Complete Blood Count

There are many reasons for a physician to order a complete blood count for a patient.  Technology today is such that whether we want to know if you are anemic, if your white blood count is elevated, if your platelet count is low, or if you have  abnormalities of any of the specific types of white blood cell, we order a complete blood count(CBC),  because the automated machine that gives us any one piece of this information gives us all of this information and more.  This has advantages, but the down side is that we often find minor abnormalities that we have to decide whether or not to further evaluate. So what does a physician really look at in a CBC.  

Hematocrit (HCT):  This is the percent of the blood that is made up of blood cells.  In earlier days this was determined by putting a specimen of whole blood in a capillary tube, plugging one end of the tube with clay, and using a centrifuge to spin the tube until all of the cells precipitate to the bottom leaving the serum is above.  The percentage was determined by a simple comparative measurement.  Now this is done by a complex machine.  Still this is a simple concept.  A low hematocrit is anemia.  A high hematocrit is called polycythemia.  This is key information.  Normal range for men is about 45-52%.  For women 37-48% (Keep in mind that normal values vary slightly from one lab to another)

Hemoglobin(Hb or Hgb):  This is the concentration of hemoglobin in the blood.  It gives similar information as the hematocrit.  A low hemoglobin means anemia.  Normal values are > 12 gm/dl in men, > 12gm/dl in women.

White Blood Cell Count (WBC):  This is the number of white blood cells per cubic millimeter of blood.  High white blood cell counts can suggest infection, leukemia, corticosteroid use, and less common problems.  Low white blood cell counts are seen in some viral illnesses, and in conditions where the bone marrow is replaced or suppressed like chemotherapy, cancers, and various infiltrative processes.  Normal values are between 4,300-10,800.

Platelet Count (Plt):  This is the number of platelets per cubic millimeter of blood.  Platelets are the first line of protection to stop bleeding.  Low platelet counts can be seen in autoimmune disorders like ITP, in severe infection, in chemotherapy or other toxic bone marrow conditions, and in other bone marrow disorders.

Differential White Blood Cell Count:  The percentage of white blood cells of each of the major types of WBCs is stated as a percentage of the total WBC, as well as an absolute number.  Neutrophils, a.k.a. polymorphonuclear WBC’s or “polys”  usually make up the largest percentage of WBC, followed by lymphocytes, monocytes, eosinophils, basophils, and in cases of leukemina and sometimes other bone marrow disorders immature white blood cells called “blasts.”  Young neutrophils are called “bands”, short for banded neutrophils, and are suggestive of bacterial infection when present in high numbers. The slang for a high percentage of neutrophiils, especially a high percentage of “bands” is that there is a “left shift” in the WBC. 

Measurements of cell size:  The CBC usually reports the Mean Corpuscular Volume (MCV), a measurement of the average size of the red blood cells.  Low MCV usually suggests iron deficiency, though other causes are possible.  High MCV can suggest vitamin B12 or folic acid deficiency, or sometimes rapid red blood cell turnover, as young red blood cells, called reticulocytes, are larger than after they are a few days old.  MCHC (Mean Corpuscular Hemoglobin Concentration) is a measurement of the concentration of hemoglobin in the red blood cells.  It also suggests iron deficiency when low.  MCH (mean corpuscular hemoglobin) is a similar measurement, with similar meaning.  Low suggests iron deficiency.  RDW is a measurement of the variability of the size of red blood cells.  Honestly I really ignore the RDW generally, but it is supposed to be helpful in determining the cause of anemia.   MPV is a measurement of average platelet size.

Don’t worry if every measurement in your CBC is not in the normal range.  It is rare to see a CBC in any given individual where every value is in the normal range.  It seems like just about everyone has at least one of the minor values outside the normal range.  If the WBC, Hct, Hb, and platelet count are normal, and none of the RBC indices are far outside the normal range, there are no immature white blood cells and the differential white blood cell count is normal or near normal, usually we are reassured that the CBC patient’s blood counts are fine.

One Response to Complete Blood Count

  1. As a medical technologist who has performed CBCs and other lab tests for over 20 years, the overview above is excellent. Please note that Hct is usually calculated by analyzers nowadays so there are instances when it is inaccurate. Hgb, on the other hand, is usually a direct measurement. Generally, the Hct will be three times the Hgb–but not for all conditions or for all people. If in doubt which one to take, take the Hgb.
    Also, regarding the RDW: It can be helpful if it is elevated. Elevated RDW indicates two or more red cell populations. Monitoring the RDW can be helpful in monitoring treatment of anemia.

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