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StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.
Jason El Brihi ; Surabhi Pathak .
Last Update: June 8, 2024 .
The complete blood count (CBC) is one of the most commonly performed laboratory investigations in medicine and provides information regarding circulating blood cell size and quantity.[1] The standard CBC includes a measured red cell count (RBC), hemoglobin (Hb), and hematocrit (Hct), calculated red cell indices, a platelet count, and a white cell count (WBC). Most analyzers now also automatically generate a leucocyte differential.
This reference describes each component of the CBC and discusses the interpretation of results. Though commonly utilized reference intervals are provided, all are lab-specific in practice. The local laboratory guidelines apply when interpreting a CBC.
Red Cell Tests
Hemoglobin (Hb) measures the amount of hemoglobin in whole blood and is expressed in grams per deciliter (g/dL).
According to the World Health Organization (WHO), normal references for hemoglobin levels are 13 to 18 g/dL in adult men and 12 to 16 g/dL in adult women who are not pregnant.
In pregnancy, the lower limit is usually decreased to 10 g/dL to accommodate physiological plasma expansion. A low Hb is called anemia, while erythrocytosis is defined as a high Hb. Hb is also used to grade the severity of anemia (see Table. 1)[2][3]
Table. 1. Grading of Anemia Severity .
Hb measurement using an automated analyzer can be affected by factors such as plasma volume. For example, patients with hypervolemia may exhibit a Hb lower than their true level, while severely dehydrated patients may have a spuriously elevated Hb. Similarly, interferents within the specimen, such as lipemia, hyperbilirubinemia, paraproteinemia, or hyperleukocytosis, may spuriously elevate the Hb. The etiology of anemia is expansive and may be due to blood loss, blood cell destruction, deficient production, or defective production.[4][5][6]
Erythrocytosis may be due to hemoconcentration or dehydration or from increased production of red cells, which may be due to a primary myeloproliferative pathology or secondary to cyanotic heart disease, respiratory disease, high altitudes, renal pathology, smoking, or erythropoietin-secreting tumors.[7]
Hematocrit (Hct) is the percentage volume of red blood cells (RBC) in the blood calculated with RBC and mean corpuscular volume (MCV): Hct = RBC x MCV/10. The normal reference interval is usually 40% to 54% in adult men and 36% to 48% in adult women. Similarly to Hb, Hct is reduced in anemia, increased in erythrocytosis, and is influenced by changes in plasma volume.[8]
Red blood cell count
RBC is the amount of red cells present per unit volume of blood and is expressed in cells per microliter (cells/μL). The normal reference interval is usually 4.6 to 6.2 million cells/μL in adult men and 4.2 to 5.4 million cells/μL in adult women. Unlike Hb, RBC does not accurately measure the blood's oxygen-carrying capacity and is not directly used to diagnose anemia. However, red cell indices are important in evaluating anemia. The RBC may be spuriously low in the setting of red cell agglutination or a sample diluted with intravenous fluid infusion.[4]
Red Cell Indices
The red cell indices are calculated parameters provided within the CBC and include red cell distribution width (RCDW), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC). They are calculated using the measured Hb, Hct, and RBC and help elucidate the etiology of anemia. MCH and MCHC are also often used for quality control within laboratories.[9]
Red cell distribution width
Red cell distribution width (RCDW) quantitatively assesses the degree of variation in red cell size, known as anisocytosis. The value is calculated as the standard deviation or the coefficient of variation of the MCV distribution curve. RCDW is expressed as a percentage, where a higher value indicates a greater variation in red cell size. The normal reference interval in adults is approximately 11.5% to 15% but varies between laboratories. RDW is useful in assessing anemia to help further delineate etiology (see Table. 2).
Mean corpuscular volume
Mean corpuscular volume (MCV) defines the average volume of the red blood cells present and is expressed in femtoliter (fL) or cubic micrometers (µm 3 ). The value is calculated by dividing Hct by RBC. The normal reference interval in adults is 80 to 100 µm 3 .
