Familial haemophagocytic lymphohistiocytosis – Diagnostics – Overview of Information and Clinical Research

Familial haemophagocytic lymphohistiocytosis is a severe condition where the immune system turns against the body, causing widespread damage and life-threatening complications. Early and accurate diagnosis is crucial, as timely detection can make the difference between life and death, opening the door to treatments that may save a child’s life.

Introduction: Who Needs Diagnostic Testing and When

Familial haemophagocytic lymphohistiocytosis, often called familial HLH or fHLH, requires immediate medical attention when symptoms appear. This is not a condition that waits or improves on its own. Parents and caregivers should seek diagnostic evaluation as soon as a child develops persistent high fever that does not respond to antibiotics, especially when accompanied by other concerning signs.^[1]

Most commonly, familial HLH affects infants and young children, with approximately seventy percent of cases appearing before a child reaches one year of age. However, symptoms can develop at any time, from before birth through childhood and even into adulthood, making it essential to remain vigilant regardless of age.^[6] Children who have a sibling previously diagnosed with HLH should undergo testing even without symptoms, as familial HLH follows a pattern where each sibling has a twenty-five percent chance of developing the disease.^[6]

The urgency of seeking diagnostics cannot be overstated. Without treatment, infants with active familial HLH typically survive less than two months after symptoms begin. The disease progresses rapidly, with the immune system’s overactivity causing damage to multiple organs simultaneously.^[1] Because the symptoms can resemble other serious conditions like severe infections or sepsis, healthcare providers need a high degree of suspicion to pursue the right diagnostic path.

⚠️ Important

If your child develops a high fever that does not improve with antibiotics, along with an enlarged liver or spleen, seek immediate medical evaluation. The similarity between HLH symptoms and common infections can lead to dangerous delays in diagnosis. Early detection significantly improves the chances of successful treatment and survival.

Doctors should consider HLH testing when children present with fever and unusual combinations of symptoms that do not fit typical infection patterns. The presence of hepatosplenomegaly, meaning an enlarged liver and spleen, together with fever and skin rashes, should raise concern. Neurological symptoms such as seizures, confusion, or unusual irritability in infants also warrant investigation, as the brain can be affected by the inflammatory process.^[4]

Classic Diagnostic Methods

Diagnosing familial HLH involves a combination of clinical observation, laboratory testing, and genetic analysis. The diagnostic process aims to distinguish HLH from other conditions that may appear similar, such as severe infections, certain cancers, or other immune system disorders.

HLH-2004 Diagnostic Criteria

The most widely used framework for diagnosing HLH comes from the HLH-2004 protocol, which was originally designed as inclusion criteria for a treatment study but has become the standard diagnostic tool worldwide. According to these criteria, a diagnosis of HLH can be made in two ways: either through genetic testing showing specific mutations, or by meeting at least five of eight clinical and laboratory criteria.^[9]

The eight criteria include fever, enlarged spleen and liver, low blood cell counts affecting at least two types of blood cells (a condition called cytopenia), elevated levels of fats in the blood called triglycerides, low levels of a clotting protein called fibrinogen, evidence of hemophagocytosis (where immune cells eat other blood cells) found in bone marrow or other tissues, reduced or absent natural killer cell activity, and extremely high levels of ferritin, an iron-storage protein.^[6]

These criteria help doctors identify the characteristic pattern of immune system overactivation. However, it is important to understand that not every child with HLH will meet all criteria at the same time, and some findings may appear later in the disease course. This is why clinical judgment and repeated testing may be necessary.

Laboratory Blood Tests

Blood tests form the cornerstone of HLH diagnosis and reveal the widespread effects of immune system dysfunction. A complete blood count typically shows anemia (low red blood cells), thrombocytopenia (low platelets), and sometimes neutropenia (low white blood cells of a specific type). These low counts occur because the overactive immune system attacks blood-producing cells in the bone marrow.^[4]

Ferritin levels are particularly important in HLH diagnosis. While ferritin normally helps store iron in the body, extremely elevated levels—often many times higher than normal—are almost universally seen in HLH. Doctors can track ferritin levels over time to monitor disease activity and response to treatment.^[6]

Triglyceride levels in the blood become elevated due to the inflammatory process affecting how the body processes fats. Fibrinogen, a protein essential for blood clotting, becomes depleted, which can lead to dangerous bleeding problems. Some patients develop a serious condition called disseminated intravascular coagulation or DIC, where the blood clotting system becomes dangerously disordered.^[6]

Liver function tests often show abnormalities, with elevated transaminases (liver enzymes called ALT and AST) indicating liver damage from the inflammatory process. The combination of elevated ferritin, transaminases, and enlarged liver and spleen is seen in nearly all HLH cases.^[6]

