CBC

Background

• The complete blood count (CBC) is one of the most commonly ordered laboratory tests in the emergency department
• Provides quantitative and qualitative information about three major cell lines: red blood cells (RBCs), white blood cells (WBCs), and platelets
• Performed by automated hematology analyzers; approximately 10–25% of samples require manual peripheral blood smear review to confirm abnormalities
• Collected in an EDTA (purple/lavender top) tube
• CBC typically refers to the basic count (WBC, RBC, hemoglobin, hematocrit, platelet count, RBC indices)
• CBC with differential additionally provides the WBC differential (neutrophils, lymphocytes, monocytes, eosinophils, basophils) and may report immature cells (bands, metamyelocytes, blasts)
• Turn-around time in most EDs is 30–60 minutes from central lab; point-of-care CBC devices can provide results in < 5 minutes

Components

Red Blood Cell Parameters

Parameter	Normal Range (Adult)	Description
RBC count	M: 4.5–5.5 × 106/μL; F: 4.0–5.0 × 106/μL	Total number of red blood cells per volume
Hemoglobin (Hgb)	M: 13.5–17.5 g/dL; F: 12.0–16.0 g/dL	Oxygen-carrying protein in RBCs; most reliable indicator of anemia
Hematocrit (Hct)	M: 38.3–48.6%; F: 35.5–44.9%	Percentage of blood volume occupied by RBCs
MCV	80–100 fL	Mean corpuscular volume; average RBC size; classifies anemia as microcytic (< 80), normocytic (80–100), or macrocytic (> 100)
MCH	27–33 pg	Mean corpuscular hemoglobin; average hemoglobin mass per RBC
MCHC	32–36 g/dL	Mean corpuscular hemoglobin concentration; average hemoglobin concentration per RBC
RDW	11.5–14.5%	Red cell distribution width; measure of variation in RBC size (anisocytosis); elevated in iron deficiency, B12/folate deficiency, mixed anemias
Reticulocyte count	0.5–2.5%	Immature RBCs; must be specifically ordered; helps classify anemia as hypo- vs. hyperproliferative

White Blood Cell Parameters

Parameter	Normal Range (Adult)	Description
WBC count	4,500–11,000/μL	Total white blood cell count
Neutrophils	40–70% (1,800–7,700/μL)	Primary defense against bacterial infections; elevated (neutrophilia) in bacterial infection, stress, steroids, inflammation; decreased (neutropenia) in bone marrow failure, chemotherapy, drug reactions
Lymphocytes	20–40% (1,000–4,800/μL)	Elevated in viral infections, CLL; decreased in HIV, immunosuppression, steroids
Monocytes	2–8% (200–800/μL)	Elevated in chronic infections (TB, endocarditis), autoimmune disease, malignancy
Eosinophils	1–4% (100–400/μL)	Elevated in allergic conditions, parasitic infections, drug reactions, adrenal insufficiency
Basophils	0.5–1% (< 100/μL)	Elevated in myeloproliferative disorders (especially CML), allergic reactions
Bands (immature neutrophils)	0–5%	Elevated ("bandemia" or "left shift") suggests acute bacterial infection or severe physiologic stress

Platelet Parameters

Parameter	Normal Range (Adult)	Description
Platelet count	150,000–400,000/μL	Low: risk of spontaneous bleeding (< 50,000 increases surgical risk; < 10,000–20,000 risk of spontaneous hemorrhage); High: thrombocytosis — reactive vs. clonal
MPV	7.5–12.0 fL	Mean platelet volume; larger platelets are younger and more metabolically active; elevated MPV with low platelets suggests peripheral destruction (e.g. ITP); low MPV with low platelets suggests bone marrow failure

Clinical Features

The CBC itself is a laboratory test, not a clinical condition. However, abnormalities in the CBC are associated with the following clinical presentations:

Anemia (Low Hgb/Hct)

