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Complete blood count (CBC)

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Complete Blood Count (CBC) Overview

The Complete Blood Count (CBC) is one of the most commonly ordered laboratory blood tests. It provides a quantitative assessment of the cellular components of blood: red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).

Modern automated hematology analyzers provide highly accurate and informative measurements of various parameters related to these cells. The CBC is a fundamental tool used for a wide range of purposes, including general health screening, diagnosing various conditions (like anemia, infection, inflammation, bleeding disorders, and leukemia), and monitoring the effects of diseases or treatments.

Any significant deviation from the established normal reference ranges in a CBC is typically considered potentially pathological and usually warrants further investigation to determine the underlying cause. While changes in the CBC (also known as a hemogram) can be non-specific in many diseases, the results are invaluable for monitoring patient status over time and assessing disease prognosis.

In disorders directly affecting the blood or bone marrow (hematopoietic system), the CBC is often of paramount diagnostic importance, guiding further testing strategies, treatment selection, and therapeutic monitoring.

Blood is made up of multiple components, including red blood cells, white blood cells, platelets, and plasma.

Components Measured in a CBC

A standard CBC typically includes the following key components:

  • Red Blood Cell (RBC) Parameters: Assess oxygen-carrying capacity and diagnose anemia or polycythemia.
    • RBC Count: Number of red blood cells.
    • Hemoglobin (HGB): Amount of oxygen-carrying protein.
    • Hematocrit (HCT): Percentage of blood volume occupied by RBCs.
    • RBC Indices: Describe the average size and hemoglobin content of RBCs (MCV, MCH, MCHC, RDW).
    • Reticulocyte Count: Measure of young RBCs, indicating bone marrow production rate (often ordered separately or as part of an extended CBC).
  • White Blood Cell (WBC) Parameters: Assess immune status, infection, and inflammation.
    • Total WBC Count: Total number of white blood cells.
    • WBC Differential: Percentage and/or absolute count of each main type of WBC (Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils).
  • Platelet (PLT) Parameters: Assess blood clotting potential.
  • Other parameters sometimes included or calculated:

It is important to note that reference ranges ("normal values") for CBC parameters can vary slightly between laboratories depending on the specific analyzers, reagents, methods used, and the reference population (e.g., different countries or regions). Age and sex also significantly influence normal ranges for several parameters.

Normal Reference Ranges (Examples)

The following tables provide typical reference ranges for adult men and women. These are examples only; always refer to the specific reference ranges provided by the laboratory that performed the test. Ranges for children vary significantly with age.

Normal CBC Ranges in Men (Example)

Example Reference Ranges - Adult Male
Indicator
Example Normal Values
Hemoglobin (HGB) 13.5 - 17.5 g/dL (or 135 - 175 g/L)
Red Blood Cells (RBC) 4.5 - 5.9 x 1012/L
Hematocrit (HCT) 41 - 53 %
Mean Corpuscular Hemoglobin (MCH) 27 - 33 pg
Mean Corpuscular Volume (MCV) 80 - 100 fL
Mean Corpuscular Hemoglobin Concentration (MCHC) 32 - 36 g/dL
RBC Distribution Width (RDW-CV) 11.5 - 14.5 %
Reticulocyte Count 0.5 - 2.5 % (relative)
White Blood Cell (WBC) Count 4.0 - 11.0 x 109/L
Neutrophils 40 - 75 % (relative); 1.8 - 7.7 x 109/L (absolute)
    Bands < 5-10 % (often included in total neutrophils)
    Segmented Neutrophils (Calculated from Total Neutrophils - Bands)
Eosinophils 0 - 6 % (relative); 0.0 - 0.5 x 109/L (absolute)
Basophils 0 - 2 % (relative); 0.0 - 0.2 x 109/L (absolute)
Lymphocytes 20 - 45 % (relative); 1.0 - 4.8 x 109/L (absolute)
Monocytes 2 - 10 % (relative); 0.1 - 0.8 x 109/L (absolute)
Platelet Count 150 - 450 x 109/L
Mean Platelet Volume (MPV) ~7 - 12 fL
Platelet Distribution Width (PDW) ~9 - 14 % (or fL, highly method dependent)
Plateletcrit (PCT) ~0.15 - 0.40 %
ESR (Erythrocyte Sedimentation Rate) 0 - 15 mm/hr (<50 yrs); 0 - 20 mm/hr (>50 yrs)

Normal CBC Ranges in Women (Example)