Bone Marrow Examination

A bone marrow biopsy allows doctors to look directly at the cells inside the bone where blood is produced. In HLH, this examination may reveal hemophagocytosis, the hallmark finding where large immune cells called macrophages are seen engulfing other blood cells. However, this finding is not always present, especially early in the disease, and its absence does not rule out HLH. The bone marrow examination also helps exclude other conditions such as leukemia or lymphoma that might cause similar symptoms.^[3]

Natural Killer Cell Function Testing

Testing the function of natural killer cells, a type of white blood cell that normally helps destroy infected or abnormal cells, provides important diagnostic information. In familial HLH, these cells do not work properly. The test measures how well these cells can kill target cells in laboratory conditions. Reduced or absent natural killer cell activity supports the diagnosis of HLH, though this test requires specialized laboratory capabilities and may not be available everywhere.^[6]

Genetic Testing

Genetic testing provides definitive diagnosis of familial HLH by identifying mutations in specific genes. The four main genes associated with familial HLH are PRF1, UNC13D, STX11, and STXBP2. These genes normally provide instructions for making proteins that help immune cells destroy their targets and then shut down properly. When both copies of one of these genes carry mutations (called biallelic pathogenic variants), the immune system cannot regulate itself correctly.^[1]

Approximately forty to sixty percent of familial HLH cases are caused by mutations in the PRF1 or UNC13D genes, while smaller numbers result from mutations in other known genes. In some affected children, the genetic cause remains unknown even after extensive testing.^[2] Genetic testing is especially important for families with a child diagnosed with HLH, as it helps identify siblings who may carry the mutations and be at risk, even if they appear healthy.

HScore Calculation

In addition to the HLH-2004 criteria, doctors may use a scoring system called HScore, which assigns points to various clinical and laboratory features to estimate the probability of HLH. This tool incorporates factors like fever pattern, organ enlargement, blood cell counts, ferritin levels, and other laboratory values. The HScore helps clinicians quantify their diagnostic suspicion and may be particularly useful in complex cases.^[7]

Distinguishing HLH from Similar Conditions

One of the most challenging aspects of diagnosing familial HLH is distinguishing it from other conditions that produce similar symptoms. Severe infections, particularly sepsis, can cause fever, low blood counts, and organ dysfunction that closely mimic HLH. The key difference is that in HLH, the symptoms persist despite appropriate antibiotic treatment for infection.^[6]

Certain genetic syndromes share features with HLH. Griscelli syndrome type 2 is one such condition where children have silvery-gray hair and immune dysfunction similar to HLH. Other syndromes with overlapping features include Chédiak-Higashi syndrome and Hermansky-Pudlak syndrome type 2. Recognizing these conditions requires attention to additional physical features beyond the typical HLH symptoms.^[6]

Blood cancers such as leukemia and lymphoma can also cause fever, enlarged organs, and abnormal blood counts. Bone marrow examination and specific blood tests help differentiate these malignancies from HLH, though in some cases, HLH can be triggered by underlying cancer.

Diagnostics for Clinical Trial Qualification

When families and doctors consider enrolling a child in a clinical trial for HLH, specific diagnostic requirements must be met. Clinical trials establish strict enrollment criteria to ensure that participants truly have the condition being studied and to create comparable groups for evaluating new treatments.

For most HLH clinical trials, participants must meet the HLH-2004 diagnostic criteria, which serve as the standardized definition of the disease for research purposes. This means either having confirmed genetic mutations in one of the HLH-associated genes, or meeting five of the eight clinical and laboratory criteria described earlier. Trial protocols typically require documentation of these criteria within a specific timeframe before enrollment.^[9]

Genetic testing results carry particular weight in clinical trial enrollment. Trials focused on familial HLH often require confirmed biallelic pathogenic variants in PRF1, STX11, STXBP2, or UNC13D genes. The genetic diagnosis must be established through testing at a certified laboratory that meets quality standards. Families should retain copies of genetic test results, as these may be needed to qualify for current or future clinical trials.^[1]

Laboratory values at the time of enrollment often must fall within specific ranges. For instance, ferritin levels might need to exceed a certain threshold, or blood cell counts must show specific degrees of cytopenia. Some trials require evidence of active disease, meaning the child must have ongoing symptoms and laboratory abnormalities at the time of enrollment, rather than being in remission from previous treatment.

⚠️ Important

Keep detailed medical records including all laboratory test results, bone marrow biopsy reports, and genetic testing documentation. These records are essential for clinical trial enrollment and may be needed quickly if a suitable trial becomes available. Ask your medical team for copies of all important test results and store them in an organized, accessible manner.