• Fatigue, weakness, exertional dyspnea, pallor, tachycardia, dizziness/syncope
• Chest pain (demand ischemia in severe anemia)
• Microcytic (MCV < 80): iron deficiency (most common), thalassemia, anemia of chronic disease, sideroblastic anemia, lead poisoning
• Normocytic (MCV 80–100): acute blood loss, anemia of chronic disease, hemolysis, renal failure, bone marrow failure
• Macrocytic (MCV > 100): B12 deficiency, folate deficiency, alcoholism/liver disease, hypothyroidism, myelodysplastic syndrome, medications (e.g. methotrexate, hydroxyurea, antiretrovirals)

Leukocytosis (Elevated WBC)

• Fever, signs of infection, localized inflammation
• Significant leukocytosis (> 25,000–30,000/μL) should raise concern for serious bacterial infection, leukemoid reaction, or hematologic malignancy
• Leukocytosis with blasts on differential warrants emergent hematology consultation for suspected leukemia

Leukopenia / Neutropenia (Low WBC)

• ANC < 1,500/μL = neutropenia; ANC < 500/μL = severe neutropenia
• Susceptibility to overwhelming bacterial and fungal infections
• Febrile neutropenia (ANC < 500 + fever ≥ 38.3°C or ≥ 38.0°C sustained for 1 hour) is an oncologic emergency

Thrombocytopenia (Low Platelets)

• Petechiae, purpura, mucosal bleeding, easy bruising
• Etiologies include: ITP, TTP, HIT, DIC, liver disease, medications, bone marrow failure, splenic sequestration

Thrombocytosis (Elevated Platelets)

• Reactive (infection, inflammation, iron deficiency, splenectomy, malignancy) vs. clonal (myeloproliferative neoplasms)
• Reactive thrombocytosis rarely causes thrombotic complications

Pancytopenia

• Reduction in all three cell lines; consider aplastic anemia, myelodysplasia, bone marrow infiltration (leukemia, lymphoma, myelofibrosis), severe megaloblastic anemia, overwhelming sepsis

Differential Diagnosis

Spurious / Falsely Abnormal Results

• Pseudothrombocytopenia: platelet clumping in EDTA tubes → falsely low platelet count; confirm by re-drawing in citrate (blue top) tube and examining peripheral smear
• Hemolyzed specimen: falsely elevated potassium, LDH; can affect Hgb measurement
• Lipemia: severely elevated triglycerides (> 2,000 mg/dL) can cause falsely elevated Hgb, MCHC, and MCH
• Cold agglutinins: can cause falsely elevated MCV and falsely decreased RBC count
• Leukocytosis with high WBC count (> 100,000/μL): may interfere with Hgb measurement
• Hemodilution: IV fluids drawn proximal to the infusion site → falsely low counts

Evaluation

ED Indications for Ordering a CBC

• Suspected infection or sepsis
• Hemorrhage, trauma, or suspected blood loss
• Suspected anemia (pallor, tachycardia, fatigue, syncope)
• Unexplained fever
• Petechiae, purpura, or abnormal bleeding
• Pre-operative evaluation
• Altered mental status
• Suspected hematologic malignancy
• Monitoring patients on chemotherapy or bone marrow–suppressing medications
• Febrile neutropenia screening in oncology patients
• Abdominal pain (though utility is limited as a standalone test)

Companion Studies Frequently Ordered with CBC

• Peripheral blood smear: morphologic evaluation (schistocytes in TTP/HUS/DIC, sickle cells, spherocytes, blasts)
• Reticulocyte count: distinguish hyperproliferative vs. hypoproliferative anemia
• Coagulation Studies (PT/INR, PTT): suspected coagulopathy or DIC
• Type and screen / crossmatch: anticipated transfusion
• Iron studies, B12, folate: workup of anemia
• Haptoglobin, LDH, bilirubin, direct Coombs: suspected hemolytic anemia
• BMP or CMP: electrolytes, renal function

Management

Management is directed at the underlying cause identified by CBC abnormalities:

Anemia

• Transfusion thresholds:
  • Hemoglobin < 7 g/dL: transfuse in most stable patients (restrictive strategy)
  • Hemoglobin < 8 g/dL: consider transfusion in patients with cardiovascular disease
  • Active hemorrhage: transfuse irrespective of Hgb level; activate massive transfusion protocol if indicated
• Each unit of pRBCs should raise Hgb by approximately 1 g/dL
• Identify and treat underlying cause (GI bleed, menorrhagia, hemolysis, nutritional deficiency, etc.)