MCV is commonly used to classify anemias as microcytic (low MCV), normocytic (normal MCV), or macrocytic (high MCV). Different etiologies are often associated with characteristic red cell morphologies (see Table. 2).[10][11][12] MCV is a mean measure of RBC size, and variations in cell size may be considered normal during anisocytosis. Hence, assessing anemia necessitates the utilization of RBC indices and peripheral smear examination rather than depending on a single parameter.
MCV may be spuriously elevated in the setting of:
Red cell agglutination, where doublet erythrocytes are automatically counted as oneHyperglycemia occurs when an excess of glucose causes hypertonic erythrocytes to swell, increasing RBC volume within the counting medium.
Hypernatremia is due to the swelling of hypertonic erythrocytes.Hyperleucocytosis, where the larger white cells are counted as red cells when calculating mean cell volume.[4][13]
Table 2. Classification of Anemia by MCV and RDW. .
Mean corpuscular hemoglobin
Mean corpuscular hemoglobin (MCH) quantifies the amount of hemoglobin per red blood cell and is expressed as picograms per cell (pg/cell). The value is calculated by dividing hemoglobin by red cell count. The normal reference interval in adults is usually 27 to 32 pg/RBC. MCH is dependent on cell volume and correlates closely with MCV. Conversely, the CBC also includes the mean corpuscular hemoglobin concentration (MCHC), which controls for cell volume and is useful in evaluating anemia.
Mean corpuscular hemoglobin concentrate
MCHC is the mean hemoglobin concentration per unit volume of red blood cells, expressed as a gram per deciliter (g/dl) or percentage of Hb per cell. The value is calculated by dividing the hemoglobin by the hematocrit. While MCH quantifies the amount of hemoglobin, MCHC indicates the hemoglobin density in each red cell, controlling for MCV.
Red blood cell quantity of Hb is regulated and is normally limited to an optimal density of approximately 34% Hb/cell. Hypochromia refers to a low MCHC, while normochromic describes a normal MCHC. Hypochromic anemia may be due to decreased Hb production, the most common cause being iron deficiency (less commonly copper deficiency, pyridoxine deficiency, or lead poisoning), or due to reticulocytosis, as reticulocytes have a lower concentration of Hb compared to mature red blood cells.
Increased MCHC can be due to red blood cell morphology abnormalities, such as spherocytosis and xerocytosis or hemoglobinopathies.
More commonly, high MCHC represents an artifact such as in the following:
Lipemia, which spuriously elevates Hb and consequently MCHC Hemolysis, where Hb is disproportionally elevated to Hct within the sample Red cell agglutination, which spuriously lowers the red cell concentration and elevates MCV Leukocytosis, which spuriously elevates MCV Hyperbilirubinemia, which spuriously lowers HbExamination of a peripheral blood smear is useful in such a case to evaluate red blood cell morphology and exclude other contributing factors, as mentioned above.[4][13][14]
White Blood Cell Tests
White blood cell count
WBC, or the absolute WBC, is the number of white blood cells present per blood microliter (μL). The normal reference interval in adults is usually 4500 to 11000 cells/μL. Leukopenia is defined as a low WBC and may be due to reduced production, increased utilization, or increased destruction of white blood cells. A spurious leukopenia may occur due to white blood cell clumping.
