HLH Diagnostic Criteria and Practical Guidance

1. Diagnostic Framework (Histiocyte Society)

• Diagnosis of hemophagocytic lymphohistiocytosis (HLH) requires either a molecular confirmation consistent with HLH or fulfillment of 5 of the 8 HLH‑2004 criteria; however, therapy may be started on strong clinical suspicion even when fewer than five criteria are present. 1, 2

2. HLH‑2004 Criteria (require 5 of 8)

All eight criteria are derived from the HLH‑2004 protocol endorsed by the Histiocyte Society. 1

3. Key Laboratory Markers

• Ferritin – Markedly elevated ferritin should always raise suspicion for HLH; adult patients often show levels 7,000–10,000 µg/L, and extreme values can exceed 100,000 µg/L. Hyperferritinemia is less specific in adults than in children, so it must be interpreted alongside other clinical features. 1, 2

• Soluble CD25 (sIL‑2R) – sCD25 outperforms ferritin for adult HLH diagnosis, achieving an area under the ROC curve of 0.90 (95 % CI 0.83–0.97) versus ferritin’s 0.78 (95 % CI 0.67–0.88). It is recommended for all patients with suspected HLH. 1

4. Hemophagocytosis Assessment

• Absence of hemophagocytosis on the initial bone‑marrow aspirate does not exclude HLH. When not observed, clinicians should obtain serial marrow samples or evaluate other tissues (spleen, lymph nodes, liver) because hemophagocytosis often appears later in the disease course. 1, 2

5. Alternative Scoring System – HScore (Adults)

• The HScore incorporates several laboratory and clinical variables, including fibrinogen level as a component. (Other variables are listed in the original score but lack cited evidence in this source.) 1

6. Supportive Diagnostic Findings

• Liver biopsy demonstrating a chronic persistent hepatitis pattern can strengthen the diagnosis of HLH in the appropriate clinical context. 1

7. Management Considerations

• Early treatment – HLH‑directed therapy may be initiated based on strong clinical suspicion even when fewer than five HLH‑2004 criteria are met, given the high mortality of untreated disease. 1, 2

• Search for underlying triggers – In adults, the most common precipitants are infections (e.g., EBV, CMV). Malignancies become increasingly prevalent with age, affecting ≈ 68 % of patients > 60 years, with 35 % due to T‑cell/NK‑cell lymphomas and 32 % due to B‑cell lymphomas. Identifying and treating the trigger is essential. 2

• Validation limitation – The HLH‑2004 criteria were originally developed for pediatric patients and have not been formally validated in adult populations, yet they remain the standard framework pending adult‑specific validation. Clinical judgment is required when applying them to adults. 1, 2

Diagnostic Criteria for Hemophagocytic Lymphohistiocytosis (HLH)

Primary Diagnostic Approaches

• The diagnosis of HLH requires either a molecular diagnosis consistent with HLH or fulfillment of 5 out of 8 specific diagnostic criteria according to the HLH-2004 guidelines, which include splenomegaly, cytopenias, hypertriglyceridemia, and hemophagocytosis in bone marrow, spleen, or lymph nodes without evidence of malignancy 3, 4
• A molecular diagnosis consistent with HLH can be made through genetic testing showing mutations in HLH-associated genes 3
• The 8 diagnostic criteria for HLH include fever, splenomegaly, cytopenias affecting ≥2 of 3 lineages in peripheral blood, hypertriglyceridemia and/or hypofibrinogenemia, hemophagocytosis, low or absent NK cell activity, ferritin ≥500 mg/L, and soluble IL-2 receptor (sCD25) ≥2400 U/mL 3, 4, 5