Leukocytosis

• Identify and treat source of infection
• Concern for leukostasis if WBC > 100,000/μL (especially in AML): emergent hematology consultation, consider leukapheresis
• Left shift (bandemia > 10%) in the context of clinical illness supports bacterial infection and may prompt earlier antibiotic administration

Neutropenia / Febrile Neutropenia

• Febrile neutropenia is a medical emergency
• Empiric broad-spectrum antibiotics within 60 minutes of presentation (e.g. cefepime, piperacillin-tazobactam, or meropenem)
• Blood cultures before antibiotics if feasible, but do not delay treatment

Thrombocytopenia

• Platelet transfusion thresholds:
  • < 10,000/μL: transfuse prophylactically (unless contraindicated, e.g. TTP, HIT)
  • < 50,000/μL: transfuse if actively bleeding or pre-procedure
  • < 100,000/μL: transfuse for CNS or ocular bleeding/procedures
• Do NOT transfuse platelets in suspected TTP or HIT — may worsen condition
• Examine peripheral smear for schistocytes if TTP/HUS suspected

Pancytopenia

• Hematology consultation
• Transfuse as needed for symptomatic anemia or bleeding
• Broad-spectrum antibiotics if febrile and neutropenic
• Avoid IM injections and rectal examinations in thrombocytopenic patients

Emerging Applications

• Red cell distribution width (RDW): Increasingly recognized as an independent prognostic marker in sepsis, heart failure, cardiac arrest, trauma, PE, and critical illness
  • Elevated RDW (> 14.5%) is associated with increased 28-day and 30-day mortality across multiple disease states
  • Serial RDW measurement over 72 hours after admission may improve prognostication in sepsis
• Neutrophil-to-lymphocyte ratio (NLR): Calculated from the differential; elevated NLR (> 6–10) is associated with worse outcomes in sepsis, ACS, PE, and various malignancies
• Monocyte distribution width (MDW): A novel CBC-derived parameter available on some automated analyzers; FDA-cleared as an early sepsis screening biomarker in the ED

Disposition

• Disposition is driven by the underlying diagnosis, not the CBC result in isolation
• Consider admission for:
  • Symptomatic anemia requiring transfusion or unstable hemoglobin
  • Febrile neutropenia (ANC < 500)
  • New thrombocytopenia < 50,000/μL, especially with bleeding
  • New pancytopenia
  • Suspected hematologic malignancy (blasts on differential)
  • WBC > 100,000/μL with concern for leukostasis
• Discharge with follow-up may be appropriate for:
  • Mild, chronic, asymptomatic anemia with known etiology and stable hemoglobin
  • Mild thrombocytopenia (> 100,000/μL) without bleeding
  • Reactive leukocytosis with identified and treated source (e.g. minor infection)
• Ensure repeat CBC and appropriate follow-up for any new abnormality detected incidentally

See Also

• Anemia
• Febrile Neutropenia
• Leukocytosis
• Thrombocytopenia
• Thrombotic Thrombocytopenic Purpura
• Disseminated Intravascular Coagulation
• Leukemia
• Transfusion Reactions
• Coagulation Studies
• Peripheral Blood Smear
• Pancytopenia

External Links

• Three neglected numbers in the CBC: RDW, MPV, and NRBC count - Cleve Clin J Med 2019
• Transfusion thresholds for guiding red blood cell transfusion - Cochrane Database Syst Rev 2021
• Monocyte distribution width: a novel indicator of sepsis - Crit Care Med 2019
• RDW increase from baseline predicts mortality in severe sepsis - Crit Care 2013
• The value of a CBC for sepsis diagnosis and prognosis - Diagnostics 2021