Red blood cell and platelet basics
At one time the CBC was done by human hands and eyes with a microscope, special cell counting slides and various stains
Why learn about blood tests? Because they’re interesting? Because your medical director makes you? Because you’d like to know what your own tests mean?
If so, read on.
The complete blood count (CBC) is the examination of the cellular components of the blood, the red blood cells (RBCs), white blood cells (WBCs) and platelets. If you remove all of these cells from the blood, then you have plasma, a cell-less fluid that contains albumin, clotting factors and antibodies. And if you remove the clotting factors from the plasma it becomes serum.
Doing the CBC
At one time the CBC was done by human hands and eyes with a microscope, special cell counting slides and various stains. Now we have sophisticated machines that accurately count, size and differentiate the blood cells and print out the results in less time than it would take to turn on and focus a microscope. Here we discuss the red blood cell and platelet portion of the CBC.
The red in the red blood cell, or erythrocyte, is from the hemoglobin, a complex molecule with an iron (Fe) core that makes up more than 90 percent of a red blood cell’s volume. Hemoglobin is tasked with oxygen pick up in the lungs and delivery to the cells. On the return trip, hemoglobin processes and delivers the cellular waste carbon dioxide back to the lungs for disposal.1
We want to know if there are too few or too many red blood cells in circulation and any change in the RBC size or color. These changes may indicate disease such as anemia, a common disorder defined by the inadequate delivery of oxygen to the body cells due to inadequate numbers of red bloods cells and/or inadequate or abnormal hemoglobin.
Red cell numbers
The red blood cell count and the hematocrit provide information on RBC numbers. The normal number of RBCs in circulation is around 4 to 6 million cells per microliter (mcl) or cubic milliliter (cmm) of blood2 (1000 microliters or cubic millimeters = 1 milliliter). The number of circulating RBCs is normally lower in females than in males. To maintain normal homeostasis under varying conditions, the body has the ability to increase the number of RBC as seen in pregnant females in preparation for the expected blood loss during deliver or for any gender living in low oxygen environments at high altitudes.
The hematocrit is the ratio or percentage of RBCs to plasma. A normal hematocrit or HCT of 40 percent means that 40 percent of the blood is comprised of RBCs.
The number of RBCs and the hematocrit may be low if we have too few RBCs due to hemorrhage as with trauma or GI bleeding; or from red cell destruction due to diseases like sickle cell anemia; or from inadequate RBC production due to low iron levels or insufficient erythropoietin.
The hormone erythropoietin is produced in the kidneys and stimulates the bone marrow to produce RBCs.3 Fortunately for patients with low erythropoietin production from their renal disease, they can receive weekly erythropoietin injections (Epogen; Procrit) to treat and prevent this type of anemia.
A decreased HCT may also occur with a normal number of circulating RBCs floating around in too much plasma fluid, such as from excessive intravenous fluid administration.
Red cell indices
The red blood cell indices provide information on the RBC size and hemoglobin content.4 Abnormal values may indicate conditions such as anemia and other diseases of the red blood cell.
Hemoglobin (Hg) content is measured by the CBC machine to give us the total circulating hemoglobin in grams of Hg per 100ml (gm/dL). In addition, the machine will provide us with two estimates of the hemoglobin contained in the average red blood cell.
The MCH or Mean Corpuscular (Cell) Hemoglobin is the average hemoglobin content of each cell and is obtained by dividing the total hemoglobin by the total RBC count: MCH = Hg/RBC total. The MCHC or Mean Corpuscular Hemoglobin Concentration is calculated by dividing the hemoglobin by the hematocrit: MCHC = Hg/HCT. These numbers help in the diagnosis of various types of anemia.
So a nice, bright red blood cell has a normal amount of hemoglobin and is called normochromic or normal in color. A pale RBC has inadedquate hemoglobin levels and is called hypochromic and is often the result of low body iron, an essential component for making hemoglobin. If there is too much hemoglobin or a normal amount of hemoglobin contained in a smaller than normal cell we get hyperchromia, such as seen with hereditary spherocytosis.
The size of the red blood cell is important. The MCV or Mean Corpuscular Volume tells us the average size of the red blood cells in circulation. Small RBCs (microcytosis) have a low MCV that is commonly due to iron deficiency, but may be due to genetic red blood cell disorders. Large RBCs (macrocytosis) have a high MCV and may be due to deficiencies of folate or vitamin B12 or other processes that impact red cell development such as chemotherapy.
We also get a look at the average shape of the red blood cells in circulation with a RDW or Red cell Distribution Width. If the RDW is abnormal it indicates the RBCs are of different sizes and shapes, not the normal shape and size we expect. This may be an early sign of anemia.
Platelets and RBCs go hand in hand to plug leaks in our circulation and keep the blood inside its human container. Platelets also participate in the body’s inflammatory response.
Platelets are not actually cells but cell fragments. Megakaryocytes are large cells formed in the bone marrow along with the red blood cells and most of the white blood cells. Once a megakaryocyte reaches maturity it fragments into about a thousand platelets that move into the blood stream.5
The CBC reports the number of platelets and their size. Too many platelets can cause too much clot formation and too few platelets can cause to little clot formation.
The mean platelet volume or MPV gives us the platelet size, just like the MCV of the RBC. Platelets that are too large or too small may not function the same as normal sized platelets. Indeed, a high MPV is a risk factor for stroke and myocardial infarct.6
The CBC is the most frequently ordered blood test in medicine due to the information it provides for a wide spectrum of common diseases. Although understanding routine blood tests will not impact your care on a 911 call, it may be useful in understanding the medical condition for the interfacility transport patient or during patient care reviews.
1. Mad Scientist. Blood Gases, Physiology of Gas Exchange. Available at www.madsci.com/manu/gas_gas.htm Accessed November 8, 2011.
2. CBC. Available at www.nlm.nih.gov/medlineplus/ency/article/003642.htm Accessed December 2, 2011.
3. Adamson, JW. The Story of Erythropoietin. Available at www.hematology.org/Publications/50-Years-in-Hematology/4726.aspx . Accessed November 11, 2011.
4. Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. Boston: Butterworths; 1990;720-723.
5. Platelets on the Web. Available at http://ouhsc.edu/platelets/Platelets/platelets%20intro.html Accessed November 20, 2011.
6. Huczek Z, Kochman J, Filipiak Kj, et al. Mean Platlet Volume on Admission Predicts Impaired Reperfusion and Lon-Term Mortality in Acute Myocardial Infarction. J Am Coll Cardiol. 2005;46;284-290