Complete Blood Count (CBC)

Have you ever had a routine blood test and wondered what all those numbers mean? The Complete Blood Count (CBC) is one of the most common blood tests doctors use to assess your health. It provides essential information about your blood cells, helping detect conditions like anaemia, infections, and underlying health issues.

The complete blood count (CBC) is India's most commonly ordered blood test. The key areas of interest in the CBC often include determining if a patient is anaemic, whether the white blood cell count indicates an infection, and whether platelet levels could impact haemostasis. 

General practitioners have indicated that they feel reasonably confident interpreting CBC results with significant abnormalities; however, they would appreciate guidance when results present only slight abnormalities or when the clinical scenario is ambiguous. 
 

Background 

Haematopoiesis – Cell development 

• All types of blood cells are created in the bone marrow from a small population of stem cells, with less than one in 5000 marrow cells being a stem cell. These cells are part of a self-replicating population. 
• Haematopoiesis refers to the creation and development of blood cells. In humans, the bone marrow is the primary site of haematopoiesis throughout life. 
• The initial differentiation of haematopoietic stem cells leads to two potential paths: lymphoid and myeloid. Stem cells then progress to progenitor cells for each mature blood cell type and are destined for a specific cell lineage. 
• Myeloid stem cells eventually differentiate into erythrocytes, megakaryocytes (which produce platelets), neutrophils, monocytes, eosinophils, and basophils. 
• Lymphoid stem cells develop into T and B lymphocytes and plasma cells. 
   

History and clinical assessment

Various features can be uncovered through history and clinical assessment. These can offer important hints for diagnosis and help contextualize the findings from the complete blood count.

Signs and symptoms pertinent to the CBC include:

▪ Pallor, jaundice

▪ Fever, lymphadenopathy

History and clinical assessment

▪ Bleeding/bruising

▪ Hepatomegaly, splenomegaly

▪ Frequency and intensity of infections, mouth ulcers, and recent viral illnesses

▪ Exposure to medications and toxins, including herbal remedies

▪ Fatigue/weight loss
 

White Blood Cells

• Total white blood cell count may be misleading
• While the total white blood cell count can serve as a helpful summary, examining the absolute counts of each type of cell is more informative than relying solely on the total count. 
• The total white count can sometimes be deceiving; for instance, significantly low neutrophil counts alongside a high lymphocyte count may yield a total white count that remains within the normal range. 
• Therefore, the total white count should not be evaluated in isolation.

Neutrophils

Neutrophils are the body’s first line of defence against infections. 

In most adults, neutrophils represent about 70% of all white blood cells. The normal range for neutrophil concentration is 2.0–8.0 × 10^9/L (this range may vary across different laboratories). A non-activated neutrophil typically has an average lifespan of about 4–10 hours in circulation. Once they migrate outside the blood vessels, neutrophils can survive for 1–2 days.

• Neutrophils are the primary white blood cells that respond to bacterial infections. The most frequent reason for a significant increase in neutrophils is due to a bacterial infection. Neutrophils begin to react within an hour after tissue injury and indicate acute inflammation.
•  A leukemoid reaction is called an abnormally high increase in neutrophils (>50 × 10^9/L) as a reactive response. In response to an infection, neutrophils typically display specific alterations. The neutrophils often appear more immature as they are released prematurely, which is referred to as a left shift. In cases of severe infection, neutrophils may display toxic granulation and other harmful changes, such as vacuolation and the presence of Döhle bodies. 
• Neutrophil levels are often elevated in any acute inflammatory response, so they can also be raised following a heart attack or other forms of tissue infarction and necrosis. Various stressors, ranging from intense physical activity to smoking, can lead to an increase in the neutrophil count. 
• Several medications have been shown to raise the neutrophil count, including steroids, lithium, clozapine, and adrenaline. Even mild anxiety can cause a slight increase in neutrophil levels due to the release of steroids. During pregnancy, there is typically a slight rise in the total neutrophil count and a left shift. Most laboratories provide pregnancy-specific reference ranges for these measurements. 
   

Neutropenia (low neutrophil count)

Neutropenia can pose a risk for life-threatening infections. It is particularly concerning when the total neutrophil count drops below 0.5 × 10^9/L, mainly if the neutropenia arises from reduced production (e.g., following chemotherapy).

