12.6.7: The Complete Blood Count (CBC) Test

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The CBC test is used as a common screening test to check for a variety of disorders such as infection, anemia, and a wide variety of other diseases. It consists of a panel of tests that examines different parts of the blood and includes the following:

A. White blood cell (WBC) count: This is a count of the actual number of white blood cells per mlof blood. Both decreases in the total WBC count (leukopenia) and increases in the total WBC count (leukocytosis) can be significant. The normal range of WBCs μL of blood in both males and females is typically between 4500 and 10,000.

1. Leukocytosis is often seen during infections, inflammation, cancer, and leukemia.

2. Leukopenia is often seen during certain autoimmune conditions, bone marrow failure some severe infections, and congenital marrow aplasia where the marrow doesn't develop normally.

B. White blood cell differential count: This determines the number of each type of leukocyte calculated as a percentage of the total number of leukocytes. There are five different types of white blood cells: neutrophils, basophils, eosinophils, monocytes, and lymphocytes. An increase or a decrease in any of the types of WBCs can be significant.

1. Neutrophils are the most abundant of the leukocytes and normally accounting for between 40% and 60% of the WBCs.

a. An increase in the percentage of neutrophils (neutrophilia) is seen with an infection. When doing a differential WBC count, neutrophils are usually divided into segs (a mature neutrophil having a segmented nucleus) and bands (an immature neutrophil with an incompletely segmented or banded nucleus). Band forms normally account for 0% to 4% of the neutrophils, but during an active infection, people are generally producing large numbers of new neutrophils and therefore will have a higher percentage of the immature band forms. (An increase in band forms above 8% is referred to as a "shift to the left" because on laboratory slips used for differential WBC counts, the heading for bands is to the left of the heading for mature neutrophils or segs.)

b. A decrease in the percentage of neutrophils (neutropenia) is seen during cancer chemotherapy, aplastic anemia, radiation therapy and exposure, certain viral infections like influenza, and some widespread bacterial infections.

2. Lymphocytes normally make up between 20 and 40% of the WBCs circulating in the blood.

a. An increase in the percentage of lymphocytes is seen during viral infections (such as infectious mononucleosis, mumps, measles and infectious hepatitis), lymphocytic leukemia, chronic bacterial infections, and multiple myeloma.

b. A decrease in the percentage of lymphocytes is seen during chemotherapy, radiation therapy and exposure, HIV infection, leukemia, and sepsis.

3. Monocytes normally account for between 2% and 8% of the WBCs.

An increase in the percentage of monocytes can be seen during parasitic infections, chronic inflammatory diseases, viral infections (such as infectious mononucleosis, measles, and mumps), and tuberculosis.

4. Eosinophils normally make up 1% and 4% of the WBCs.

An increase in the percentage of eosinophils can be seen during parasitic infection, cancer, allergic reactions, and collagen vascular disease.

5. Basophils normally represent between 0.5% and 1% of the WBCs.

An increase in the percentage of basophils can be seen during acute allergic reactions, chronic infections, and some forms of leukemia.

C. Red blood cell (RBC): This is a count of the actual number of red blood cells (erythrocytes) per ml of blood. Normal levels are 4.5 – 5.5 X 106 cells per ml in males and 4.0 – 4.9 X 106 per ml in females. Both decreases in the total RBC count (erythropenia) and increases in the total RBC count (erythrocytosis) can be significant.

1. Erythropenia is seen in the case of anemia.

2. Erythrocytosis is seen when too many RBCs are being made or when conditions such as diarrhea, dehydration, or burns cause excessive bodily fluid loss.

D. Hemoglobin: This test measures the amount of the oxygen-carrying protein hemoglobin in the blood. Hemoglobin levels mirror the RBC count results. Normal levels in males are between 13.5 and 16.5 grams per deciliter (g/dL) and between 12.0 and 15.0 g/dL in females.

E. Hematocrit: This test measures the percentage of red blood cells in a given volume of whole blood. Hematocrit levels mirror the RBC count results. Normal levels in males are between 41% and 50% and between 36% and 44% in females.

F . Platelet count: This count measures the number of platelets in a given volume of blood. Normal platelet levels are between 100,000 and 450,000 per ml. Both increases and decreases can point to abnormal conditions either excess bleeding or clotting. The platelet count decreases when greater numbers are being used, such as with bleeding or when a person has certain inherited disorders, systemic lupus erythematosis, or pernicious anemia.

G. Mean platelet volume (MPV): This is a measurement of the average size of the platelets in the sample being tested; newly formed platelets are larger, so an increased MPV is seen when increased numbers of platelets are being produced.

H. Mean corpuscular volume (MCV): This is a measurement of the average size of the RBCs. Normal MCV is between 80 and 100. The MCV is elevated when RBCs are larger than normal (macrocytic), e.g., in anemia caused by a deficiency in vitamin B12; the MCV is decreased when the RBCs are smaller than normal (microcytic), e.g., in iron deficiency anemia.

I. Mean corpuscular hemoglobin (MCH): This is a calculation of the average amount of oxygen-carrying hemoglobin inside a red blood cell. Normal MCH is between 26 and 34. Because macrocytic RBCs are larger, they tend to have a higher MCH; smaller microcytic RBCs have a lower value.

J. Mean corpuscular hemoglobin concentration (MCHC): This is a calculation of the average concentration of hemoglobin inside a red blood cell. Normal MCHC is between 31% and 37%. In disorders such as such as in iron deficiency anemia where the hemoglobin is abnormally diluted inside the RBCs, decreased MCHC values (hypochromia) are seen; in conditions where the hemoglobin is abnormally concentrated inside the RBCs, such as in burn patients, increased MCHC values (hyperchromia) are seen.

K. Red cell distribution width (RDW): This is a calculation of the variation in the size of the RBCs. Normal distribution width is 11 - 15. In certain anemias, such as pernicious anemia, the amount of variation in RBC size (anisocytosis) and variation in RBC shape (poikilocytosis) causes an increase in the RDW.

Contributors and Attributions

Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)