The CBC gives us a picture of the immune system, infections, vitamin & mineral status, and the ability to stop bleeding. Each component is evaluated separately below.
WHITE BLOOD CELL COUNT (WBCS):
In adults, the normal WBC ranges from 4,500-11,000 cells/mL. A ‘Differential’ is when the lab technician (or machine) differentiates the WBCs based on their type.
Your WBC can be elevated (leukocytosis) if you smoke, if you are taking certain medications (Phenytoin, Steroids, Lithium, Catecholamines, and alcohol), or if you have an infection. If it is very elevated it could be an indicator of certain types of cancers.
Your WBC can be low (leukopenia) if you are of African ethnicity, have an infection, inadequate production of WBCs due to a bone marrow problem, or due to endotoxins during severe infection (septic shock),
RED BLOOD CELLS
Hemoglobin (Hgb): This is the actual measurement of the amount of hemoglobin in a given volume of blood. The normal range is: 12-15 mg/dL. Increased Hgb could be due to dehydration, increased production of hemoglobin. Decreased Hgb could be due to iron deficiency, B12/Folate deficiency, chronic inflammation (Anemia of Chronic Disease), kidney disease, hemoglobinopathy (sickle cell, thalassemia, etc), or destruction of RBCs.
Hematocrit (Hct): The is the percentage of volume that the red blood cells make up when the blood specimen is spun down by centrifuge. The normal range is: The HCT typically follows along with Hgb and causes of low or high Hct correspond with those listed above for Hgb.
Rule of 3’s: Hgb should be 3 times the Hct (i.e. Hgb of 9 gm/dl should make Hct 27%). The Hgb should increase 1 gm/dl & 3% Hct for each unit of PRBC’s if you need a transfusion.
RBC count parallels the Hgb and Hct unless thalassemia is present. Thalassemias (normal RBC, low Hgb & Hct). MCV/RBC ratio <13 is characteristic of the thalassemias
Hgb electrophoresis: This test is the Gold Standard for evaluating a hemoglobinopathy. Hemoglobinopathy is an abnormality in globin (protein) chain structure (sickle cell) or globin chain synthesis (thalassemias). Reported as % of 100%
Mean Corpuscular Volume (MCV): Is the average (or ‘mean’) volume of millions of RBC’s and is useful for classifying anemias based on size.
- Microcytic (<80): the most common cause of microcytosis is iron deficiency.
- Normocytic (80-100): there are a number of causes of normocytic anemias such as thalassemia, spherocytosis,
- Macrocytic (>100): macrocytosis is typically due to B12 or folate deficiency or a thyroid problem
- Dimorphic RBC population is when you have some small cells and some large cells. Remember, it is an average.
Mean Corpuscular Hemaglobin Concentration (MCHC): Correlates with Hgb concentration in RBC’s. Hypochromasia – RBC’s have a central area of pallor. Spherical RBC’s have an increased MCHC and no central area of pallor
RDW (RBC Distribution Width): valuable tool for identifying anemias. The RDW is useful in detecting significant variation in size of RBC’s (anisocytosis). Iron deficiency is the only microcytic anemia with an increased RDW
Erythropoiesis: The stimulation of new red blood cells is dependent on the hormone EPO which is synthesized in the peritubular capillaries of the kidneys. EPO stimulates RBC production, not Hgb synthesis.
Stimuli prompting enhanced release of EPO leading to accelerated erythropoiesis (RBC hyperplasia) includes – Low PaO2 (hypoxemia), Severe anemia, Left shifted Oxygen Dissociation Curve (tissue hypoxia), Benign and malignant renal disorders stimulate its release (renal cysts, renal adenocarcinoma, hepatocellular carcinoma)
You can tell that there is accelerated erythropoiesis because there is an increased reticulocyte count and a presence of polychromasia. These cells require 2-3d to mature to RBCs.
Reticulocyte count: When your body is stimulating the production of RBCs, the first cell type it kicks out is the reticulocyte (immature RBC). This test is the most cost effective method of determining whether the bone marrow is responding appropriately to the anemia (effective erythropoiesis). It is normally reported as a percent (0.5-1.5% is normal).
The Reticulocyte Count must be corrected for the degree of anemia. Corrected count = (Hct/45) x reticulocyte count). The normal Hct is 45. A good response is >3%. A bad response is <2%. If Polychromasia is present then divide the corrected reticulocyte count by 2.
Serum Iron: is depicted by the bars in the picture. The black area represents the iron which is bound to transferrin. The height of the bar is the Transferrin level. The iron concentration is determined by stripping the iron off of the transferrin with acid.
Total Iron Binding Capacity (TIBC) is determined by adding iron to fill the remaining sites.
Ferritin is the soluble, storage form of iron that correlates well with total body stores of iron. There is an inverse relationship between ferritin stores in bone marrow and transferrin.
Bar ‘A’ is normal. Iron is 100. TIBC is 300. % Saturation is 33%.
Bar ‘B’ is Iron deficiency: Iron is ~20, TIBC is 500, % Saturation is 4%
Bar ‘C’ is Anemia of Chronic Disease: Iron is ~20, TIBC is low (because total body stores are high), and % saturation is 10%, serum ferritin is high
Bar ‘D’ is Iron Overload: Iron is 200, TIBC is 200, % Saturation is high, and serum ferritin is high. Examples are hemochromatosis, hemosiderosis, lead poisoning, sideroblastic anemias
Platelets are essential in the coagulation process. The ‘aggregate’ and clump together to help plug holes in blood vessels. Platelet function is inhibited by Aspirin & anti-inflammatory (NSAID) medications. If the platelets do not function properly or if their number is very low then you have a higher risk of bleeding.
Normal = 150,000-400,000 cells/ml. Thrombocytopenia is when the platelets are less than 150,000. Thrombocytosis is when platelets are greater than 400,000.
Coombs Test: Miscellaneous- Utilizes anti-immunoglobulins (from a rabbit) that bind to the Fc region of human antibodies (Ab’s). Their binding leaves the Fab region (of the human antibody) free to react with an antigen (Ag). The rabbit anti-immunoglobulins bind with human Ab’s and form cross-links (across the zeta potential) and cause agglutination
Direct- detects the presence of IgG Ab’s attached to RBC’s in the blood. Measures bound Ab’s. Anti-immunoglobulins are added to the blood. If IgG antibodies are present, there is agglutination. Example: in HDN, maternal IgG get into the babies blood and attack the RBC’s. The anti-immunoglobulins are added and if agglutination occurs, the test is positive for maternal IgG in babies blood
Indirect- detects the presence of IgG Ab’s in the serum that are against a given Ag. Measures serum Ab’s. Ab’s may be present but don’t cause agglutination due to the zeta potential. Addition of anti-Ig Ab’s will cause agglutination. Example: detects the presence of anti-Rh IgG Ab’s in the blood of an Rh-negative woman. Rh(+) erythrocytes are added to the serum of the Rh(-) woman then, the anti-Ig Ab’s are added. If agglutination occurs, the test is positive for anti-Rh Ab’s in the woman