 When neutropenia occurs due to increased destruction or margination (such as in viral infections), defining what constitutes a critical level is harder. In such cases, patients often do not experience significant bacterial infections, even with relatively low neutrophil counts. Neutropenia can be categorized as:
 

Classification Neutrophil count

• Mild 1.0–2.0 × 10^9/L
• Moderate 0.5–1.0 × 10^9/L
• Severe < 0.5 × 10^9/L
   

Significant levels of Neutrophils – Low

▪ < 1.0 × 10^9

/L (high risk of infection)

Most common causes:

▪ Viral (obvious or hidden)

▪ Autoimmune/idiopathic

▪ Medications
 

Red flags

▪ Patient is particularly unwell

▪ Severity of neutropenia

▪ Rate at which neutropenia is changing

▪ Lymphadenopathy, hepatosplenomegaly
 

Management of patients with neutropenia

• Neutrophils <1.0 × 10^9/L: The likelihood of serious bacterial infections increases as the neutrophil count falls below 1.0 × 10^9/L but becomes most pronounced when it drops below 0.5 × 10^9/L. 
• A thorough evaluation of the patient's situation is crucial, and patients who present with illness and/or fever and have a count below 1.0 × 10^9/L typically require urgent referral. Those who are feverish but otherwise stable should still be discussed with a Hematologist.
•  If the patient is stable and afebrile, they must be advised to seek medical care promptly if their condition worsens or they develop a fever. 
• Stable patients should have follow-up blood counts performed within 48 hours; if the neutropenia remains at this level or worsens, consultation with a Hematologist is needed.

Neutrophils – Elevated

Most probable causes include:

- Infection or inflammation

- Tissue necrosis

- Any form of stress or rigorous exercise

- Medications

- Pregnancy

- Smoking 

- Chronic Myeloid Leukemia (CML)

Important considerations include:

- The individual being particularly ill

- Severity of the neutrophilia

- The speed at which neutrophilia is changing

- Occurrence of a left shift

Consistent elevation of neutrophils may indicate chronic myeloid leukemia (CML). The characteristic features include a moderate rise in the neutrophil count (usually >50 × 10^9/L), accompanied by a left shift and notable myelocytes. Basophils and/or eosinophils may also be present. Chronic mild neutrophilia without a left shift is generally unlikely to be attributed to CML. 

Lymphocytes

Lymphocytes help fight viruses and keep the immune system strong.

• Lymphocytes typically comprise 20–40% of the circulating white blood cells. The normal range for lymphocyte concentration is between 1.0–4.0 × 10^9/L. T
• Two main morphological categories of lymphocytes can be identified under a light microscope: large granular and small lymphocytes. 
• However, functionally distinct subsets of lymphocytes do not correlate with their morphological appearance.
   

Lymphocytopenia (low lymphocyte count):  

Low numbers of lymphocytes typically do not have significant implications.  

• Characteristic reductions in lymphocyte counts are generally observed in the later stages of HIV infection due to the destruction of T lymphocytes (CD4+ T cells).  
• The use of steroids can lead to decreased lymphocyte levels.  
• On rare occasions, lymphocytopenia can be caused by specific chemotherapy regimens or cancers.  
• Individuals exposed to high doses of radiation, such as those involved in events like Chornobyl, may experience severe lymphocytopenia.  
   

Lymphocytosis (high lymphocyte count):  

• Elevations in the absolute lymphocyte count are commonly attributed to acute infections, such as those caused by the Epstein-Barr virus or viral hepatitis.  
• Less frequently, increased lymphocytes may also result from pertussis and toxoplasmosis or, though rare, chronic intracellular bacterial infections like tuberculosis or brucellosis.  
   

A rise in lymphocyte count may also occur due to:  

▪ Smoking (reactive)  

▪ Hyposplenism (usually following splenectomy)  

The acute stress response—typically seen in a hospital environment and uncommon in the general community—usually resolves within 24 hours:  

▫ Acute cardiac occurrences  

▫ Trauma  

▪ Autoimmune thyroiditis  
 

Monocytes:  

• Monocytes comprise about 3-8% of all circulating white blood cells.  
• They remain in the bloodstream for approximately one to three days before migrating into tissues (around 8-12 hours) to reach sites of infection.  
• The normal range for monocyte concentration is between 0-1.0 × 10^9/L.  
• Monocytes that move from the bloodstream into other tissues are called macrophages.  
• Macrophages play a role in both specific immunity and phagocytosis.  

Monocytopenia (low monocyte count):  

• An isolated reduction in total monocyte count is an unusual occurrence.  
• Occasionally, monocyte depletion happens during overwhelming bacterial infections; however, this is more incidental than a definitive clinical marker.  
• Hairy cell leukaemia, a rare and indolent lymphoproliferative disorder, may sometimes be linked to monocytopenia, although other abnormalities (notably neutropenia and lymphocytosis) are usually present.  
   

Monocytosis (high monocyte count):  

Increased monocyte counts are most frequently related to infections and inflammatory conditions, often accompanying other blood count changes.  

