How should I interpret a CBC with normal white blood cell count, elevated eosinophils, thrombocytosis, and mildly low MCHC, and what follow‑up is recommended?

Interpretation of CBC with Elevated Eosinophils, Thrombocytosis, and Low MCHC

This CBC shows three abnormalities—absolute eosinophilia (0.6 × 10³/µL), thrombocytosis (533 × 10³/µL), and mildly low MCHC (31.3 g/dL)—that require systematic evaluation to exclude myeloproliferative neoplasms, reactive causes, and combined nutritional deficiencies.

Initial Interpretation

Eosinophilia (0.6 × 10³/µL; reference 0.0–0.4)

• Absolute eosinophil count of 0.6 × 10³/µL represents mild eosinophilia (normal upper limit 0.4 × 10³/µL), which in North America is most commonly caused by allergic processes, parasitic infections, or medications. 1

• The vast majority of eosinophilia cases are reactive rather than neoplastic, and extensive laboratory workup is not justified unless detailed history and physical examination indicate specific risks. 1

• Key historical features to elicit include:

  • Travel history or dietary exposures suggesting helminth infection 1
  • Medication review for drug-induced eosinophilia 1
  • Symptoms of atopy (asthma, rhinitis, dermatitis) 1
  • Respiratory symptoms (cough, dyspnea) or skin manifestations that may indicate hypereosinophilic syndrome 2

Thrombocytosis (533 × 10³/µL; reference 150–450)

• Thrombocytosis above 450 × 10³/µL requires differentiation between reactive causes and myeloproliferative neoplasms (MPNs). 3

• When thrombocytosis exceeds 1000 × 10³/µL or persists despite treatment, chronic myeloid leukemia (CML) or other MPNs must be excluded. 4

• The combination of thrombocytosis with eosinophilia raises concern for myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase (TK) fusion genes, particularly PDGFRA, PDGFRB, or FGFR1 rearrangements. 5, 2

Low MCHC (31.3 g/dL; reference 31.5–35.7)

• Mildly low MCHC is often a laboratory artifact or reflects early iron deficiency, though it can coexist with other causes of macrocytosis or dysplasia. 6

• An elevated red cell distribution width (RDW) of 13.9% (upper normal 15.4%) does not suggest combined deficiency, but folate or B12 status should still be assessed if macrocytosis develops. 6

Diagnostic Algorithm

Step 1: Exclude Myeloproliferative Neoplasms with Eosinophilia

Because thrombocytosis and eosinophilia can indicate myeloid/lymphoid neoplasms with TK fusion genes, the following must be performed:

• Peripheral blood smear review to assess for immature myeloid cells, basophilia, dysplastic features, or blast cells. 4, 5

• BCR-ABL1 testing (Philadelphia chromosome) by PCR or FISH to exclude CML, which presents with thrombocytosis, basophilia, and eosinophilia in 90–95% of cases. 4, 3

• PDGFRA and PDGFRB rearrangement testing is mandatory when eosinophilia is present, as these fusion genes define a distinct WHO category of myeloid/lymphoid neoplasms with eosinophilia that respond to tyrosine kinase inhibitors. 5, 2

• JAK2 V617F mutation testing should be considered if polycythemia vera or essential thrombocythemia is suspected, though the normal hemoglobin and hematocrit make polycythemia vera unlikely. 3

Step 2: Evaluate for Reactive Causes

If genetic testing excludes neoplastic causes, pursue reactive etiologies:

For Eosinophilia:

• Stool examination for ova and parasites (three samples) if travel, dietary, or other exposure risks are identified. 1

• Allergy testing (serum IgE, specific IgE panels) may be performed concurrently with parasitic workup if atopy is suspected. 1

• Medication review for drugs causing eosinophilia (antibiotics, anticonvulsants, NSAIDs). 1

For Thrombocytosis:

• Inflammatory markers (CRP, ESR) to assess for reactive thrombocytosis due to infection, inflammation, or malignancy. 7

• Iron studies (ferritin, transferrin saturation) because iron deficiency is a common cause of reactive thrombocytosis. 7

• Imaging (chest X-ray, abdominal ultrasound) if occult malignancy is suspected, though tumor-associated eosinophilia usually accompanies widely metastatic disease. 1

Step 3: Monitor for Persistence

• Repeat CBC with differential in 4–6 weeks to determine whether eosinophilia and thrombocytosis resolve or persist. 1

• If absolute eosinophil count remains >1.5 × 10³/µL for longer than 6 months, idiopathic hypereosinophilic syndrome must be ruled out with organ-directed evaluation (echocardiography, pulmonary function tests, skin or gastrointestinal biopsy as indicated). 1, 2

• If thrombocytosis persists or worsens despite treatment of reactive causes, bone marrow biopsy with cytogenetics is indicated to exclude MPN. 3

Critical Pitfalls

• Do not dismiss thrombocytosis and eosinophilia as purely reactive without excluding PDGFRA/PDGFRB rearrangements, as these patients respond dramatically to imatinib and have excellent prognosis when treated early. 5, 2

• A white blood cell count within normal range does not exclude CML or other MPNs, particularly in early chronic phase disease. 4

• Basophilia (2% in this case, 0.2 × 10³/µL absolute) is common in CML and should heighten suspicion for BCR-ABL1 positivity. 4

• The mildly low MCHC may mask coexisting iron deficiency, which can normalize MCV and obscure macrocytosis from folate or B12 deficiency; obtain iron studies and consider folate/B12 testing if anemia develops. 6

Recommended Follow-Up

Immediate (within 1–2 weeks):

• Peripheral blood smear review 4, 5
• BCR-ABL1 testing (PCR or FISH) 4, 3
• PDGFRA/PDGFRB rearrangement testing (FISH or RT-PCR) 5, 2
• Stool for ova and parasites (if exposure history) 1
• Iron studies (ferritin, transferrin saturation) 7

Short-term (4–6 weeks):

• Repeat CBC with differential to assess persistence 1
• Inflammatory markers (CRP, ESR) if not already obtained 7

Hematology/oncology referral is indicated if:

• Genetic testing is positive for BCR-ABL1, PDGFRA, PDGFRB, or JAK2 mutations 4, 5, 3, 2
• Peripheral smear shows immature myeloid cells, dysplasia, or blasts 4, 5
• Thrombocytosis or eosinophilia persists for >6 months without identified reactive cause 1
• Cytopenias develop in two or more lineages, suggesting myelodysplastic syndrome 5