Complete Blood Count (CBC)

Key Points

• The CBC is the most commonly ordered blood test — measures RBC, WBC, hemoglobin (Hgb), hematocrit (Hct), and platelets; a CBC with differential adds the relative percentage of each WBC type.
• Leukocytosis (WBC >11,000/mm³) indicates infection or inflammation; leukopenia (WBC <4,500/mm³) increases infection risk.
• Anemia is defined by low hemoglobin: <12 g/dL (female) or <13 g/dL (male).
• Thrombocytopenia (platelets <150,000/mm³) increases bleeding risk; platelet count <50,000/mm³ is a critical threshold for many procedures.
• The WBC differential identifies which type of WBC is elevated or decreased — bacterial infections typically increase neutrophils; viral infections increase lymphocytes.

Reference Ranges

CBC Panel

Component	Normal Range (Female)	Normal Range (Male)	Critical Values
RBC	4.5–5.0 million/mm³	4.7–6.2 million/mm³	<3.0 or >6.5 million/mm³
WBC	4,500–11,000/mm³	4,500–11,000/mm³	<2,000 or >30,000/mm³
Hemoglobin (Hgb)	12–16 g/dL	13–17 g/dL	<7 g/dL or >20 g/dL
Hematocrit (Hct)	35–47%	42–53%	<21% or >65%
Platelets	150,000–400,000/mm³	150,000–400,000/mm³	<50,000 or >1,000,000/mm³

WBC Differential

WBC Type	Normal %	Function	Elevated In
Neutrophils	50–70%	Engulf bacteria and fungi; first-line defense	Bacterial infection, stress, inflammation
Lymphocytes	20–40%	B cells (antibodies), T cells (cell-mediated immunity), NK cells	Viral infections, chronic infection
Monocytes	2–8%	Engulf bacteria; tissue repair; chronic inflammation	Chronic infections, inflammatory conditions
Eosinophils	1–4%	Parasitic infection response; allergic reactions	Parasites, allergies, hypersensitivities
Basophils	0–1%	Allergic reactions (histamine release)	Allergic reactions (rare)

Interpretation of Abnormal Values

WBC Abnormalities

Finding	Term	Common Causes	Nursing Priority
WBC >11,000/mm³	Leukocytosis	Bacterial infection, stress response, corticosteroids, leukemia	Assess for infection source; monitor temperature, cultures
WBC <4,500/mm³	Leukopenia	Chemotherapy, viral infection, bone marrow suppression	Neutropenic precautions if ANC <500/mm³
Neutrophils >70%	Neutrophilia	Bacterial infection, stress, steroids	Assess for acute bacterial infection
Neutrophils <50%	Neutropenia	Chemotherapy, aplastic anemia, viral infection	Infection risk — protective isolation if ANC <1,000

Absolute Neutrophil Count (ANC)

ANC = WBC × (% neutrophils + % bands) / 100. ANC <500/mm³ → severely neutropenic — highest infection risk; implement strict neutropenic precautions. ANC 500–1,000/mm³ → moderately neutropenic — standard precautions.

Hemoglobin and Hematocrit Abnormalities

Hgb Value	Interpretation	Nursing Action
< 7 g/dL	Severe anemia — critical	Blood transfusion likely ordered; monitor for cardiac symptoms
7–10 g/dL	Moderate anemia	Assess for dyspnea, tachycardia, fatigue; may require transfusion based on symptoms
10–12 g/dL (female)	Mild anemia	Monitor symptoms; oral iron or folate supplementation
> 18 g/dL	Polycythemia	Assess for headache, ruddy complexion, thrombosis risk

Platelet Abnormalities

Platelet Level	Interpretation	Nursing Action
> 400,000/mm³	Thrombocytosis	Assess for thromboembolism risk
150,000–400,000/mm³	Normal	—
50,000–150,000/mm³	Mild thrombocytopenia	Avoid IM injections; apply pressure after venipuncture; monitor for bruising
< 50,000/mm³	Significant thrombocytopenia	No invasive procedures without precautions; assess for petechiae, purpura, mucosal bleeding
< 20,000/mm³	Severe thrombocytopenia — critical	Spontaneous bleeding risk — strict fall precautions; notify provider immediately

Clinical Significance

When CBC is Ordered

• Infection/sepsis workup: WBC with differential identifies bacterial vs. viral cause; cultures ordered if sepsis suspected
• Anemia evaluation: Hgb/Hct + MCV identifies type (iron deficiency, B12/folate, hemolytic)
• Chemotherapy monitoring: WBC nadir monitoring (typically 7–14 days post-chemo) to time neutropenic precautions
• Pre-procedure: Platelet count before invasive procedures (epidural requires >150,000/µL)
• Transfusion trigger: Hgb <7–8 g/dL typically triggers transfusion decision

Nursing Assessment

NCLEX Focus

Key priorities: (1) WBC >30,000 or <2,000 = call provider immediately; (2) platelet <50,000 = bleeding precautions; (3) Hgb <7 = potential transfusion; (4) report WBC differential shift — increased bands (immature neutrophils = “left shift”) = acute bacterial infection; (5) ANC <500 = neutropenic precautions.

• Correlate CBC values with clinical symptoms — values alone do not drive intervention; symptoms matter
• Assess for signs of infection (fever, chills, redness, drainage) when WBC elevated
• Assess for signs of bleeding (petechiae, purpura, ecchymosis) when platelets low
• Assess for signs of anemia (fatigue, dyspnea, pallor, tachycardia, dizziness) when Hgb low

Related Concepts

• anemia-overview-and-transfusion-thresholds — Hgb/Hct values from CBC are the primary diagnostic tool for anemia; transfusion decisions based on CBC results.
• leukopenia-and-neutropenia — WBC differential on CBC identifies neutropenia; ANC calculation guides neutropenic precautions.
• thrombocytopenia-bleeding-risk-and-management — Platelet count from CBC guides bleeding precautions and procedure safety.
• blood-sampling-modalities-and-preanalytical-safety — Proper CBC specimen collection technique prevents preanalytic errors.
• immune-system — WBC differential reflects immune system function and response to infection.

Self-Check

1. A patient’s CBC shows WBC 18,500/mm³ with 85% neutrophils and 10% bands. What is the clinical significance of this “left shift” finding?
2. A patient’s platelet count is 38,000/mm³. What nursing interventions are priority to prevent harm?
3. A post-chemotherapy patient has a WBC of 1,200/mm³ with 30% neutrophils. What is the ANC, and what precautions should be implemented?