Learning Objectives
By the end of this section, students will be able to:
1. Describe the purpose of a complete blood count (CBC).
2. Identify the main components of a CBC.
3. Explain the difference between a total white blood cell count and a white blood cell differential.
4. Describe the basic functions of the major white blood cell types and how their relative proportions can change in different conditions.
5. Recognize the key measurements used to assess red blood cells.
6. Use reference values to determine whether CBC results are within normal ranges.
Complete Blood Count (CBC)
A CBC is a group of tests that measure the number and characteristics of different cells in the blood (MedlinePlus, 2020).
A CBC is commonly used to screen for conditions such as:
- infection
- anemia
- blood disorders
1. Components of the Complete Blood Count
The CBC measures the following key components of the blood:
Table 1 – CBC Components
| Component | Function | What it Measures / Why it Matters |
| Red Blood Cells (RBCs) | Carry oxygen from the lungs to the rest of the body | Indicates oxygen-carrying capacity; low levels may suggest anemia |
| ↳ Hemoglobin | Oxygen-carrying protein within red blood cells | Reflects how much oxygen the blood can carry |
| ↳ Hematocrit | Proportion of blood made up of red blood cells | Helps assess anemia or dehydration |
| White Blood Cells (WBCs) | Fight infections and other diseases | Total WBC count reflects immune activity; a differential identifies the number of each WBC type |
| Platelets | Help blood clot and stop bleeding | Low levels increase bleeding risk; high levels may indicate clotting issues |
2. White Blood Cells
White Blood Cell (WBC) Count
The WBC count measures the total number of white blood cells in the blood.
Normal range:
4,500–11,000 cells/µL
- Leukocytosis (high WBC count): often seen in infection, inflammation, and some cancers
- Leukopenia (low WBC count): may be associated with bone marrow disorders, immune conditions, or severe infections
White Blood Cell Differential
A white blood cell differential shows the percentage of each type of white blood cell. This helps identify patterns associated with infection or disease (Table 2).
Table 2 – White Blood Cell Differential
| White Blood Cell Type | Normal Range (% of total WBCs) |
| Neutrophils | 40–69% |
| Lymphocytes | 20–40% |
| Monocytes | 2–8% |
| Eosinophils | 1–4% |
| Basophils | 0.5–1% |
White Blood Cell Function and Interpretation
Each type of white blood cell has a specific function in the immune system. Changes in these cell types can help identify the underlying cause of infection or disease (Table 3).
Although certain white blood cells are associated with specific conditions, more than one cell type may increase/decrease at the same time. Look for the dominant pattern, not just one value.
Table 3 – White Blood Cell Function and Interpretation
| White Blood Cell Type | Primary Function | What an Increase Usually Suggests |
| Neutrophils | First responders; engulf and destroy bacteria (phagocytosis) | Bacterial infection, inflammation, cancer |
| Lymphocytes | Adaptive immunity; B cells produce antibodies, T cells kill infected cells | Viral infection (infectious mononucleosis, mumps, measles), some leukemias (e.g., ALL, CLL), chronic bacterial infections |
| Monocytes | Become macrophages; clean up debris | Chronic infection, inflammation |
| Eosinophils | Defend against parasites; response to allergies | Parasitic infection, allergies, cancer |
| Basophils | Release histamine; involved in inflammation and allergic reactions | Allergic reactions, chronic inflammation, some leukemias (e.g., CML) |
Important Concept: Left Shift
An increase in immature white blood cells (called a “left shift”) may indicate increased production of white blood cells, as seen in infections and some leukemias.
3. Red Blood Cells
Red blood cell (RBC) measurements are closely related and are often interpreted together to assess oxygen-carrying capacity and blood volume (Table 4).
Table 4. Red Blood Cell Measures
| Measurement | What it Represents | What Changes May Indicate |
| RBC count | Number of red blood cells | ↓ Low: anemia • ↑ High: dehydration or increased RBC production |
| Hemoglobin | Oxygen-carrying capacity of the blood | ↓ Low: reduced oxygen transport (anemia) |
| Hematocrit | Proportion of blood made up of RBCs | ↓ Low: anemia • ↑ High: dehydration |
4. Platelets
Platelets are involved in blood clotting.
- Low platelet levels → increased risk of bleeding
- High platelet levels → increased risk of clotting
5. Interpreting CBC Results
Reference values provide the normal ranges for key components of the CBC. These ranges are used to determine whether results are within normal limits or indicate a potential abnormality (Table 5).
Table 5 – Reference Values
| Measurement | Normal Range |
| RBC count | AMAB¹: 4.3–5.7 million/µL; AFAB²: 4.0–5.5 million/µL |
| Hemoglobin | AMAB: 130–170 g/L; AFAB: 110–160 g/L |
| Hematocrit | AMAB: 40–55%; AFAB: 36–48% |
| WBC count | 4.5–11 × 10⁹/L |
| Platelets | 150–400 × 10⁹/L |
¹ Assigned Male at Birth
² Assigned Female at Birth
6. Notes on CBC Interpretation
Additional CBC measurements (such as red blood cell size and volume indices) are often included in clinical reports but are not required for this lab. The focus here is on key values used to identify patterns in blood disorders.
7. Using This Information
These values and patterns will be used in the blood case studies to:
- identify abnormal results
- recognize patterns in blood cell changes
- determine the most likely diagnosis
Test Your Knowledge
References
MedlinePlus. [Internet]. Bethesda (MD): Complete blood count (CBC). U.S. National Library of Medicine [updated 2020 Jun 24]. Available from: https://medlineplus.gov/lab-tests/complete-blood-count-c
Kaiser, G. (n.d.). The complete blood count (CBC) test. LibreTexts, Community College of Baltimore County, Catonsville Campus. Adapted from LibreTexts.