Laboratory Tests for Diagnosing Hemophagocytic Lymphohistiocytosis (HLH)
The diagnosis of HLH requires either genetic testing showing mutations in HLH-associated genes or fulfillment of 5 out of 8 specific diagnostic criteria according to the HLH-2004 guidelines, which include fever, splenomegaly, cytopenias, hypertriglyceridemia/hypofibrinogenemia, hemophagocytosis, low NK cell activity, elevated ferritin, and elevated soluble IL-2 receptor. 1
Core Laboratory Tests for HLH Diagnosis
Complete Blood Count (CBC): To identify cytopenias affecting ≥2 of 3 lineages in peripheral blood 2:
- Hemoglobin < 90 g/L (< 100 g/L in infants < 4 weeks)
- Platelets < 100 × 10^9/L
- Neutrophils < 1.0 × 10^9/L
Ferritin: Levels ≥ 500 mg/L meet the diagnostic criteria, but values > 7,000-10,000 mg/L (sometimes > 100,000 mg/L) are more characteristic of HLH in adults 2, 1
Fasting Triglycerides: Levels ≥ 3.0 mmol/L (≥ 265 mg/dL) 2
Fibrinogen: Levels ≤ 1.5 g/L 2
Soluble IL-2 Receptor (sCD25): Levels ≥ 2400 U/mL; reported as an excellent diagnostic test for adult HLH with an area under the curve of 0.90 2, 1
NK Cell Activity: Low or absent NK cell activity according to local laboratory reference 2
Bone Marrow Aspiration/Biopsy: To identify hemophagocytosis (macrophages engulfing other blood cells) 2
- If initial bone marrow examination is negative, serial marrow aspirates or biopsies from other organs (spleen, lymph nodes, liver) may be necessary 2
Additional Laboratory Tests
Liver Function Tests: Elevated AST/ALT may support diagnosis and are included in the HScore 2
Coagulation Studies: Beyond fibrinogen, to assess for coagulopathy 2
Cerebrospinal Fluid Analysis: For protein and cell count if neurological symptoms are present 2
Genetic Testing: For primary HLH diagnosis, particularly in children or those with family history 3, 1
- Tests for mutations in PRF1 (FHL2), UNC13D (FHL3), STX11 (FHL4), and other HLH-associated genes 2
Flow Cytometry: To detect NK/T-cell degranulation defects and determine expression of relevant proteins like perforin and SAP (XLP1) 2
HScore Components
The HScore is an alternative scoring system developed specifically for adults with suspected secondary HLH, which includes 2:
- Known underlying immunosuppression
- Temperature
- Organomegaly
- Number of cytopenias
- Ferritin level
- Triglyceride level
- Fibrinogen level
- AST level
- Hemophagocytosis on bone marrow aspirate
Important Clinical Considerations
Hyperferritinemia should always prompt inclusion of HLH in the differential diagnosis, though it is less specific in adults than in children 2, 1
Hemophagocytosis alone is neither sensitive nor specific for HLH and may be present in other conditions like septicemia 2, 3
In the context of malignancy, several HLH features may be present due to the neoplasm itself (fever, organomegaly, cytopenias, elevated LDH, coagulation disturbances) 2
Do not delay treatment for HLH if strongly suspected, even if all 5 criteria are not yet fulfilled 2, 1
In patients with suspected malignancy-associated HLH, additional testing should include screening for blasts in peripheral blood and bone marrow, and appropriate imaging studies 2
In male patients with lymphoma and EBV-driven HLH, genetic or flow cytometric analysis for XLP1 should be strongly considered 2
Pitfalls to Avoid
Relying solely on hemophagocytosis for diagnosis, as it may be absent initially or present in other conditions 2, 3
Confusing elevated ferritin from transfusion-related iron overload with HLH-related hyperferritinemia 2
Failing to recognize that the HLH-2004 criteria were developed for children and are not formally validated in adults, though they remain the standard diagnostic approach 2, 1
Delaying treatment while waiting for all diagnostic criteria to be met in a critically ill patient 2, 4