Isolated rises in monocyte count, without accompanying alterations in other blood counts, are rare but can be linked to:  

▪ Chronic infections, including tuberculosis  

▪ Chronic inflammatory diseases (e.g., Crohn’s disease, ulcerative colitis, rheumatoid arthritis, systemic lupus erythematosus)  

▪ Dialysis  

▪ An early indication of chronic myelomonocytic leukaemia (rare) 
 

Basophils:  

• Basophils are the least prevalent type of white blood cells, constituting about 0.01-0.3% of the total.  
• The normal range for basophils is between 0-0.2 × 10^9/L.  
• While the full function of basophils is not entirely understood, they are known to engage in phagocytosis and histamine production.  

Basopenia (low basophil count):  

Basopenia is challenging to confirm due to the inherently low normal basophil levels.  

Basophilia (high basophil count):  

• A significantly elevated basophil count is rarely observed.  
• When it is present, it may suggest a myeloproliferative disorder or other less common causes.  
• Re-testing with a CBC one or two weeks later might provide further insights.  

Eosinophils:  

• Eosinophils account for about 1-6% of white blood cells in most individuals.  
• The normal range for eosinophil concentration is between 0-0.5 × 10^9/L.  
• Eosinophils circulate for 8-12 hours and persist in tissues for 8-12 days when not stimulated.  
   

Eosinopenia (low eosinophil count):  

• Eosinopenia is challenging to demonstrate in practice due to the low prevalence of eosinophils in most healthy individuals.  
• Consequently, a low eosinophil count should not raise concern.  

Eosinophilia (high eosinophil count):  

In developed nations, the most frequent causes are allergic conditions like asthma and hay fever, while globally, parasitic infections are the leading cause of increased eosinophils.  
 

Follow-up:  

• Total eosinophil count Follow-up:  
• < 1.0 × 10^9/L: Trivial, disregard.  
• Up to 1.5 × 10^9/L: Likely, disregard.  
• > 1.5 × 10^9/L: Consider potential causes.
   

Platelets  

Platelets are tiny cells that help your blood clot and prevent excessive bleeding.

• Platelets are generated by budding off from megakaryocytes in the bone marrow. Each megakaryocyte gives rise to approximately 5,000 to 10,000 platelets. 
• These platelets typically circulate in the bloodstream for about one to two weeks before being eliminated by the spleen and liver.  
• A normal platelet count falls between 150–450 × 10^9/L.  
• The interpretation of a low platelet count necessitates an evaluation within the clinical context. Severe thrombocytopenia is linked to a higher risk of bleeding and requires immediate assessment.
• This condition is critical if it is accompanied by other abnormalities in the CBC or if the platelet count is decreasing. Bleeding is rare at platelet counts more significant than 50 × 10^9/L unless there is a concurrent platelet function defect, which is uncommon.  
   

Causes  

Thrombocytopenia may be falsely elevated due to various reasons, which should be ruled out before investigating clinical causes, such as:  

▪ A partially clotted sample.  

▪ Platelet aggregation in the blood collection tube.  

Isolated thrombocytopenia can result from:  

▪ Viral infections  

▪ Idiopathic thrombocytopenic purpura (ITP)  

▪ Certain medications  

▪ Liver disease  

▪ Autoimmune conditions  

▪ Hypersplenism  

▪ HIV infections  

▪ Pregnancy  

▪ Bone marrow disorders  
 

Follow-up  

• Low platelet counts are relatively frequent, but it is crucial to interpret the results in context, seeking additional indicators for guidance.
•  Additional testing that may be recommended for patients with consistently low platelet counts includes a blood film, ANA (anti-nuclear antibodies), HIV, Hepatitis B and C tests, and liver enzyme assessments. 
• In some instances of chronic thrombocytopenia, testing for Helicobacter pylori serology and anticardiolipin antibodies may also be warranted. 
• During an examination, splenomegaly and lymph node involvement should also be assessed.  
   

Suggested follow-up for thrombocytopenia:  

Platelet count referral  

• < 30 × 10^9/L Requires immediate referral  
• 30–100 × 10^9/L Discretion based on context; urgent referral needed if bleeding occurs. Further investigation is necessary if it remains low or progresses. Referral is advised if no cause is identified.  
• 100–145 × 10^9/L Follow-up counts are necessary; investigation needed if persistently low. Referral is advised if it worsens without an obvious cause.  
   

Elevated platelets (High platelet count)  

• Increased platelet counts may be reactive and do not always indicate a clinical issue. 
• Generally, reactive thrombocytosis is not linked to an elevated thrombotic risk. In rare circumstances, thrombocytosis may indicate a myeloproliferative disorder, such as essential thrombocythaemia or myelofibrosis. 
• These conditions should be seriously considered if there is a persistently elevated platelet count (lasting more than six months) or an extremely high count without a clear reactive cause. They may also be associated with splenomegaly; the platelet count is usually above 600 × 10^9/L (often much higher).  

