What are the diagnostic criteria and recommended workup for hemophagocytic lymphohistiocytosis (HLH)?

HLH Diagnosis

Diagnose HLH in adults using the HLH-2004 criteria: meeting 5 of 8 clinical/laboratory parameters OR having a molecular diagnosis consistent with HLH establishes the diagnosis, though clinical judgment may warrant initiating treatment even when fewer than 5 criteria are met. 1

Diagnostic Criteria: HLH-2004

The diagnosis requires 5 of the following 8 criteria 1:

1. Fever
2. Splenomegaly
3. Cytopenias (≥2 of 3 lineages):
   • Hemoglobin < 90 g/L
   • 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 (no evidence of malignancy)
6. Low or absent NK cell activity (per local laboratory reference)
7. Ferritin ≥ 500 μg/L
8. Soluble CD25 (sIL-2 receptor) ≥ 2400 U/mL

Critical Diagnostic Considerations

Ferritin as a Red Flag

Hyperferritinemia should always trigger consideration of HLH. 1 While the diagnostic threshold is ≥500 μg/L, ferritin levels of 7,000-10,000 μg/L are characteristic in adults, and levels >10,000 μg/L are >90% sensitive and specific in children. 1 However, ferritin is less specific in adults, requiring integration with other clinical features.

Soluble CD25 (sIL-2 receptor)

sCD25 is a superior diagnostic marker compared to ferritin with an area under the curve of 0.90 versus 0.78 for ferritin. 1 This is a cost-effective test that should be obtained early in the workup.

Hemophagocytosis Is Not Required

Absence of hemophagocytosis does not exclude HLH. 1 If initial bone marrow is non-diagnostic, consider:

• Serial bone marrow aspirates over time
• Sampling other organs (spleen, lymph nodes)
• Proceeding with treatment if clinical suspicion remains high

Essential Workup Components

Initial Laboratory Assessment

Order the following to evaluate HLH-2004 criteria:

• Complete blood count with differential
• Ferritin
• Triglycerides (fasting)
• Fibrinogen
• Soluble CD25 (sIL-2 receptor)
• NK cell function testing
• Liver enzymes (AST elevation supports diagnosis) 1

Additional Supportive Findings

Look for these features that strengthen the diagnosis 1:

• CNS involvement: CSF pleocytosis (mononuclear cells), elevated CSF protein
• Hepatic findings: Liver biopsy showing chronic persistent hepatitis pattern
• Other laboratory abnormalities: Hypoproteinemia, hyponatremia, elevated VLDL/low HDL
• Clinical features: Lymphadenopathy, jaundice, edema, skin rash

Mandatory Trigger Search

A meticulous search for underlying triggers must begin immediately and continue despite ongoing HLH treatment. 1 This is critical because:

In adults, the most common triggers are:

• Infections (most prevalent overall)
• Malignancies (particularly with increasing age):
  • T-cell/NK-cell lymphomas (35%)
  • B-cell lymphomas (32%)
  • Leukemias (6%)
  • Hodgkin lymphoma (6%)
  • Other hematologic neoplasms (14%)
  • Solid tumors (3%) 1

Workup for triggers includes:

• Comprehensive infectious disease evaluation (viral serologies, cultures, imaging)
• CT chest/abdomen/pelvis to evaluate for occult malignancy
• Bone marrow biopsy (evaluates both hemophagocytosis AND underlying malignancy)
• Consider PET-CT for lymphoma evaluation
• Rheumatologic workup if autoimmune features present

Genetic/Molecular Testing

Obtain genetic testing for HLH-associated genes, particularly in younger adults or those with family history. 1 A molecular diagnosis alone establishes the diagnosis without requiring clinical criteria.

Alternative Diagnostic Tools

HScore

The HScore is a validated alternative diagnostic tool 1 that incorporates:

• Known immunosuppression (0-18 points)
• Temperature (0-49 points)
• Organomegaly (0-38 points)
• Number of cytopenias (0-34 points)
• Ferritin level (0-50 points)
• Triglycerides (0-64 points)
• Fibrinogen (0-30 points)
• AST (0-19 points)
• Hemophagocytosis on bone marrow (0-35 points)

This can be useful when HLH-2004 criteria are incomplete, though the guidelines prioritize HLH-2004 criteria with clinical judgment.

Critical Pitfalls to Avoid

1. Don't wait for all 5 criteria: Treatment may need to start with high clinical suspicion even if <5 criteria are met 1
2. Don't dismiss the diagnosis with negative bone marrow: Hemophagocytosis may be absent early or require serial sampling 1
3. Don't stop searching for triggers: Continue investigating for occult malignancy and infections throughout treatment 1
4. Don't rely on ferritin alone: While highly elevated ferritin is suggestive, it's less specific in adults and must be interpreted with other criteria 1
5. Don't forget CNS evaluation: Consider lumbar puncture if neurologic symptoms present 1

Special Populations

Macrophage Activation Syndrome (MAS): This term refers to HLH arising from autoimmune/autoinflammatory conditions (Still's disease, lupus, vasculitis). 1 Treatment may differ from other forms of secondary HLH, so identifying this subtype is important.