How is dengue-associated hemophagocytic lymphohistiocytosis (HLH) diagnosed—does it require a complete blood count (CBC) and bone marrow examination?

Diagnosis of Dengue-Associated HLH

Diagnose dengue-associated HLH using the HLH-2004 criteria (requiring 5 of 8 criteria), supplemented by CBC, ferritin, triglycerides, fibrinogen, and bone marrow examination when needed—though bone marrow is not always mandatory if other criteria are met and clinical suspicion is high. 1

Diagnostic Approach

Initial Screening Parameters

Start with these readily available tests when you suspect dengue-associated HLH:

• CBC with differential: Look for cytopenias affecting ≥2 cell lines 1

  • Hemoglobin <90 g/L
  • Platelets <100 × 10⁹/L
  • Neutrophils <1.0 × 10⁹/L

• Ferritin: The single most important screening test 1, 2

  • Values >500 μg/L meet HLH-2004 criteria
  • Values >2000-6000 μg/L are highly suggestive
  • Values >10,000 μg/L are >90% sensitive and specific in children, though less specific in adults
  • Marked hyperferritinemia should always trigger HLH evaluation

• Triglycerides: Fasting levels ≥3.0 mmol/L (≥265 mg/dL) 1

• Fibrinogen: ≤1.5 g/L 1

• Liver function tests: Elevated AST is common and part of the HScore 1, 3

• Soluble CD25 (sIL-2 receptor): ≥2400 U/mL if available—this has excellent diagnostic performance (AUC 0.90) 1

Clinical Red Flags Specific to Dengue-Associated HLH

Suspect HLH when dengue patients present with:

• Prolonged or recurrent fever >7 days beyond typical dengue course 4
• Persistent fever with progressive cytopenias despite supportive care 5, 3
• Splenomegaly: This is a particularly useful screening finding—a 2025 study showed splenomegaly combined with leucopenia and thrombocytopenia had 94% accuracy for detecting HLH in dengue 2
• Hepatosplenomegaly together is even more specific 1, 3
• Anemia without intravascular hemolysis or massive bleeding 4

When to Perform Bone Marrow Examination

Bone marrow aspiration is NOT always required but strengthens the diagnosis when performed 1:

• Perform bone marrow when you need additional diagnostic certainty or when other criteria are borderline
• Finding hemophagocytosis counts as 1 of the 8 HLH-2004 criteria and adds 35 points to the HScore 1
• Critical caveat: Absence of hemophagocytosis does NOT exclude HLH 6—hemophagocytic activity may not be present initially or may require serial aspirates over time 1
• If bone marrow is non-diagnostic, consider sampling other organs (spleen, lymph nodes, liver) 1

Formal Diagnostic Criteria

Use HLH-2004 criteria as your framework—diagnosis requires 5 of 8 1:

1. Fever
2. Splenomegaly
3. Cytopenias (≥2 lineages)
4. Hypertriglyceridemia and/or hypofibrinogenemia
5. Hemophagocytosis in bone marrow/spleen/lymph nodes
6. Low/absent NK cell activity
7. Ferritin ≥500 μg/L
8. Soluble CD25 ≥2400 U/mL

Alternative: HScore Calculation

The HScore provides a probability-based approach and may be more practical in adults 1, 3:

• Score >241 indicates >99% probability of HLH 3
• Incorporates: immunosuppression status, temperature, organomegaly, cytopenias, ferritin, triglycerides, fibrinogen, AST, and hemophagocytosis
• Does not require all parameters to calculate

Critical Diagnostic Dilemma: Severe Dengue vs. Dengue-Associated HLH

This distinction is crucial because treatment differs fundamentally 4:

Features Favoring HLH Over Severe Dengue Alone:

• Persistent fever beyond day 7 4
• Progressive pancytopenia rather than isolated thrombocytopenia 3
• Extreme hyperferritinemia (>2000-10,000 μg/L) 3, 7
• Marked transaminitis disproportionate to typical dengue 7
• Hypofibrinogenemia with elevated D-dimer 5
• Altered mental status 3
• Median day of HLH diagnosis: day 23 of illness in one ICU series 3

Dynamic Assessment Strategy

Monitor recovery trajectories longitudinally rather than relying solely on static thresholds 6:

• Patients with self-limited dengue-associated hyperinflammation show:

  • Spontaneous defervescence
  • Progressive reticulocyte recovery
  • Rapid platelet rebound
  • Marked decline in ferritin levels

• Patients with true HLH show:

  • Persistent fever despite supportive care
  • Deteriorating hematological parameters
  • Rising or persistently elevated ferritin

Common Pitfalls

1. Waiting for bone marrow confirmation: Don't delay treatment if 5 criteria are met clinically—bone marrow may be negative early 1, 6

2. Dismissing HLH because triglycerides are normal: One case report showed HLH with normal triglycerides but other criteria met 7

3. Assuming all hyperferritinemia is HLH: Ferritin is less specific in adults; integrate multiple clinical features 1

4. Missing viral coinfections: Check for EBV and CMV—present in >1/3 of pediatric cases and may worsen severity 8

5. Overlooking the diagnosis in non-endemic regions: Maintain suspicion even in areas without typical dengue transmission, especially after unusual weather events 7

Practical Diagnostic Algorithm

1. Day 1-7 of dengue: Standard dengue management
2. Day 7+: If persistent fever + progressive cytopenias → Check ferritin, triglycerides, fibrinogen, AST
3. If ferritin >2000 μg/L + ≥2 cytopenias + splenomegaly → Calculate HScore or count HLH-2004 criteria
4. If 5 criteria met or HScore >241 → Diagnose HLH and initiate treatment
5. If borderline (3-4 criteria) → Consider bone marrow aspiration + soluble CD25 + monitor recovery trajectory closely
6. If clinical deterioration despite meeting <5 criteria → Consider empiric HLH-directed therapy based on clinical judgment 1

1, 2, 5, 3, 4, 8, 7