Diagnosing hemophagocytic lymphohistiocytosis requires careful evaluation of multiple clinical signs and laboratory tests, as this rare condition can easily be mistaken for other illnesses due to its similarities with common infections and other diseases.

Introduction: When to Seek Diagnostics

Hemophagocytic lymphohistiocytosis is extremely rare, affecting approximately 1 in 50,000 people worldwide, though the true number may be higher because many cases go undiagnosed or are mistaken for other conditions.^[1] Because this condition is so uncommon, many healthcare providers may not immediately recognize its symptoms, which can lead to delays in diagnosis. These delays can have serious consequences, as the disease can progress rapidly without proper treatment.^[6]

Anyone experiencing persistent fever that does not respond to antibiotics should consider seeking medical evaluation, especially if accompanied by other concerning symptoms. Parents should be particularly vigilant with infants and young children, as about 70% of those with primary, or familial, HLH develop symptoms within their first year of life.^[1] However, the condition can appear at any age, including in teenagers and adults, particularly when triggered by infections, cancers, or autoimmune diseases.^[6]

Early diagnosis is crucial because hemophagocytic lymphohistiocytosis can quickly become life-threatening. Without treatment, the median survival can be as low as two months.^[6] When symptoms suggest HLH, time matters greatly. The faster the condition is identified and treatment begins, the better the chances of survival and recovery.

People who should seek diagnostic evaluation include those with unexplained fevers lasting more than a week, especially when combined with enlarged organs like the liver or spleen, unexplained rashes, or signs of easy bruising and bleeding. Family members of someone diagnosed with primary HLH should also be aware that siblings have a 25% chance of developing the condition if it runs in the family, since it follows an autosomal recessive pattern (meaning both parents carry a defective gene).^[7]

Classic Diagnostic Methods for Identifying HLH

Diagnosing hemophagocytic lymphohistiocytosis presents unique challenges because its initial symptoms often look like many other more common illnesses. A child with HLH might appear to have a simple infection at first, making it easy to miss the underlying serious condition. The disease can even mimic certain cancers like leukemia and lymphoma in its early stages, which makes proper testing essential to avoid misdiagnosis.^[4]

Clinical Presentation and Physical Examination

The diagnostic journey typically begins with a thorough physical examination and review of symptoms. Doctors look for a specific pattern of clinical signs that together point toward HLH rather than other conditions. The most prominent features include persistent high fever that does not improve with antibiotics, an enlarged spleen (called splenomegaly), and an enlarged liver (called hepatomegaly).^[1] Many patients also develop swollen lymph nodes, skin rashes, and yellowing of the skin and eyes known as jaundice.^[4]

Other observable symptoms may include pale skin, easy bruising with purple or red spots, and unusual bleeding. Some people develop swollen or bleeding gums that can even lead to tooth loss. Infants may have feeding problems and fail to gain weight appropriately. Neurological symptoms are particularly concerning and can include seizures, confusion, changes in mental status, difficulty with coordination (called ataxia), weakness on one side of the body, or irritability.^[9]

Essential Laboratory Blood Tests

Blood tests form the backbone of HLH diagnosis. These tests reveal abnormalities in blood cell counts and markers of inflammation that help distinguish HLH from other conditions. One of the most critical findings is pancytopenia, which means low counts of all three main types of blood cells: red blood cells (causing anemia), white blood cells (particularly neutrophils), and platelets (causing thrombocytopenia).^[7] When platelets drop to dangerously low levels, as they often do in HLH, the blood cannot clot properly, putting patients at risk for internal bleeding that could be fatal.^[14]

Doctors also measure specific inflammatory markers in the blood. Ferritin, a protein that stores iron, becomes dramatically elevated in HLH—often reaching levels far higher than seen in other conditions. In fact, extremely high ferritin levels combined with enlarged liver and spleen are found in almost all HLH patients.^[7] Another useful marker is soluble IL-2 receptor alpha, which can be tracked over time to monitor disease activity and response to treatment.^[7]

