Leukocyte Disorders
Leukocyte disorders
Interpret absolute counts, not percentages. Neutrophilia arises from demargination (stress, epinephrine, exercise - minutes), increased marrow production (infection, G-CSF, steroids), or decreased egress. A leukemoid reaction (WBC often 30–50 × 10^9/L with left shift, toxic granulation, Döhle bodies) has a high leukocyte alkaline phosphatase score, whereas CML has a low score and BCR-ABL1. Chronic neutrophilic leukemia is a diagnosis of exclusion requiring CSF3R mutation and absence of BCR-ABL1. Neutropenia reflects decreased production (chemotherapy, B12 deficiency, MDS, aplastic anemia, congenital), increased destruction (drug-induced immune, autoimmune, viral), or margination. Drug-induced agranulocytosis (clozapine, methimazole, sulfonamides) is immune-mediated and presents with fever and mucosal ulceration. Large granular lymphocytic leukemia causes chronic neutropenia with clonal T cells.
Recognize inherited morphologic anomalies: Pelger-Huët (bilobed neutrophils), hypersegmentation (B12/folate), Chediak-Higashi (giant granules), Alder-Reilly (dense granules), May-Hegglin (Döhle-like inclusions with thrombocytopenia), and leukocyte adhesion deficiency (CD18; delayed cord separation, neutrophilia without pus).
Acute leukemia requires ≥20% blasts in blood or marrow, or specific defining genetics such as t(15;17), t(8;21), or inv(16). Assign lineage with flow cytometry and cytochemistry: MPO/Sudan black B for myeloid, CD3/CD7 for T, CD19/CD79a for B. Distinguish reactive from neoplastic lymphocytosis by morphology, immunophenotype, and clonality.
Trap: steroid-induced neutrophilia with a left shift can mimic infection; check the medication list and prior counts.
Rule of thumb: with a left-shifted neutrophilia, order BCR-ABL1 and LAP - CML is the diagnosis to exclude.