Step by Step
WBC
White blood cells (4,500–11,000/µL)
Elevated WBC suggests infection, inflammation, or leukemia. Decreased WBC suggests bone marrow suppression, viral illness, or chemotherapy effects. The differential further breaks this down: neutrophils (55–70%), lymphocytes (20–40%), monocytes (2–8%), eosinophils (1–4%), and basophils (0.5–1%).
RBC/Hgb/Hct
Red blood cells, hemoglobin, and hematocrit
RBC (4.5–5.5 million/µL in males, 4.0–5.0 in females): decreased suggests anemia; increased suggests polycythemia vera or dehydration. Hemoglobin (12–17 g/dL): decreased suggests anemia, presenting as pallor, fatigue, and dyspnea. Hematocrit (37–52%) is roughly 3 times the hemoglobin value — a useful cross-check between the two.
Plt
Platelets (150,000–400,000/µL)
Decreased platelets (thrombocytopenia) increase bleeding risk. Increased platelets (thrombocytosis) increase clotting risk.
MCV
MCV — classifying the type of anemia
Mean corpuscular volume (80–100 fL) helps classify anemia by red blood cell size: decreased MCV indicates microcytic anemia (commonly from iron deficiency); increased MCV indicates macrocytic anemia (commonly from B12 or folate deficiency).
Applied Walkthrough
1
A patient with a bacterial infection shows an elevated WBC count, with the differential revealing a predominance of neutrophils — consistent with an acute bacterial process rather than a viral one.
2
A different patient reports fatigue and pallor; their labs show decreased RBC, hemoglobin, and hematocrit — all consistent with anemia.
3
To classify this anemia further, the clinician checks MCV: a low MCV points toward microcytic anemia, most commonly caused by iron deficiency, while a high MCV would instead point toward macrocytic anemia, more commonly caused by B12 or folate deficiency.
4
Meanwhile, a separate patient's platelet count comes back low (thrombocytopenia) — putting them at increased bleeding risk, a very different clinical concern from either the infection or anemia scenarios above.
Exam Application
Exams test whether you know the normal ranges for the five key CBC values, whether you can interpret high versus low results for each, and whether you can use MCV to classify anemia as microcytic versus macrocytic.
⚠ Common Trap
The most common trap is assuming all forms of anemia have the same underlying cause — checking MCV is essential, since microcytic anemia (low MCV, often iron deficiency) and macrocytic anemia (high MCV, often B12/folate deficiency) point toward very different diagnoses and treatments.
✓ Quick Self-Check
1. What does an elevated WBC count typically suggest?
Infection, inflammation, or leukemia.
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2. What does decreased hemoglobin typically indicate, and what symptoms does it cause?
Anemia; pallor, fatigue, and dyspnea.
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3. What is the relationship between hematocrit and hemoglobin?
Hematocrit is roughly 3 times the hemoglobin value.
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4. What does a low platelet count (thrombocytopenia) increase the risk of?
Bleeding.
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5. What does MCV help classify, and what does a low MCV suggest?
The type of anemia; a low MCV suggests microcytic anemia, commonly from iron deficiency.
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