Additional blood chemistry tests reveal elevated triglycerides (a type of fat in the blood) and low fibrinogen (a protein needed for blood clotting). Some patients develop a dangerous condition called disseminated intravascular coagulation or DIC, where blood clots form throughout the body while simultaneously causing severe bleeding.^[7] Liver function tests typically show abnormalities, with elevated enzymes called transaminases (ALT and AST) and lactate dehydrogenase.^[7]

Specialized Immune Function Testing

One of the most valuable diagnostic tests for HLH measures the function of natural killer cells, or NK cells, which are specialized white blood cells that help fight infections. Decreased NK cell activity occurs in up to 90% of patients with HLH, making this one of the most useful laboratory tests available.^[9] However, this test requires specialized laboratory capabilities and may not be available at all medical centers.

Doctors may also evaluate how well cytotoxic T cells (another type of immune cell) are functioning. In HLH, these cells become overactive but ineffective, producing excessive amounts of chemical messengers called cytokines. This leads to what is sometimes called a “cytokine storm,” causing widespread inflammation and tissue damage throughout the body.^[6]

Tissue Biopsies and Microscopic Examination

To confirm the diagnosis, doctors often need to examine tissue samples under a microscope. The hallmark finding is hemophagocytosis, which means that certain immune cells called macrophages or histiocytes are engulfing and destroying blood cells. This abnormal activity must be documented in bone marrow, spleen, or lymph nodes to establish the diagnosis.^[9]

A bone marrow biopsy is the most common procedure used to look for hemophagocytosis. During this test, a doctor removes a small sample of bone marrow tissue, usually from the hip bone, to examine under a microscope. A liver biopsy may also be performed if liver involvement is suspected.^[4] In some cases, doctors may examine lymph nodes that have been surgically removed or may take samples from the spleen during other procedures.

Importantly, hemophagocytosis may not be visible in the early stages of disease or in every tissue sample examined. This means that not seeing it does not rule out HLH if other criteria are met. Repeated sampling may be necessary in some cases.^[19]

Imaging Studies

While no specific imaging patterns definitively diagnose HLH, various scans help assess organ involvement and complications. Doctors may perform computed tomography (CT) scans or ultrasound examinations to measure the size of the liver and spleen and to look for enlarged lymph nodes throughout the body.^[9]

When neurological symptoms are present, doctors often obtain imaging of the brain and may collect cerebrospinal fluid through a lumbar puncture (also called a spinal tap). This procedure involves inserting a needle between bones in the lower back to collect fluid that surrounds the brain and spinal cord, which can be examined for signs of HLH affecting the central nervous system.^[4]

Formal Diagnostic Criteria

To standardize diagnosis across different medical centers, the Histiocyte Society established specific criteria. Doctors use a system where HLH can be diagnosed if at least five of the following eight criteria are met:^[9]

• Fever (persistent high temperature)
• Splenomegaly (enlarged spleen)
• Low blood cell counts affecting at least two of three cell types (red blood cells, white blood cells, or platelets)
• High triglycerides in the blood or low fibrinogen
• Evidence of hemophagocytosis in bone marrow, spleen, or lymph nodes
• Low or absent NK cell activity
• Extremely high ferritin levels
• Elevated soluble IL-2 receptor levels

Some medical centers also use a scoring system called HScore, which assigns points to various clinical and laboratory findings to calculate the probability that a patient has HLH.^[10]

Genetic Testing

For patients suspected of having primary or familial HLH, genetic testing plays a crucial role in confirming the diagnosis and guiding treatment decisions. Scientists have identified numerous genetic mutations that cause primary HLH, affecting genes with names like PRF1, UNC13D, STX11, STXBP2, and others.^[1] These genes provide instructions for making proteins that help immune cells function properly.