Hemoglobin and red cell indices  

Haemoglobin  

Low haemoglobin levels commonly indicate anaemia. Most laboratories classify a haemoglobin level below 130 g/L for men as low. In contrast, for women, the lower limit of the reference range is typically stated as 115 g/L, although some laboratories might use a value of 120 g/L.  

Anaemia demands careful evaluation. The broad range of possible diagnoses can be significantly narrowed by assessing the mean cell volume (MCV) and the reticulocyte count, leading to an initial classification of anaemia as:  

▪ Microcytic, with MCV < 80 fl  

▪ Normocytic, with MCV between 80 – 100 fl  

▪ Macrocytic, with MCV > 100 fl  
 

What does a CBC test measure?

1. Red Blood Cells (RBCs): Carry oxygen throughout the body.
2. Haemoglobin (Hb): A protein in RBCs responsible for oxygen transport.
3. Hematocrit (Hct): The proportion of RBCs in your blood.
4. White Blood Cells (WBCs): Fight infections and immune response.
5. Platelets: Help in blood clotting and wound healing.
6. Mean Corpuscular Volume (MCV): Indicates the size of your RBCs.
7. Mean Corpuscular Hemoglobin (MCH) & Mean Corpuscular Hemoglobin Concentration (MCHC): Indicate the amount and concentration of haemoglobin in RBCs.
8. Red Cell Distribution Width (RDW): Measures the variation in RBC size.
    

Results  

Normal ranges for a complete blood count (CBC) test can vary slightly but generally include the following values for adults:
 

CBC Normal Range

The reference ranges for CBC values may differ based on age, sex, health conditions, and laboratory standards.
 

Limitations of reference ranges for the CBC 

For the general population, standard reference ranges are usually sufficient for diagnostic purposes; however, several challenges can complicate interpretation in certain situations. 

• Establishing blood count reference ranges is challenging due to numerous factors influencing blood count parameters and their prevalence within the population. 
• These factors include iron deficiency, thalassaemia, medications, alcohol consumption, and minor infections. 
• Moreover, ethnic variations in specific parameters, gender differences, and pregnancy changes must be considered. Some factors are acknowledged in published ranges (e.g., gender, pregnancy), while others are not. Additionally, variations among different haematology analyzers can affect some blood count parameters. 
• Given these considerations, the approach to determining blood count reference intervals differs from that for many biochemical parameters, where reference intervals typically cover the 2.5th to 97.5th percentiles. 
• Blood count reference intervals have been established using a combination of local and published data and a practical approach. This results in many reference intervals encompassing more than 95% of “normal” individuals, meaning that marginal results must be evaluated in context. Similarly, a normal blood count does not eliminate the possibility of early disease states, such as iron deficiency. 

Borderline abnormalities should be assessed in a clinical context

• All haematology results must be analyzed with a comprehensive history, physical examination, and previous test results. 
• Follow-up counts can often clarify marginal results, as many significant clinical conditions will show progressive changes. 
• The CBC is commonly included in routine "well-person" checks or screening tests for life or health insurance applicants.
•  While some organizations advocate for “well-person” checks, others contend that blood tests are unnecessary for healthy individuals. In asymptomatic patients, the probability of relevant test results is low, resulting in a high incidence of false positives.
•  Furthermore, conducting investigations for individuals without a clear clinical indication can waste resources (time and money) that could be better utilized for those with unmet healthcare needs.
   

Conclusion

Understanding your CBC test results can help you take charge of your health. If you receive abnormal readings, it’s essential to consult your doctor for further evaluation. Regular monitoring of your blood health can help detect potential health issues early. For a hassle-free experience, get your CBC test done at home with TruePal’s at-home health check-up services. Book your appointment today and take the first step toward better health!
 

FAQs

How do you get ready for a CBC?  

If you’re having only a complete blood count done, you can continue to eat and drink normally before the test. However, if your blood sample is being used for additional tests, you may need to fast for a specific period beforehand. Consult your healthcare provider about the necessary preparations.

What can you expect following a CBC?  

For a complete blood count, a healthcare professional will collect a blood sample by inserting a needle into a vein in your arm, typically at the crease of your elbow. The blood sample is then sent to a laboratory. Once the test is completed, you can resume your normal activities immediately.

What does a CBC blood test evaluate?  

A CBC test can indicate whether you have:  

•  Anaemia (decreased levels of red blood cells or haemoglobin).  
• Erythrocytosis (elevated levels of red blood cells).  
• Leukocytosis (increased white blood cell count).  
• Leukopenia (reduced white blood cell count).  
• Thrombocytosis (heightened platelet count).  
• Thrombocytopenia (lowered platelet count).