a significant increase in lymphocytes “diluting out” the neutrophils. One process frequently used is to start the manual differential with one neutrophil and then count a random 99 cells. The thought is to communicate to the physician that there are indeed neutrophils present – even though they might be outnumbered 100 to 1.
Multichannel instruments use another method. They separate out a much larger number of cells (perhaps as high as 50,000) and report the absolute number of cells, not the percentage. This absolute differential is usually reported as either ABS or #. This method prevents any misleading interpretation. So here is rule #2 – never ever look at the percentage differential as it can be misleading.
Here is an example of this:
|WBC||Neutrophil %||Lymph %||Absolute Neutrophil||Absolute Lymphocyte|
|Reference range||50-80||20-40||2.0-8.0x 109/L||0.8-4.0x 109/L|
|9.0 x109/L||90||10||8.0x 109/L||0.9x 109/L|
|4.0x 109/L||90||10||3.6x 109/L||0.4x 109/L|
|25.0x 109/L||90||10||22.5x 109/L||2.5x 109/L|
Notice that in the example, an acceptable white cell count gives you an increase in neutrophils percentage and a decrease in lymphocyte percentage but the absolute differential proves that neither cell is truly increased or decreased. In the second example the percentage of neutrophils is totally incorrect in that there is a normal number of neutrophils and a decrease in lymphocytes. Finally, the last line gives you the reverse - an increase in neutrophils and a normal number of lymphocytes.
So why use the manual differential at all? Because there is no instrument that is as good as the eye of an experienced laboratory practitioner. So, most physicians use the absolute differential for the numbers and the manual differential for a qualitative examination of the cells.