Example Reference Ranges - Adult Female (Non-pregnant)
Indicator
Example Normal Values
Hemoglobin (HGB) 12.0 - 16.0 g/dL (or 120 - 160 g/L)
Red Blood Cells (RBC) 4.0 - 5.4 x 1012/L
Hematocrit (HCT) 36 - 48 %
Mean Corpuscular Hemoglobin (MCH) 27 - 33 pg
Mean Corpuscular Volume (MCV) 80 - 100 fL
Mean Corpuscular Hemoglobin Concentration (MCHC) 32 - 36 g/dL
RBC Distribution Width (RDW-CV) 11.5 - 14.5 %
Reticulocyte Count 0.5 - 2.5 % (relative)
White Blood Cell (WBC) Count 4.0 - 11.0 x 109/L
Neutrophils 40 - 75 % (relative); 1.8 - 7.7 x 109/L (absolute)
    Bands < 5-10 % (often included in total neutrophils)
    Segmented Neutrophils (Calculated from Total Neutrophils - Bands)
Eosinophils 0 - 6 % (relative); 0.0 - 0.5 x 109/L (absolute)
Basophils 0 - 2 % (relative); 0.0 - 0.2 x 109/L (absolute)
Lymphocytes 20 - 45 % (relative); 1.0 - 4.8 x 109/L (absolute)
Monocytes 2 - 10 % (relative); 0.1 - 0.8 x 109/L (absolute)
Platelet Count 150 - 450 x 109/L
Mean Platelet Volume (MPV) ~7 - 12 fL
Platelet Distribution Width (PDW) ~9 - 14 % (or fL, highly method dependent)
Plateletcrit (PCT) ~0.15 - 0.40 %
ESR (Erythrocyte Sedimentation Rate) 0 - 20 mm/hr (<50 yrs); 0 - 30 mm/hr (>50 yrs)

Interpretation of Results

Interpretation of CBC results involves comparing the patient's values to the laboratory's reference ranges and considering the clinical context. Deviations from the normal ranges can indicate various conditions:

  • Low RBC count, HGB, HCT: Suggests anemia. RBC indices (MCV, MCH, MCHC, RDW) help classify the type of anemia (e.g., microcytic hypochromic in iron deficiency, macrocytic in B12/folate deficiency).
  • High RBC count, HGB, HCT: Suggests polycythemia or dehydration.
  • Low WBC count (Leukopenia): May indicate infection (viral, overwhelming bacterial), bone marrow problems, autoimmune disease, or drug effects. Neutropenia (low neutrophils) is a key concern for infection risk.
  • High WBC count (Leukocytosis): Often indicates infection (especially bacterial if neutrophils are high), inflammation, stress, or leukemia/lymphoma (if counts are very high or abnormal cells are present). Specific elevations (neutrophilia, lymphocytosis, eosinophilia) point towards different causes.
  • Low Platelet count (Thrombocytopenia): Increases bleeding risk. Causes include immune destruction (ITP), bone marrow problems, infections (sepsis, viral), medications, hypersplenism.
  • High Platelet count (Thrombocytosis): Can be reactive (to inflammation, infection, iron deficiency) or neoplastic (Essential Thrombocythemia, other myeloproliferative neoplasms). Increases clotting risk.

Individual results must be interpreted by a healthcare professional familiar with the patient's history and clinical situation.

Preparation for the Test

Generally, no special preparation is needed for a standard CBC test.

  • Fasting: Not usually required.
  • Medications: Inform your healthcare provider about all medications and supplements you are taking, as some can affect blood cell counts.
  • Hydration: Normal hydration is recommended. Dehydration can falsely elevate RBC count, HGB, and HCT.

How the Test is Performed

  • Sample Type: Whole blood.
  • Collection: A blood sample is drawn from a vein, typically in the arm (venipuncture), into a collection tube containing an anticoagulant (usually EDTA, recognizable by a purple/lavender top) to prevent clotting.
  • Analysis: The sample is analyzed using an automated hematology analyzer, which counts and measures the different blood cells and parameters. A blood smear may also be examined microscopically if indicated by automated results or clinical suspicion.

References

  1. National Heart, Lung, and Blood Institute (NHLBI). (n.d.). Complete Blood Count. NIH. Retrieved from https://www.nhlbi.nih.gov/health/blood-tests/complete-blood-count
  2. Lab Tests Online. (n.d.). Complete Blood Count (CBC). Retrieved from https://labtestsonline.org/tests/complete-blood-count-cbc
  3. Mayo Clinic Staff. (n.d.). Complete blood count (CBC). Mayo Clinic Patient Care & Health Information. Retrieved from https://www.mayoclinic.org/tests-procedures/complete-blood-count/about/pac-20384919
  4. Bain, B. J., Bates, I., & Laffan, M. A. (Eds.). (2016). *Dacie and Lewis Practical Haematology* (12th ed.). Elsevier. [Note: Comprehensive hematology laboratory textbook]
  5. Hoffman, R., Benz Jr, E. J., Silberstein, L. E., Heslop, H. E., Weitz, J. I., & Anastasi, J. (Eds.). (2017). *Hematology: Basic Principles and Practice* (7th ed.). Elsevier. [Note: Comprehensive clinical hematology textbook]