Natural killer cell function testing may be required as part of trial qualification, particularly for studies investigating how treatments affect immune cell activity. Bone marrow examination results showing hemophagocytosis may strengthen a patient’s eligibility, though as noted earlier, the absence of this finding does not necessarily disqualify a patient if other criteria are met.

Age restrictions are common in clinical trials. Some trials focus specifically on infants and young children, while others may include patients up to certain ages. The age at diagnosis versus the age at trial enrollment can both be relevant criteria. Trials may also specify whether patients who have previously received certain treatments are eligible or excluded.

Organ function testing results help determine if a patient can safely participate in a trial. Kidney function tests, liver function tests, heart function assessments, and neurological evaluations may all be required before enrollment. Trials testing new medications need to ensure that participants have adequate organ function to metabolize and eliminate the drugs safely.

For trials comparing new treatments to standard therapy, patients typically cannot have already received the standard treatment being used as the comparison. This ensures that researchers can fairly evaluate whether the new approach offers benefits over established methods. However, other trials specifically enroll patients who have already tried standard treatments without success.

Disease registries, such as the INTO-HLH Registry, collect comprehensive diagnostic and treatment information from HLH patients worldwide. While not clinical trials themselves, these registries help researchers identify patients who might be eligible for future trials and provide valuable data about the natural course of the disease. Participation typically requires documented diagnosis meeting HLH-2004 criteria and consent to share medical information in a de-identified manner.^[13]

Prognosis and Survival Rate

Prognosis

The outlook for children with familial haemophagocytic lymphohistiocytosis has improved significantly with advances in treatment, though the disease remains extremely serious. Several factors influence prognosis, including the speed of diagnosis, the severity of organ damage at presentation, the response to initial treatment, and access to specialized care including the ability to undergo hematopoietic stem cell transplantation.

Without treatment, the prognosis for familial HLH is extremely poor. The disease progresses rapidly, and untreated infants typically survive less than two months after symptoms appear. Death usually results from progressive organ damage, severe infections that occur because of immune system dysfunction, or dangerous bleeding caused by low platelet counts and clotting problems.^[1]

Early recognition and prompt initiation of treatment dramatically change the trajectory of the disease. Modern chemoimmunotherapy protocols such as HLH-94 and HLH-2004, which use medications to suppress the overactive immune system, have significantly improved outcomes. These treatments can control the acute inflammatory crisis and stabilize patients, creating a window of opportunity for definitive therapy.^[1]

The presence of neurological symptoms at diagnosis worsens prognosis. Brain involvement indicates more widespread disease and can lead to lasting complications including seizures, developmental delays, coordination problems, and cognitive difficulties. Even with successful treatment of the underlying immune disorder, some children experience long-term neurological effects.^[1]

Allogeneic hematopoietic stem cell transplantation remains the only curative therapy for familial HLH. This procedure replaces the patient’s faulty immune system with healthy donor cells. Success of transplantation depends on multiple factors including the patient’s condition at the time of transplant, the quality of donor match, and the experience of the transplant center. Children who reach transplantation in good condition, with disease well-controlled by initial therapy, have better outcomes than those who arrive at transplant with active, uncontrolled disease.^[1]

If left untreated or if treatment is delayed, primary HLH is usually fatal within a few months. However, even with treatment, the prognosis can be limited to a few years unless bone marrow transplantation can be successfully performed. For secondary forms of HLH that occur without the genetic mutations seen in familial disease, prompt detection and aggressive treatment may offer better outcomes, though this depends heavily on the underlying trigger and the patient’s overall health.^[3]

Survival Rate

Survival rates for familial HLH have improved substantially with the development of better treatment protocols and increased awareness leading to earlier diagnosis. However, specific survival percentages vary depending on multiple factors including whether the patient receives optimal treatment and undergoes successful stem cell transplantation.

Historical data shows that without any treatment, median survival for infants who develop active familial HLH is less than two months from the onset of symptoms. This stark statistic emphasizes why immediate recognition and treatment initiation are so critical. The development of life-threatening complications happens rapidly in untreated cases.^[1]

The introduction of modern chemoimmunotherapy protocols followed by allogeneic hematopoietic stem cell transplantation has significantly improved survival. While exact survival percentages are not specified in the available sources, the shift from nearly universal mortality within months to the possibility of long-term survival represents a major advancement in treating this devastating disease.^[1]

It is important to understand that survival statistics represent averages across many patients and cannot predict outcomes for any individual child. Each case is unique, influenced by factors such as the specific genetic mutation present, the age at disease onset, the extent of organ involvement, how quickly treatment begins, complications that may arise during therapy, and the success of finding a suitable stem cell donor. Families should discuss their child’s specific situation and prognosis with their medical team, who can provide personalized information based on the individual circumstances.

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