Cross-cutting · Clinical Diagnosis

Interpreting the CBC

A board-focused walkthrough of reading the CBC — using MCV, RDW, and the reticulocyte count to classify anemia, and reading the WBC differential and platelet count — to land the diagnosis and the next best step.

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How the boards test the CBC

The complete blood count (CBC) reports three cell lines — red cells, white cells, and platelets — plus the RBC indices. STEP 2 rarely asks you to recall exact reference numbers; instead it hands you a set of indices or a smear description and expects the diagnosis and the next best step.

Build every answer in the same order: indices → morphology → diagnosis → management.

The workhorse index is the MCV, which sorts anemia into micro-, normo-, and macrocytic buckets.
The RDW (variation in red-cell size) separates iron deficiency (high RDW) from thalassemia trait (normal RDW).
For a normocytic anemia, the reticulocyte count is the pivotal split.
For the WBC, the differential and morphology matter far more than the total count.
For platelets, first decide destruction vs underproduction.

Numbers and red flags worth knowing

Anemia: Hgb <13 g/dL (men), <12 g/dL (women) — classify by MCV first
MCV: <80 = microcytic; 80–100 = normocytic; >100 = macrocytic
RDW: high in iron deficiency; normal in thalassemia trait
Reticulocyte count/index: the key split for normocytic anemia — high = hemolysis or blood loss; low = marrow hypoproduction
Leukocytosis: WBC >11,000 — read the differential, not just the total
Left shift (bands, metamyelocytes) → bacterial infection
Neutropenia: ANC <1,500; <500 = severe → neutropenic-fever risk
Thrombocytopenia: platelets <150,000; spontaneous bleeding risk when <10,000–20,000
Pancytopenia (all three lines low) → marrow failure/infiltration, B12/folate deficiency, or hypersplenism

Anemia by MCV — buckets and first move

MCV	Category	Classic causes	First workup step
<80	Microcytic	Iron deficiency, thalassemia, chronic disease, sideroblastic, lead	Iron studies (ferritin, TIBC, Fe)
80–100	Normocytic	Acute bleed, hemolysis, chronic disease, CKD, aplastic, early iron def	Reticulocyte count
>100	Macrocytic	B12/folate deficiency, alcohol, liver disease, hypothyroidism, MDS, drugs	B12 + folate; read the smear

Peripheral blood smear showing a neutrophil with six or more nuclear lobes, indicated by an arrow, in megaloblastic anemia — Hypersegmented neutrophil (≥6 lobes) — the classic smear clue to B12/folate deficiency in a macrocytic anemia. · Wikimedia Commons — Paul Weisz Carrington, M.D. — Public domain, via Wikimedia Commons

Microcytic anemia — TAILS

Microcytic anemia = "TAILS":

T — Thalassemia (normal RDW, ↑ RBC count, target cells, Mentzer index <13)
A — Anemia of chronic disease (may be normo- or microcytic; ↑ ferritin, ↓ TIBC)
I — Iron deficiency (↓ ferritin, ↑ TIBC, ↑ RDW; most common overall)
L — Lead poisoning (basophilic stippling; child with pica)
S — Sideroblastic anemia (ringed sideroblasts; alcohol, B6 deficiency, isoniazid)

*Mentzer index = MCV ÷ RBC count: <13 favors thalassemia, >13 favors iron deficiency.*

Vignette — microcytic anemia in an older adult

Stem: A 62-year-old man reports fatigue. Hgb 9.5, MCV 72, RDW high, platelets 520,000. Ferritin is low; TIBC is high.

Diagnosis: iron deficiency anemia. In an older adult, this is occult GI blood loss until proven otherwise.

Next best step: colonoscopy (plus EGD) — bidirectional endoscopy to exclude a GI malignancy, not simply starting iron and moving on. Empiric oral iron is reasonable only when the source is obvious (e.g., a menstruating young woman with menorrhagia).

Buzzword to catch: the reactive thrombocytosis (platelets >450,000) — a classic tag-along of iron deficiency, not a separate marrow disorder.

Giemsa-stained blood film in iron deficiency anemia showing small, pale red cells with enlarged central pallor — Iron deficiency: microcytic, hypochromic red cells with widened central pallor — matches a low MCV and high RDW. · Wikimedia Commons — Dr Graham Beards — CC BY-SA 3.0, via Wikimedia Commons

Vignette — reading the white cell smear

Stem: A 70-year-old man has an isolated lymphocytosis — WBC 45,000 with 80% mature-appearing lymphocytes — and the smear shows smudge cells. He is asymptomatic.

Diagnosis: chronic lymphocytic leukemia (CLL).

Next best step: peripheral blood flow cytometry — confirms a clonal CD5+ / CD19+ / CD23+ B-cell population; a bone marrow biopsy is usually not required to make the diagnosis.

Contrast on the smear:

Circulating blasts + Auer rods → AML (Auer rods = myeloid) → next step is bone marrow biopsy
Lymphoblasts (no Auer rods), pancytopenia, a child → ALL (TdT+; most common childhood cancer)
Atypical (reactive) lymphocytes in a young adult → EBV mononucleosis

Smear buzzwords → diagnosis

Smudge cells → CLL
Auer rods → AML (especially APL / M3)
Circulating blasts → acute leukemia
Atypical / reactive lymphocytes → EBV mono (also CMV, acute HIV)
Hypersegmented neutrophils → B12/folate (megaloblastic) anemia
Basophilic stippling → lead poisoning (or thalassemia, sideroblastic)
Eosinophilia — "NAACP": Neoplasm, Allergy/Asthma, Addison's, Connective tissue/Collagen-vascular, Parasites
Basophilia → chronic myeloid leukemia (CML)
Left shift + toxic granulation → bacterial infection
Leukemoid reaction (WBC >50,000, high LAP score) vs CML (low LAP, BCR-ABL)

Iron studies — telling microcytic anemias apart

Test	Iron deficiency	Anemia of chronic disease	Thalassemia trait
Ferritin	Low	High / normal	Normal
Serum iron	Low	Low	Normal
TIBC / transferrin	High	Low	Normal
Transferrin saturation	Low	Low / normal	Normal
RDW	High	Normal	Normal
RBC count	Low	Low	Normal / high

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