What are the diagnostic criteria for Hemophagocytic Lymphohistiocytosis (HLH)?

Diagnostic Criteria for Hemophagocytic Lymphohistiocytosis (HLH)

HLH is diagnosed when either a molecular diagnosis consistent with HLH is established OR when 5 out of 8 specific clinical and laboratory criteria are met according to the HLH-2004 guidelines. 1

The Two Pathways to Diagnosis

Pathway 1: Molecular Diagnosis

• Genetic testing demonstrating mutations in HLH-associated genes (PRF1, UNC13D, STX11, STXBP2, RAB27A, LYST, SH2D1A, BIRC4) establishes the diagnosis without requiring clinical criteria 1, 2

Pathway 2: Clinical Criteria (5 of 8 Required)

The 8 diagnostic criteria are: 1, 2

1. Fever (persistent, unexplained)

2. Splenomegaly (clinical or radiographic)

3. Cytopenias affecting ≥2 of 3 lineages:

   • Hemoglobin < 90 g/L (< 100 g/L in infants < 4 weeks)
   • Platelets < 100 × 10⁹/L
   • Neutrophils < 1.0 × 10⁹/L

4. Hypertriglyceridemia and/or hypofibrinogenemia:

   • Fasting triglycerides ≥ 3.0 mmol/L (≥ 265 mg/dL)
   • Fibrinogen ≤ 1.5 g/L

5. Hemophagocytosis in bone marrow, spleen, or lymph nodes (with no evidence of malignancy)

6. Low or absent NK cell activity (according to local laboratory reference)

7. Ferritin ≥ 500 mg/L

8. Soluble CD25 (sCD25/soluble IL-2 receptor) ≥ 2400 U/mL

Critical Diagnostic Considerations

When to Suspect HLH Despite Not Meeting Full Criteria

Treatment may be initiated even when fewer than 5 criteria are met if clinical suspicion is high. 1 This is particularly important because HLH is rapidly fatal without treatment, and waiting for all criteria can delay life-saving therapy.

Hyperferritinemia as a Key Diagnostic Clue

• Ferritin > 10,000 mg/L is 90% sensitive and 96% specific for HLH and should immediately trigger comprehensive HLH evaluation 3
• Adult HLH typically presents with ferritin 7,000-10,000 mg/L, occasionally exceeding 100,000 mg/L 1, 2
• While the diagnostic threshold is only 500 mg/L, such modest elevations are nonspecific; values > 10,000 mg/L are far more diagnostically useful 1, 3

Soluble IL-2 Receptor (sCD25) Performance

• sCD25 has superior diagnostic accuracy compared to ferritin (AUC 0.90 vs 0.78) and is an excellent low-cost test for adult HLH 1, 2
• This marker may be particularly valuable when ferritin is equivocal or when rapid diagnosis is needed

The Challenge of Hemophagocytosis

• Hemophagocytosis may be absent initially and should not delay diagnosis or treatment 1, 2
• If initial bone marrow is negative, obtain material from other organs (liver, spleen, lymph nodes) or perform serial marrow aspirates over time 1, 2
• The absence of hemophagocytosis does not exclude HLH if other criteria are met

Alternative Diagnostic Tool: The HScore

For adults with suspected secondary HLH, the HScore provides a validated alternative to HLH-2004 criteria. 1, 2 This scoring system was developed specifically for adults and includes:

• Known underlying immunosuppression (HIV or long-term immunosuppressive therapy): 0 or 18 points
• Temperature: 0 (< 38.4°C), 33 (38.4-39.4°C), or 49 (> 39.4°C) points
• Organomegaly: 0 (none), 23 (hepatomegaly OR splenomegaly), or 38 (both) points
• Number of cytopenias: 0 (1 lineage), 24 (2 lineages), or 34 (3 lineages) points
• Ferritin: 0 (< 2000 mg/L), 35 (2000-6000 mg/L), or 50 (> 6000 mg/L) points
• Triglycerides: 0 (< 1.5 mmol/L), 44 (1.5-4 mmol/L), or 64 (> 4 mmol/L) points
• Fibrinogen: 0 (> 2.5 g/L) or 30 (≤ 2.5 g/L) points
• AST: 0 (< 30 U/L) or 19 (≥ 30 U/L) points
• Hemophagocytosis on bone marrow: 0 (no) or 35 (yes) points 1, 2

Supportive Findings That Strengthen the Diagnosis

Additional clinical and laboratory abnormalities that support HLH include: 1, 2

• CSF pleocytosis (mononuclear cells) and/or elevated CSF protein
• Liver biopsy showing chronic persistent hepatitis pattern
• Cerebromeningeal symptoms
• Lymph node enlargement
• Jaundice, edema, skin rash
• Hepatic enzyme abnormalities
• Hypoproteinemia, hyponatremia
• Elevated VLDL with low HDL

Critical Pitfalls to Avoid

Validation Limitations

• The HLH-2004 criteria were developed for children and remain based on expert opinion for adults, not formal validation 1, 2, 4
• Despite this limitation, they remain the standard diagnostic approach in clinical practice

Diagnostic Mimickers

• HLH can be extremely difficult to distinguish from sepsis/septic shock, systemic inflammatory response syndrome, or severe viral infections in critically ill patients 5, 4
• Many diagnostic features are nonspecific, requiring integration of multiple clinical findings rather than relying on any single criterion 1, 4

Laboratory Turnaround Time

• NK cell functional assays and sCD25 levels often require send-out to specialized laboratories, potentially delaying diagnosis by days 3, 4
• Ferritin is available same-day at most institutions and should be used as an immediate screening tool 3

The Danger of Delayed Diagnosis

• Do not wait for all diagnostic criteria or specialized test results before initiating treatment when clinical suspicion is high 1, 2
• HLH is rapidly fatal without treatment, and mortality increases significantly with diagnostic delays 5, 4
• Two patients in one case series died within days of HLH diagnosis from DIC and multiorgan failure 5