What is the most likely diagnosis and recommended management for a patient with leukocytosis (WBC 14.6 K/µL) due to neutrophilia (absolute neutrophil count 11.5 ×10³/µL) and borderline monocytosis, normal red blood cell indices, and HbA1c 5.6%?

Leukocytosis with Neutrophilia: Most Likely Reactive Process

This patient's laboratory findings most likely represent a reactive neutrophilia from physiologic stress, infection, or inflammation rather than a hematologic malignancy, and the immediate management should focus on identifying and treating the underlying cause rather than pursuing hematologic workup at this time.

Clinical Context and Interpretation

The patient presents with:

• Moderate leukocytosis (WBC 14.6 K/µL) with neutrophilia (ANC 11.5 ×10³/µL, 78.9%)
• Borderline monocytosis (1.0 ×10³/µL; upper limit 0.9)
• Normal hemoglobin, platelets, and red cell indices
• Normal HbA1c (5.6%)

Why This is Likely Reactive

The degree of leukocytosis (14.6 K/µL) and neutrophilia falls within the range commonly seen with physiologic stress, infection, or inflammatory conditions 1. Neutrophilic leukocytosis is frequently observed after physical or mental stress, and almost all immune reactions due to infection, allergy, or autoimmune diseases are associated with changes in leukocyte populations 1.

Key distinguishing features against malignancy:

• No anemia (HGB 12.6 g/dL is within normal range)
• Normal platelet count (291 K/µL)
• Normal MCV and other red cell indices
• Monocyte count only marginally elevated (1.0 vs. 0.9 upper limit)
• No left shift mentioned (no immature myeloid cells reported)

Differential Diagnosis Priority

1. Infection/Inflammation (Most Likely)

According to guideline recommendations, fever, chills, leukocytosis, and left-shift of neutrophils are specific indications suggesting infection 2. The neutrophilia pattern here is consistent with bacterial infection or acute inflammatory process.

Look for:

• Fever, chills, or recent infectious symptoms
• Signs of localized infection (respiratory, urinary, skin/soft tissue)
• Recent surgery, trauma, or tissue injury
• Inflammatory conditions (autoimmune disease flares)

2. Physiologic Stress Response

Physical or mental stress commonly causes neutrophilic leukocytosis 1. Recent studies show that patients with major trauma, cerebrovascular accidents, or major surgery frequently develop persistent leukocytosis (mean peak WBC 26.4K) with bandemia 3.

Look for:

• Recent physical exertion or emotional stress
• Acute medical events (MI, stroke, pulmonary embolism)
• Post-surgical state
• Medications (corticosteroids, lithium, epinephrine)

3. Hematologic Malignancy (Lower Probability)

While chronic myelomonocytic leukemia (CMML) requires sustained monocytosis ≥0.5 ×10⁹/L with monocytes ≥10% of leukocyte count 4, this patient's monocyte percentage is only 6.9%, below the 10% threshold required for CMML diagnosis. Additionally, CMML typically presents with anemia and/or thrombocytopenia, which are absent here 4.

Atypical CML would require dysplastic neutrophilia with immature myeloid cells ≥10% of WBC count and prominent dysgranulopoiesis 5, which is not reported in this differential.

Recommended Management Algorithm

Immediate Steps:

1. Clinical Assessment

   • Obtain detailed history focusing on:
     • Infectious symptoms (fever, cough, dysuria, wound drainage)
     • Recent trauma, surgery, or acute medical events
     • Medication history (especially corticosteroids)
     • Chronic inflammatory conditions
   • Physical examination for:
     • Fever, vital sign abnormalities
     • Signs of infection or inflammation
     • Splenomegaly or lymphadenopathy (would suggest malignancy)

2. Targeted Diagnostic Testing

   • If infection suspected: Blood cultures, urinalysis/culture, chest X-ray, inflammatory markers (CRP, procalcitonin) 2
   • If inflammatory process suspected: ESR, CRP, autoimmune serologies as indicated
   • Peripheral blood smear review to assess for:
     • Left shift (bands, metamyelocytes, myelocytes)
     • Dysplastic features
     • Immature cells or blasts

When to Pursue Hematologic Workup:

Refer to hematology if:

• Persistent leukocytosis >3 months without identified cause (CMML requires sustained monocytosis >3 months) 4
• Monocyte percentage increases to ≥10% on repeat testing 4
• Development of cytopenias (anemia, thrombocytopenia)
• Presence of immature myeloid cells or dysplastic features on smear
• Splenomegaly or unexplained constitutional symptoms
• Progressive increase in WBC count

Hematologic workup would include:

• Bone marrow biopsy with cytogenetics and molecular testing (TET2, SRSF2, ASXL1, RAS pathway mutations) 4
• Flow cytometry to assess monocyte subsets 6

Follow-up Strategy:

• If reactive cause identified: Treat underlying condition and recheck CBC in 2-4 weeks after resolution
• If no clear cause identified: Repeat CBC with differential in 2-4 weeks
  • If persistent without explanation, consider inflammatory markers and peripheral smear review
  • If still unexplained after 3 months, refer to hematology 4

Important Caveats

Common pitfalls to avoid:

• Do not immediately pursue bone marrow biopsy for mild-moderate leukocytosis without excluding reactive causes 1
• Recognize that hospitalized patients can develop persistent inflammation-immunosuppression and catabolism syndrome (PICS) with prolonged leukocytosis (mean duration 14.5 days) without active infection 3
• The monocyte absolute count of 1.0 is only marginally elevated; monocyte percentage (6.9%) is more important and is below the 10% threshold for CMML 4
• Normal HbA1c rules out diabetes as a contributing inflammatory factor 7

Red flags requiring urgent hematology referral:

• Presence of blasts on peripheral smear
• Severe anemia (Hgb <10 g/dL) or thrombocytopenia (platelets <100 K/µL) 4
• Constitutional symptoms (fever, night sweats, weight loss) without identified infection
• Rapidly progressive leukocytosis