Why is the Complete Blood Count (CBC) Important?

CBC TEST

Why is the Complete Blood Count (CBC) Important?

Blood is a fluid composed of blood cells suspended in plasma. Blood is red due to the large volume of red blood cells. Blood circulates throughout the body, acting as a medium between body cells, transporting oxygen and nutrients to various parts of the body, and carrying carbon dioxide for expulsion through the lungs and waste products for excretion by the kidneys. Additionally, blood is part of the body’s immune defense system and helps regulate body temperature.

The Complete Blood Count (CBC) is a basic test used for routine annual check-ups and patient care. It measures the quantity and characteristics of all three types of blood cells: red blood cells (RBCs), white blood cells (WBCs), and platelets.

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1.Red Blood Cells (RBCs)

Red blood cells are shaped like a donut, with a central biconcave depression but no hole. Inside the RBCs is a substance called hemoglobin, which is responsible for transporting oxygen from the lungs to the rest of the body and carrying carbon dioxide back to the lungs for expulsion.

The RBC examination is divided into four parts:
1.1 Hemoglobin (Hb) Level

This measures the concentration of hemoglobin in the blood. Since hemoglobin is inside the red blood cells, its level is directly related to the amount of RBCs and the packed red cell volume (Hematocrit). Males generally have higher Hb levels than females.

  • Decreased Hb indicates anemia, which can result from many causes, such as blood loss, nutritional deficiencies (especially iron, vitamin B12, or folate), inherited blood disorders like thalassemia, or bone marrow diseases that reduce RBC production. A reduced Hb level requires a doctor’s consultation to find the cause and provide appropriate treatment.

  • Increased Hb can be seen in cases of dehydration or polycythemia (thickening of the blood) due to chronic oxygen deprivation or abnormal bone marrow function.

1.2 Hematocrit (Hct)

This measures the volume of red blood cells as a percentage of the total blood volume. It is directly related to the RBC count and the Hb level. The causes of low or high Hct are the same as those for abnormal Hb levels.

1.3 Red Blood Cell Count (RBC)

This is the total number of red blood cells in the blood, which is directly counted by an automated blood analyzer. It is directly related to Hb and Hct levels. The causes of low or high RBC counts are the same as those for abnormal Hb levels.

1.4 Red Blood Cell Indices

These measurements examine the size and hemoglobin content of the red blood cells:

  • Mean Corpuscular Volume (MCV): This is the volume or size of the red blood cell. Common causes of decreased MCV (small RBCs) include iron deficiency anemia and thalassemia. Common causes of increased MCV (large RBCs) include vitamin B12 or folate deficiency.

  • Mean Corpuscular Hemoglobin (MCH): This is the average mass (quantity) of hemoglobin in each red blood cell. It is directly related to MCV, and thus, its changes share similar causes.

  • Mean Corpuscular Hemoglobin Concentration (MCHC): This is the average concentration of hemoglobin within each red blood cell. A decreased MCHC is related to low Hb and small RBC size, sharing similar causes. An increased MCHC may occur when the RBC volume is decreased due to a change in cell shape (losing the donut shape and becoming spherical), such as in autoimmune hemolytic anemia.

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2. White Blood Cells (WBCs)

White blood cells are larger than RBCs. The normal WBC count is approximately 4,000–10,000 cells per microliter. The WBC count changes in relation to the body’s state of inflammation or infection. WBCs are produced in the bone marrow and their function is to destroy pathogens and foreign substances. They do this either by engulfing the pathogens directly or by producing antibodies (proteins that fight foreign invaders).

  • Low WBC count, especially a low neutrophil count, increases the risk of infection. Common causes of low WBCs include drugs, chemicals, radiation, viruses, and bone marrow diseases (e.g., bone marrow failure, infection, or infiltration by cancer).

  • High WBC count is commonly found as the body’s response to inflammation or infection. An increase can also be seen in leukemia (blood cancer), which can present with abnormally low to very high WBC levels.

The different types of white blood cells have unique characteristics, and their increased quantity reflects the body’s response to different stimulating factors:

Neutrophil
2.1 Neutrophil

These are the most numerous WBCs (around 40–80% of the total). Their primary function is to respond to acute inflammation and bacterial infections. The body rapidly increases their number to directly engulf pathogens. Neutrophils also release substances that cause inflammation or infection, leading to a fever.

lymphocyte

2.2 Lymphocyte

These are small WBCs that produce antibodies against both viruses and bacteria. Their numbers increase during viral infections and certain types of leukemia.

monocyte

2.3 Monocyte

These are the largest WBCs. They destroy pathogens and foreign invaders and are more effective at engulfing bacteria than neutrophils. Their numbers often increase in response to viral infections, tuberculosis, fungal infections, and certain leukemias.

Eosinophil

2.4 Eosinophil

These are present in small amounts in the bloodstream. They respond to foreign substances that cause allergic reactions. Elevated levels are found in people with allergies, asthma, drug hypersensitivity, or parasitic infections.

Basophil

2.5 Basophil

These are the least numerous WBCs. They produce substances that prevent blood clotting and release compounds that help dilate blood vessels.

platelet

3. Platelet

Platelets are fragments of blood cells, very small compared to RBCs and WBCs. Their main function is to prevent bleeding from blood vessels and help stop blood flow after an injury. The normal count is approximately 100,000 to 400,000 per microliter of blood. Platelets are produced by cells in the bone marrow.

  • A low platelet count (Thrombocytopenia), especially below 50,000, causes prolonged and excessive bleeding after an injury. If the level drops below 20,000, bleeding can occur spontaneously without injury, particularly in mucosal areas (e.g., bleeding gums, nosebleeds) or in vital organs such as the gastrointestinal tract or the brain.

    • Causes of low platelets include drugs, chemicals, radiation, autoimmune destruction of platelets, viruses, and an abnormally enlarged spleen. It can also be found in bone marrow diseases that reduce platelet production, such as bone marrow failure or aplasia, or cancer infiltration of the bone marrow.

  • A high platelet count (Thrombocytosis) occurs when factors stimulate increased production, such as chronic inflammation or infection, the presence of cancer, or acute blood loss or RBC destruction. If high platelet levels occur spontaneously without any stimulating factor, it is often due to a bone marrow disorder that automatically causes increased platelet production.

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