Leukocyte Disorders And Disease States

Reactive Versus Malignant Leukocytosis

The MLS exam constantly asks you to separate a benign reaction from a malignancy. A left shift is the appearance of immature neutrophils (bands, metamyelocytes, occasionally myelocytes) in the blood, driven by infection or inflammation. Reactive neutrophilia is accompanied by toxic granulation, Dohle bodies (blue cytoplasmic RNA inclusions), and cytoplasmic vacuolation. These three changes together point to a reactive process, not leukemia.

Key reactive patterns to recognize:

Infectious mononucleosis shows reactive lymphocytes (Downey cells) with abundant indented cytoplasm; the heterophile (Monospot) test is positive. The exam wants you to separate a reactive (atypical) lymphocyte from a malignant lymphoblast: reactive cells are large with abundant pale-blue cytoplasm that scallops or indents around adjacent red cells, mature clumped chromatin, and no prominent nucleoli; lymphoblasts have a high nuclear-to-cytoplasmic ratio, fine open chromatin, and visible nucleoli.

Calling reactive lymphocytes "abnormal" and reflexively suspecting leukemia is a frequent error - context (a young patient with fever, pharyngitis, and a positive heterophile test) supports a benign viral process.

Absolute counts matter more than percentages on the exam. To get an absolute cell count, multiply the total WBC by the cell's fraction: a WBC of 10.0 x10^9/L with 80% lymphocytes gives an absolute lymphocyte count of 8.0 x10^9/L. A relative lymphocytosis (high percentage) can occur simply because neutrophils are low, so always confirm with the absolute number before interpreting.

Acute Leukemias

The defining threshold under WHO classification is >=20% blasts in blood or marrow. The two broad categories:

• Acute myeloid leukemia (AML): myeloblasts, often with Auer rods (pathognomonic, fused primary granules). Myeloperoxidase (MPO) and Sudan Black B positive.
• Acute lymphoblastic leukemia (ALL): lymphoblasts; MPO negative, TdT positive. Most common childhood leukemia; peak age 2-5 years.

Auer rods are never seen in lymphoblasts, so their presence rules in a myeloid process. Acute promyelocytic leukemia (APL, AML-M3) carries t(15;17) and a high risk of DIC; it is treated with all-trans retinoic acid (ATRA). The cytochemical stains used to separate acute leukemias are testable: myeloperoxidase (MPO) and Sudan Black B are positive in myeloblasts and negative in lymphoblasts; nonspecific esterase marks the monocytic lineage; periodic acid-Schiff (PAS) shows a block-positive pattern in lymphoblasts; and terminal deoxynucleotidyl transferase (TdT) marks lymphoblasts.

Modern practice confirms lineage with flow-cytometry immunophenotyping (for example CD13/CD33 for myeloid, CD19/CD10 for B-ALL, CD3 for T-ALL), but the classic cytochemistry pairings still appear on the exam.

Age and clinical pattern help: ALL is the most common malignancy of childhood with a peak at 2-5 years and frequent lymphadenopathy and hepatosplenomegaly, while AML is more common in adults. Both present with the consequences of marrow failure - anemia, thrombocytopenia (bleeding), and neutropenia (infection) - because blasts crowd out normal hematopoiesis.

Chronic Leukemias

CML is distinguished from a leukemoid reaction by a low leukocyte alkaline phosphatase (LAP) score and the Philadelphia chromosome; a leukemoid reaction has a high LAP score.

Myeloproliferative And Myelodysplastic Disorders

The myeloproliferative neoplasms (MPNs) are clonal overproductions of mature cells. Besides CML, know polycythemia vera (panmyelosis with markedly elevated red cell mass, low EPO, JAK2 V617F mutation), essential thrombocythemia (platelets often > 1000 x10^9/L), and primary myelofibrosis (marrow fibrosis with teardrop cells, or dacryocytes, and a leukoerythroblastic blood picture).

Myelodysplastic syndromes (MDS) are the opposite story - ineffective, dysplastic hematopoiesis producing cytopenias despite a cellular marrow, with features such as hypogranular neutrophils, pseudo-Pelger-Huet cells (bilobed nuclei), and ring sideroblasts; MDS may progress to AML.

Lymphomas And Plasma Cell Disorders

• Hodgkin lymphoma: Reed-Sternberg cells (binucleate "owl-eye").
• Multiple myeloma: plasma cell proliferation, rouleaux on the smear, monoclonal M-spike, Bence-Jones protein, lytic bone lesions.

Multiple myeloma's rouleaux (red cells stacked like coins) reflects the high paraprotein load and also elevates the erythrocyte sedimentation rate. The exam frequently contrasts rouleaux (orderly stacks, increased protein) with autoagglutination (irregular clumps, cold agglutinins) - a distinction that also explains spurious analyzer indices in the final section of this chapter.

Worked Example

A 60-year-old has WBC 80 x10^9/L with neutrophils, myelocytes, metamyelocytes, basophilia, and a low LAP score. The low LAP plus basophilia plus the full myeloid spectrum points to CML, confirmed by BCR-ABL1, rather than a leukemoid reaction (which would show a high LAP and toxic changes).

Common Traps

• Confusing reactive lymphocytes (large, abundant cytoplasm) with malignant lymphoblasts (high N:C, fine chromatin).
• Forgetting smudge cells are an artifact characteristic of CLL.
• Assuming high LAP means leukemia - it actually points away from CML toward a reactive leukemoid reaction.