Finding a specific genetic mutation confirms that HLH is inherited rather than acquired, which has important implications for family members and treatment planning. However, not all cases of primary HLH have an identified genetic cause yet, and some genetic forms may not be detected by current testing methods.^[5] Early genetic testing is particularly important because identifying inherited forms helps doctors determine whether bone marrow transplantation will be necessary for long-term survival.^[10]

Distinguishing HLH from Other Conditions

One of the greatest challenges in diagnosing HLH is distinguishing it from conditions with similar presentations. Doctors must carefully rule out infections that can cause similar symptoms, particularly severe bacterial infections causing sepsis. Many viral, bacterial, and fungal infections can trigger secondary HLH or mimic its symptoms.^[2]

Blood cancers, especially leukemia and lymphoma, must also be excluded through appropriate testing. Some children with HLH may initially appear to have cancer, making bone marrow examination and other cancer-specific tests necessary.^[4] Autoimmune diseases can also present with overlapping features, adding another layer of diagnostic complexity.

Specific genetic syndromes share features with HLH and must be considered. For example, Griscelli syndrome is a rare genetic condition that can include HLH along with distinct features like silvery-gray hair and neurological problems. Other related syndromes include Chediak-Higashi syndrome and Hermansky-Pudlak syndrome.^[2]

Diagnostics for Clinical Trial Qualification

When patients with HLH are considered for enrollment in clinical trials, additional diagnostic procedures and criteria may apply beyond those used in routine clinical practice. Clinical trials study new treatments or approaches to managing HLH, and they require very specific and standardized methods of confirming diagnosis and measuring disease activity.

Most clinical trials require that HLH diagnosis be confirmed according to the formal HLH-2004 diagnostic criteria before a patient can participate. This means documentation of at least five of the eight standard diagnostic criteria, with particular emphasis on laboratory confirmation rather than clinical assessment alone.^[10] Some trials may require even more stringent documentation, such as mandatory genetic testing results or confirmed hemophagocytosis visible on tissue biopsy.

For trials studying primary HLH, genetic confirmation of a disease-causing mutation is often required. This ensures that the study population is homogeneous and that results can be interpreted specifically for inherited forms of the disease. The genetic testing must typically be performed in certified laboratories following standardized protocols.^[10]

Clinical trials also frequently require baseline measurements of specific biomarkers that will be monitored throughout the study. These might include precise measurements of ferritin levels, soluble IL-2 receptor concentrations, and cytokine levels in the blood. NK cell function testing and T cell activity assays may be performed at specialized research laboratories to ensure consistency across all study participants.^[2]

Imaging studies for trial enrollment are often more comprehensive than routine clinical care. Trials may require baseline MRI scans of the brain to document any central nervous system involvement, even in patients without obvious neurological symptoms. CT scans or ultrasound measurements of organ sizes may need to be performed using standardized protocols at specific time points.^[9]

Some clinical trials studying new targeted therapies require additional specialized testing. For example, trials investigating treatments that block specific cytokines may require measurement of those exact cytokines before and during treatment. Trials testing drugs that affect interferon-gamma, a key inflammatory messenger in HLH, would require baseline testing of this substance and related pathways.^[10]

Exclusion criteria for clinical trials may also involve diagnostic testing. Participants might need testing to confirm they do not have certain infections, cancers, or other medical conditions that could interfere with the study or make it unsafe for them to receive the experimental treatment. This might include comprehensive infection screening, cancer-specific tests, and evaluation of organ function through blood tests and imaging.

The timing of diagnostic testing relative to treatment is also standardized in clinical trials. Baseline tests must often be completed within a specific timeframe before the first dose of study medication. Follow-up diagnostic tests are then scheduled at predetermined intervals to assess response to treatment and monitor for complications or disease progression.

Bone marrow examinations may be required at specific time points during a clinical trial, not just at diagnosis. Serial bone marrow biopsies help researchers understand how the disease responds to treatment at a cellular level and can provide valuable information about why some patients respond better than others.

Quality control of diagnostic testing is particularly important in clinical trials. Laboratories performing tests for trial participants often must meet specific accreditation standards and may need to participate in proficiency testing programs. Some trials require that certain key tests be performed at central reference laboratories rather than local hospitals to ensure consistency of methods and interpretation across all study sites.^[18]