An elevated white blood cell count is called leukocytosis and may be caused by an inflammatory stressor or myeloproliferative pathology. A spurious leucocytosis may occur in the setting of:
Reticulocytosis, where nucleated red blood cells are counted as white blood cells Platelet clumping, where aggregate platelets are counted as white blood cells Fibrin clumps CryoproteinemiaIdentification of a WBC abnormality is followed by an assessment of the white cell differential to identify which white blood cell types are affected. The differential may be reported as a percentage of the total WBC or as an absolute count for each mature white cell type.[15]
Neutrophil count
The neutrophil count is the number of neutrophils per microliter of blood and is expressed as a percentage of the WBC or, as an absolute number, absolute neutrophil count. The normal reference interval in adults is usually 40% to 60% of the absolute WBC or 1500 to 8000 cells/µL. Neutrophilia, which refers to an increase in the neutrophil count, can be due to infection, trauma, tissue necrosis, physical or emotional stress, smoking, myeloproliferative disease, chronic inflammatory disease (such as inflammatory bowel disease, systemic lupus erythematosus, and rheumatoid arthritis), vasculitides, chronic hepatitis, diabetic ketoacidosis, acute gout, congenital conditions (such as Down syndrome and hereditary idiopathic neutrophilia), blood loss, hemolytic anemia, obesity, pregnancy, or medications.
Neutropenia, which refers to a decrease in the neutrophil count, can be due to bone marrow failure (due to myeloproliferative diseases, lymphoproliferative disease, metastatic malignancy, myelodysplasia or irradiation), megaloblastic anemia, hereditary, autoimmune disease (such as systemic lupus erythematosus and rheumatoid arthritis), hypersplenism, hemodialysis, viral infection (such as cytomegalovirus, measles, rubella, human immunodeficiency virus, Dengue fever, infectious mononucleosis), bacterial infections (such as typhoid fever or septicemia), protozoal infections, idiopathic, or drug-induced.
Lymphocyte count
The lymphocyte count is the number of lymphocytes per microliter of blood and is expressed as a percentage of the WBC or as an absolute number. The normal reference interval in adults is usually 20% to 40% of the absolute WBC or 1000 to 4000 cells/µL. Lymphocytosis, which refers to an increase in the lymphocyte count, may be due to viral infection, pertussis, toxoplasmosis, tuberculosis, brucellosis, hyposplenism, or lymphoproliferative disease. Lymphocytopenia, which refers to a decrease in the lymphocyte count, may be due to viral infection, bacterial infection, autoimmune disease (such as systemic lupus erythematosus, rheumatoid arthritis, Sjogren syndrome, and inflammatory bowel disease), malignancy, myeloproliferative disease, lymphoproliferative disease, primary immunodeficiency diseases or medications.[16][17]
Monocyte count
The monocyte count is the number of monocytes per microliter of blood and is expressed as a percentage of the WBC or as an absolute number. The normal reference interval in adults is usually 2% to 8% or 200 to 1000 cells/µL. Monocytosis, an increase in the monocyte count, can be caused by viral or parasitic infection, brucellosis, tuberculosis, listeriosis, endocarditis, granulomatous disease, autoimmune disease (such as inflammatory bowel disease, systemic lupus erythematosus, and rheumatoid arthritis), malignancy, myeloproliferative disease, lymphoproliferative disease, hemolytic anemia, cigarette smoking, or medications. Monocytopenia, a decrease in the monocyte count, is observed in infection, autoimmune disease (such as systemic lupus erythematosus), aplastic anemia, lymphoproliferative disease (particularly hairy cell leukemia), or induced by medications.
Eosinophil count
The eosinophil count is the number of eosinophils per microliter of blood and is expressed as a percentage of the WBC or as an absolute number. The normal reference interval in adults is usually 0% to 4% or 0 to 500 cells/µL. Eosinophilia, an increase in the eosinophil count, may be due to malignancy, myeloproliferative disease, lymphoproliferative disease, parasitic infection (such as Toxocara sp, Ascaris, Strongyloides stercoralis, Schistosomiasis, Trichinella, Filariasis, Cysticercosis or Echinococcus), hypersensitivity reactions, connective tissue disease (such as polyarteritis nodosa, rheumatoid arthritis), atopic disease, skin disorders (such as psoriasis or scabies), idiopathic hypereosinophilic syndrome, Loeffler syndrome, pulmonary infiltration with eosinophilia, vasculitides (especially eosinophilic granulomatosis with polyangiitis) or induced by medications. Eosinopenia, a decrease in the eosinophil count, is seen in conditions such as acute stress, inflammation, or drug-induced (most notably glucocorticoids).
