Complete Blood Count (CBC) Components
Standard CBC Components
A complete blood count (CBC) for an adult patient with no significant medical history includes the following core components: red blood cell count, hemoglobin, hematocrit, red blood cell indices (mean cell volume, mean cell hemoglobin, mean cell hemoglobin concentration, and red cell distribution width), white blood cell count with differential, platelet count, and reticulocyte count. 1
Red Blood Cell Parameters
- Hemoglobin or hematocrit: Measures oxygen-carrying capacity of blood 2
- Red blood cell (RBC) count: Total number of red blood cells per volume of blood 1
- Red blood cell indices: 1
- Mean cell volume (MCV): Average size of red blood cells, used to classify anemia as microcytic (<80 fL), normocytic (80-100 fL), or macrocytic (>100 fL) 3
- Mean cell hemoglobin (MCH): Average hemoglobin content per red blood cell, a reliable marker for iron deficiency 3
- Mean cell hemoglobin concentration (MCHC): Average hemoglobin concentration in red blood cells 3, 1
- Red cell distribution width (RDW): Measures variation in red blood cell size; elevated RDW with normal MCV suggests early iron deficiency or mixed nutritional deficiencies 3
- Reticulocyte count: Measures immature red blood cells to assess bone marrow response; low or normal count indicates impaired erythropoiesis, while elevated count suggests increased red cell production 3
White Blood Cell Parameters
- White blood cell (WBC) count: Total number of white blood cells 1, 4
- Differential count: Provides percentages and absolute numbers of different white blood cell subtypes (neutrophils, lymphocytes, monocytes, eosinophils, basophils) 1, 4
Platelet Parameters
Clinical Context for CBC Ordering
Multiple clinical guidelines recommend CBC as part of routine evaluation in specific contexts:
- Hypertension evaluation: CBC is included in basic laboratory testing for newly diagnosed hypertension 2
- Diabetes management: CBC with platelets should be performed annually in patients with diabetes 2
- Heart failure assessment: CBC is a standard laboratory test for patients with chronic heart failure 2
- Hematologic evaluation: CBC with differential is the initial test when evaluating for bleeding disorders, anemia, or other hematologic conditions 2
Important Clinical Pearls
The CBC must be interpreted in conjunction with clinical history and physical examination to maximize diagnostic utility. 5 Key considerations include:
- Peripheral blood smear examination may be necessary when CBC reveals abnormalities to assess for morphologic changes 6
- Iron studies (serum ferritin, transferrin saturation) should be obtained when anemia is detected, particularly with low MCH or elevated RDW 3
- Urgent hematology consultation is warranted when CBC shows abnormalities in two or more cell lines, severe anemia (hemoglobin <10 g/dL), thrombocytopenia with elevated LDH, or other concerning findings 7
The automated CBC has evolved to provide comprehensive hematologic assessment efficiently, making it one of the most frequently ordered and clinically valuable laboratory tests in medicine. 1, 8