CBC Pattern Approach: Leukocytosis and Thrombocytopenia
Pattern recognition on the CBC is the fastest route to a differential diagnosis.
Leukocytosis
- Neutrophilia with left shift (raised bands/metamyelocytes, toxic granulation, Dohle bodies) suggests acute bacterial infection or inflammation.
- Lymphocytosis: reactive (viral, pertussis, stress) versus chronic lymphoproliferative disease; look for smudge cells and persistent elevation.
- Eosinophilia (above ~0.5 x10^9/L or 6%): allergy, parasites, drug reaction, or eosinophilic disorders.
- Blasts: any circulating blast is abnormal and requires urgent hematology review — denotes acute leukemia or myelodysplasia until proven otherwise.
- NRBCs: correct the WBC when nucleated red cells exceed ~10 per 100 WBC.
Thrombocytopenia
- Below 150 x10^9/L. First exclude EDTA-induced pseudothrombocytopenia (platelet clumps on the film) and repeat in citrate.
- Decreased production: marrow disorder; look at the three cell lines together (pancytopenia points to marrow failure).
- Increased destruction: immune (ITP), DIC (with low fibrinogen, raised D-dimer, prolonged PT/APTT), TTP/HUS (with schistocytes and hemolysis), hypersplenism.
- Dilutional: massive transfusion.
Key checks
WBC with differential, platelets, reticulocyte count, blood film morphology, and — when indicated — D-dimer, fibrinogen, LDH, and a peripheral smear review.