Differential Diagnosis: Thrombocytosis with Leukocytosis and Anemia
The combination of thrombocytosis, leukocytosis, and anemia most commonly indicates chronic myeloid leukemia (CML) in chronic phase, but also requires consideration of other myeloproliferative neoplasms, myelodysplastic/myeloproliferative overlap syndromes, and reactive causes—particularly infection and iron deficiency.
Primary Hematologic Malignancies
Chronic Myeloid Leukemia (CML)
CML is the classic presentation of this triad. Diagnosis is based on leukocytosis with thrombocytosis, immature granulocytes (metamyelocytes to myeloblasts), and basophilia 1, 2. The Philadelphia chromosome t(9;22) or BCR-ABL1 fusion confirms diagnosis and is present in 95% of cases 2. Splenomegaly occurs in >50% of patients, though 50% remain asymptomatic at presentation 1, 2.
Key diagnostic steps:
- Peripheral blood differential showing left shift of granulopoiesis with basophilia
- Philadelphia chromosome detection via cytogenetics or BCR-ABL1 by RT-PCR/FISH
- Bone marrow examination to assess blast percentage and exclude accelerated/blast phase
Myelodysplastic/Myeloproliferative Overlap Syndromes
Atypical CML (aCML) presents with leukocytosis and dysplastic neutrophilia (immature myeloid cells ≥10% of WBC count) with prominent dysgranulopoiesis, but is BCR-ABL1 negative 3. These patients commonly harbor ASXL1, SETBP1, ETNK1, and EZH2 mutations 3.
MDS/MPN-NOS represents overlap syndromes not meeting criteria for defined categories. These can present with leukocytosis, thrombocytosis, and anemia, with <20% blasts 3. Cytogenetic abnormalities occur in 40-50% of cases 3.
Mixed MDS-MPS (4.4% of MDS cases) shows features of both myelodysplasia and myeloproliferation 4. Two patterns emerge:
- Megakaryocytic proliferation with thrombocythemia alongside MDS features
- Granulocytic proliferation with leukocytosis and MDS features
- Ring sideroblasts and myelofibrosis are frequent findings 4
Other Myeloproliferative Neoplasms
Essential thrombocythemia (ET) primarily presents with thrombocytosis (platelets ≥450 × 10⁹/L) but can have mild leukocytosis 5, 6. Diagnosis requires bone marrow showing megakaryocyte proliferation with large, mature morphology and presence of JAK2, CALR, or MPL mutations 5. However, significant leukocytosis with anemia makes this less likely.
Primary myelofibrosis can present with leukocytosis ≥11 × 10⁹/L, anemia, and thrombocytosis, but typically shows leukoerythroblastosis and bone marrow fibrosis 5.
Secondary/Reactive Causes
Infection
Infection is a common and often overlooked cause of thrombocytosis with leukocytosis 7. In a study of 801 thrombocytosis cases, infectious causes accounted for nearly half of secondary thrombocytosis 7.
Clinical clues suggesting infection:
- Fever, tachycardia, weight loss
- Hypoalbuminemia, neutrophilia
- Indwelling prosthesis, quadriplegia/paraplegia, dementia, diabetes
- More rapid normalization of platelet count than non-infectious causes 7
Iron Deficiency Anemia
Severe iron deficiency can cause extreme thrombocytosis and leukocytosis 8. A case report documented thrombocytosis of 2500 × 10⁹/L with leukocytosis of 35 × 10⁹/L in a patient with hemoglobin of 5.4 g/dL from iron deficiency 8. Both abnormalities rapidly resolved with iron supplementation 8.
Diagnostic approach:
- Microcytic anemia with low ferritin (<30 μg/L without inflammation, <100 μg/L with inflammation) 9
- History of blood loss, malnutrition, or malabsorption
- Rapid resolution with iron replacement confirms diagnosis
Diagnostic Algorithm
Step 1: Peripheral blood smear examination
- Look for immature granulocytes, basophilia, dysplasia, blasts
- Assess red cell morphology (microcytic vs. macrocytic)
Step 2: If blast cells or dysplasia present:
- Bone marrow aspiration and biopsy
- Cytogenetics for Philadelphia chromosome and other abnormalities
- BCR-ABL1 testing by RT-PCR or FISH
- Molecular testing for JAK2, CALR, MPL, and next-generation sequencing panel
Step 3: If no dysplasia/blasts and clinical features suggest reactive cause:
- Assess for infection: fever, inflammatory markers (CRP, ESR), cultures
- Iron studies: ferritin, transferrin saturation, serum iron
- Consider other inflammatory conditions
Step 4: If initial workup negative but counts persist:
- Bone marrow examination to exclude early myeloproliferative neoplasm
- Molecular testing may reveal clonal hematopoiesis
Critical Pitfalls
Do not assume reactive cause without excluding CML. The Philadelphia chromosome test is simple and definitive—failure to check BCR-ABL1 can delay life-saving tyrosine kinase inhibitor therapy 2.
Extreme thrombocytosis (>1000 × 10⁹/L) does not exclude reactive causes. Iron deficiency can produce platelet counts >2000 × 10⁹/L 8.
MDS can present with thrombocytosis and leukocytosis, not just cytopenias 4. The presence of elevated counts does not exclude myelodysplasia—look for dysplastic features on smear.
Accelerated phase CML includes thrombocytosis as a defining criterion (15-29% blasts with thrombocytosis unrelated to therapy) 1, 10. Check blast percentage even when other counts are elevated.