What are the elements of a hemogram (complete blood count, CBC) and what are the clinical implications of decreased or increased levels of each component, such as red blood cells, white blood cells, platelets, and hemoglobin, in the human body?

Understanding the Hemogram (Complete Blood Count)

A hemogram, also known as a complete blood count (CBC), is a fundamental blood test that evaluates three major cell lines—red blood cells, white blood cells, and platelets—providing critical information about bone marrow function and overall health status. 1

Core Components of the Hemogram

The CBC consists of several key measurements that assess different aspects of blood cell production and function 2:

Red Blood Cell (RBC) Parameters

Hemoglobin (Hb) is the preferred measure of oxygen-carrying capacity, as it is more reproducible across laboratories and not affected by storage time or patient variables like serum glucose 1. Normal values vary by age and gender: adult males/post-menopausal females average 15.5 ± 2.0 g/dL, while menstruating females average 14.0 ± 2.0 g/dL 1.

Hematocrit (Hct) represents the percentage of blood volume occupied by red blood cells, with normal values of 47 ± 6% in adult males and 41 ± 5% in menstruating females 1. However, hematocrit is less reliable than hemoglobin because it increases falsely with sample storage and in hyperglycemic states 1.

Mean Corpuscular Volume (MCV) measures average red blood cell size 1:

• Microcytic (< 80 fL): Indicates iron deficiency, thalassemia, anemia of chronic disease, or sideroblastic anemia 1
• Normocytic (80-100 fL): Suggests hemorrhage, hemolysis, bone marrow failure, chronic inflammation, or renal insufficiency 1
• Macrocytic (> 100 fL): Points to vitamin B12/folate deficiency, alcoholism, myelodysplastic syndrome, or certain drugs like hydroxyurea 1

Reticulocyte count measures immature red blood cells and evaluates bone marrow response to anemia 1. A low reticulocyte count indicates inadequate red cell production from iron deficiency, erythropoietin deficiency, or inflammation, while a high count suggests active blood loss or hemolysis 1.

White Blood Cell (WBC) Parameters

Total white blood cell count assesses immune function and bone marrow activity 1. The differential count breaks down specific cell types:

• Neutrophils: Elevated in bacterial infections, inflammation, and stress; decreased in viral infections, bone marrow suppression, or autoimmune conditions 1
• Lymphocytes: Increased in viral infections and certain leukemias; decreased in immunodeficiency states 1
• Monocytes: Elevated in chronic infections and inflammatory conditions 1
• Eosinophils: Increased in allergic reactions and parasitic infections 1
• Basophils: Elevated in myeloproliferative disorders 1

Platelet Parameters

Platelet count evaluates clotting capacity and bone marrow function 1. Normal ranges are typically 150,000-450,000/μL 1.

Mean Platelet Volume (MPV) measures average platelet size and can predict cardiovascular disease risk 3.

Clinical Implications of Abnormal Values

Decreased Red Blood Cells/Hemoglobin (Anemia)

When hemoglobin drops below 80% of normal values (Hct < 33% in females, < 37% in males), anemia workup should be initiated 1. The consequences include:

• Cardiovascular stress: Increased heart rate, cardiac output, and risk of heart failure 1
• Tissue hypoxia: Fatigue, exercise intolerance, dyspnea, headache, vertigo, and chest pain 1
• Mortality risk: Anemia is associated with unexpectedly high rates of death, particularly in chronic kidney disease patients 1
• Quality of life: Persistent fatigue that is not relieved by rest and interferes with daily activities 1

Increased Red Blood Cells (Polycythemia)

Elevated hemoglobin/hematocrit increases blood viscosity, leading to:

• Thrombotic complications: Stroke, myocardial infarction, and deep vein thrombosis 4
• Hyperviscosity symptoms: Headache, dizziness, visual disturbances, and hypertension 4

Decreased White Blood Cells (Leukopenia)

Abnormalities in two or more cell lines warrant hematology consultation 1. Low WBC causes:

• Infection susceptibility: Particularly bacterial and fungal infections when neutrophils are low 1
• Delayed wound healing: Impaired immune response 1
• Potential bone marrow failure: May indicate malignancy, aplastic anemia, or drug toxicity 1, 5

Increased White Blood Cells (Leukocytosis)

Elevated WBC can indicate:

• Acute infection or inflammation: Physiologic response to stress 1
• Hematologic malignancy: Leukemia detection, particularly when blast cells are present 1, 5
• Cardiovascular risk: Chronic elevation predicts increased CVD mortality 3

Decreased Platelets (Thrombocytopenia)

Platelet counts below 150,000/μL require evaluation for autoimmune causes, medication effects, or bone marrow dysfunction 1. Consequences include:

• Bleeding risk: Petechiae, bruising, mucosal bleeding, and life-threatening hemorrhage when severe 1
• Surgical complications: Increased perioperative bleeding 1

Increased Platelets (Thrombocytosis)

Elevated platelet counts cause:

• Thrombotic events: Increased risk of arterial and venous thrombosis 3, 4
• Myeloproliferative disorders: May indicate underlying bone marrow disease 4

Critical Diagnostic Considerations

Hemoglobin is superior to hematocrit for monitoring because it has lower coefficients of variation and remains stable during sample storage 1. When evaluating anemia, iron status must be assessed with serum ferritin (tissue iron stores) and transferrin saturation (available iron for erythropoiesis) 1.

The reticulocyte index distinguishes production defects from destruction/loss: A low index (< 2.0) indicates inadequate bone marrow response, while a high index suggests appropriate compensation for blood loss or hemolysis 1.

Abnormalities in multiple cell lines demand immediate hematology referral as they may indicate bone marrow failure, malignancy, or systemic disease requiring specialized management 1.