Important Considerations for Diagnosis

• Clinical judgment is essential in diagnosing HLH, as the HLH-2004 criteria were developed for children and are not formally validated in adults, though they remain the standard diagnostic approach 4
• Hyperferritinemia is a key marker for HLH, with values >7,000-10,000 mg/L (and sometimes >100,000 mg/L) being more characteristic of HLH in adults 4, 5
• Soluble IL-2 receptor (sCD25) has been reported as an excellent diagnostic test for adult HLH, with an area under the curve of 0.90, compared to 0.78 for ferritin 3
• Hemophagocytosis may not be present initially, and further searches in other organs or serial marrow aspirates over time are recommended if not found on initial bone marrow examination 3, 4

Alternative Diagnostic Tool: HScore

• The HScore is an alternative scoring system developed specifically for adults with suspected secondary HLH, which includes parameters such as known underlying immunosuppression, organomegaly, number of cytopenias, ferritin level, triglyceride level, fibrinogen level, AST level, and hemophagocytosis on bone marrow aspirate 3, 4, 5, 6

Supportive Diagnostic Findings

• Additional findings that support the diagnosis of HLH include spinal fluid pleocytosis, elevated spinal fluid protein, liver histology resembling chronic persistent hepatitis, cerebromeningeal symptoms, lymph node enlargement, jaundice, edema, skin rash, hepatic enzyme abnormalities, hypoproteinemia, hyponatremia, and elevated VLDL and low HDL 3, 4, 5

Pitfalls and Caveats

• Do not delay treatment for HLH, as it may be strongly considered and treatment initiated even if all 5 criteria are not yet fulfilled 4
• Hyperferritinemia is less specific for HLH in adults than in children, and a meticulous search for underlying triggers should be conducted even during ongoing HLH treatment 4, 5, 6, 7

HLH‑2004 Diagnostic Criteria and Adult Diagnostic Considerations

Diagnostic Framework

• The diagnosis of hemophagocytic lymphohistiocytosis (HLH) requires either molecular confirmation of an HLH‑associated genetic mutation or fulfillment of five of eight clinical and laboratory criteria (fever, splenomegaly, cytopenias, hypertriglyceridemia/hypofibrinogenemia, hemophagocytosis, low/absent NK‑cell activity, ferritin ≥ 500 µg/L, soluble CD25 ≥ 2400 U/mL) as defined by the HLH‑2004 guidelines. 8

Most Valuable Diagnostic Markers in Adults

• Soluble CD25 (sCD25) is superior to ferritin for adult HLH diagnosis, showing an area under the ROC curve of 0.90 (95 % CI 0.83–0.97) versus ferritin’s 0.78 (95 % CI 0.67–0.88); it should be measured in all patients with suspected HLH. (Diagnostic performance evidence) 8

• Absence of hemophagocytosis on the initial bone‑marrow aspirate does NOT exclude HLH; serial marrow examinations or evaluation of spleen, lymph nodes, or liver are recommended because hemophagocytosis may appear later in the disease course. (Clinical observation) 8

Alternative Scoring System: HScore (Adults)

• In critically ill adults, an HScore ≥ 168 yields 100 % sensitivity and 94.1 % specificity, slightly outperforming the HLH‑2004 criteria. (Validation data) 8

• The HScore incorporates nine weighted parameters: underlying immunosuppression, temperature range, organomegaly (hepatomegaly/splenomegaly), number of cytopenias, ferritin level, triglyceride level, fibrinogen level, AST level, and presence of hemophagocytosis on bone‑marrow aspirate. (Scoring components) 8

Validation Limitations and Clinical Judgment

• The HLH‑2004 criteria were originally developed for pediatric patients and have never been formally validated in adults, yet they remain the standard diagnostic framework for adult HLH. (Guideline limitation) 8

Supportive Findings (Non‑Formal Criteria)

• Cerebrospinal fluid (CSF) abnormalities—pleocytosis or elevated protein—support the diagnosis of HLH. 8

• Liver histology resembling chronic persistent hepatitis is a supportive finding. 8

• Extracutaneous manifestations such as cerebromeningeal symptoms, lymphadenopathy, jaundice, edema, or skin rash add diagnostic weight. 8

• Metabolic and laboratory abnormalities including hypoproteinemia and hyponatremia are frequently observed and reinforce clinical suspicion. 8