Hematology: Back to Basics

Blood is made up of several components: plasma makes up about 55% of blood content, with the other 45% consisting mainly of red blood cells (RBCs). Together, white blood cells (WBCs) and platelets make up less than 1% of the blood. Circulating through the body and supplying essential nutrients to the cells, such as sugars, oxygen and hormones, the blood also transports metabolic waste products away from the cells.

The fluidic part of blood, called plasma, contains essential proteins, sugars, fats, hormones, mineral salts and vitamins. Red blood cells, also called erythrocytes, are disc-shaped cells that carry oxygen in their hemoglobin centers from the lungs to the tissue and carbon dioxide back to the lungs to be exhaled, making them critical components of healthy bodily functioning. White blood cells, also called leukocytes, are a key component of the body’s immune system and protect against diseases, fighting viruses, bacteria and other health threats that enter the body. Platelets, also known as thrombocytes, are small cells formed in the red bone marrow that help the blood clot by aggregating and occluding at the point of injury.

Deciphering the CBC

The Complete Blood Count (CBC) test is the most common laboratory hematology analyzer test conducted worldwide. It measures the production of blood cells, gives information on essential components of the blood, and identifies a patient’s oxygen-carrying capacity. Specifically, red blood cells (RBCs), white blood cells (WBCs), and platelets are analyzed. These tests are valuable in detecting and diagnosing certain diseases such as anemia, blood cancers, infections, hemorrhages, allergies, and immunodeficiencies, as well as monitoring side effects of specific drugs or treatments. Evaluating and interpreting the results of the CBC is critical to providing patients with the highest quality care.

Example sub-tests of the CBC:

• Red blood cell count
  • This test measures the number of RBCs in 1 cubic mm of whole blood. A low RBC count can indicate anemia, blood loss, hemolysis, or bone marrow suppression. A high RBC count is the result of a condition causing low oxygen levels in the blood, such as certain blood cancers, heart disease, sleep apnea or pulmonary fibrosis. Normal levels are 4.6-5.9 million in men, and 4.1-5.4 million in women.
• White blood cell count
  • This test measures the concentration of leukocytes in the blood. A high WBC count can suggest the presence of infection, while a low count can signal various conditions, including HIV/AIDS, rheumatoid arthritis, vitamin deficiencies or lupus. Normal levels of WBCs are 4.3 – 10.8 x 10⁹/L
• WBC differential count
  • This test provides the concentrations and proportions of the 5 main types of WBC namely Neutrophils, Lymphocytes, Monocytes, Eosinophils and Basophils. Each cell is given as concentration in the blood (cells/µL) and proportion out of the total WBC (%).
• Hematocrit (Hct) test
  • This test measures the volume proportion of RBC in the blood and is given in percentage. The measurement depends on the number and size of red blood cells. Hematocrit count will be low after a hemorrhage or excessive IV infusion and will be high if the patient is dehydrated. Normal levels in men are 40-55% and 36-48% in women.
• Hemoglobin (Hgb) test
  • Hemoglobin is the oxygen-carrying protein of the blood that helps transport oxygen from the lungs to the body via the arteries. This test determines the concentration of hemoglobin in whole blood and is usually given in grams/dL. Decreased hemoglobin arises for the same reasons as decreased RBCs. Normal levels are 13-17 g/dl in men, and 12-16 g/dl in women.
• Mean corpuscular volume (MCV) test
  • This test measures the average volume of an RBC and is given in femtolitre. The MCV is equal to the hematocrit divided by RBC count X 10. Low values show that the cells are microcytic and can indicate iron deficiency, lead poisoning, or thalassemia. High values reveal macrocytic cells and suggest conditions such as folate or B12 deficiency, megaloblastic anemia, or liver disease. Normal ranges are 80-100 fL.
• Red cell distribution width (RDW or RCDW) test
  • This tests the distribution of RBCs sizes (volumes) and is a quantitative estimate of the homogeneity of individual cell size. Abnormal levels can indicate conditions such as anemia, and specifically thalassemia, a genetic disease that causes severe anemia, heart disease and liver disease. Normal ranges are 11.5% to 14.5%.
• Platelet count
  • This test measures the concentration of platelets in whole blood. If testing shows a high count (called thrombocythemia), this can be due to anemia, cancer or infection, while a low count (called thrombocytopenia) can indicate aplastic anemia or leukemia. Thrombocytopenia can prevent wounds from healing and result in severe bleeding. The normal range of platelets is 150,000-380,000/µL.

Enhancing your understanding of this hematology analysis report is critical for providing quality care to patients. As the most commonly performed laboratory test, the CBC delivers information about the production of all components of the blood and evaluates the patient’s oxygen carrying capacity and immune system functioning. The sub-tests are helpful in detecting and diagnosing a myriad of conditions and medication side effects.