Basophil count
The basophil count is the number of basophils present per microliter of blood and is expressed as a percentage of the WBC or as an absolute number. The normal reference interval in adults is usually 0.5% to 1% or 0 to 200 cells/µL. Basophillia, an increase in the basophil count, may be due to malignancy, myeloproliferative disease, lymphoproliferative disease, endocrinopathy, viral infection, tuberculosis, chronic inflammatory disease (such as inflammatory bowel disease or rheumatoid arthritis), allergy, anaphylaxis or drug-induced. Isolated basophilia is extremely uncommon and warrants workup to exclude a myeloproliferative process. Basopenia, a decrease in the basophil count, may be due to acute infection, hyperthyroidism, ovulation, or medications.[16][17]
Platelet Tests
Platelet count
The platelet count is the amount of platelets per unit volume of blood and is expressed in cells per microliter of blood or cubic millimeter. The normal reference interval in adults is 150,000 to 400,000/μL or mm 3 . Platelet counts may appear low due to platelet clumping in the presence of ethylenediaminetetraacetic acid (EDTA) in blood specimen collection tubes, causing pseudo thrombocytopenia. Repeating the platelet count is recommended when a specimen is collected in a tube containing a different anticoagulant, such as citrate or oxalate.[15]
Thrombocytopenia, a decrease in the platelet count, can be due to decreased production, increased splenic sequestration, or increased peripheral utilization and is defined by a platelet count of less than 150,000/μL or mm 3 .[18] However, major bleeding is unusual unless the platelet count decreases below 50,000/μL or mm 3 . The risk of spontaneous major bleeding increases exponentially below 20,000/μL or mm 3 , and a dramatic increase in the risk of spontaneous cerebral bleeding is seen at counts below 10,000/μL or mm 3 . Thrombocytosis, an increase in the platelet count, is defined as a platelet count above 450,000/μL or mm 3 , however, is rarely symptomatic unless the platelet count is above 1,000,000/μL or mm 3 . This may be due to primary myeloproliferative disorders, secondary to bone marrow stimulation, or decreased sequestration post-splenectomy.[19][20] The platelet count may be spuriously elevated in bacteremia, fungemia, or malaria, where the analyzer counts microorganisms in the blood as platelets. This may also occur in the presence of red blood cell fragments, such as in hemolysis.[4][21] Other platelet tests include mean platelet volume (MPV), platelet distribution width (PDW), and plateletcrit (PCT); however, these are not typically reported with the standard CBC.
The components of the CBC can help identify a wide range of pathologies and remain one of the most commonly ordered and useful tests in clinical medicine. While automated analyzers have allowed for fast and accurate results, spurious results may arise in several situations (see Table. 3). Differentiating between true changes and spurious abnormalities can impact clinical decision-making and patient care. Results must be interpreted in the clinical context and according to local laboratory guidelines, as reference intervals are specific to each lab. An examination of a peripheral blood smear is essential in assessing any abnormal CBC result and is recommended.
Table 3. Summary of spurious results in the complete blood count from automated analyzers.
Any abnormality on the CBC should lead to examination to rule out interfering factors that may contribute to spurious reading, as listed in Table 3. Evaluation of a peripheral blood smear can help exclude factors that may interfere with abnormal CBC readings. Familiarity with the reference range is based on local laboratory parameters and to interpret any abnormalities in the clinical context. Communication within the clinical team is crucial for identifying and understanding the cause of an abnormal CBC.
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Disclosure: Jason El Brihi declares no relevant financial relationships with ineligible companies.
Disclosure: Surabhi Pathak declares no relevant financial relationships with ineligible companies.
