It is sometimes difficult to establish the diagnosis of Haemophagocytic Lymphohistiocytosis (HLH), and the combination of the physical symptoms and certain laboratory tests is required. (Note: The understanding of the pathology underlying HLH/FHL disease is evolving, and recommended “diagnostic” criteria are likely to be revised in the future.)

• Low or absent NK (natural killer) cell function.
• Prolonged fever.
• Blood cell abnormalities (low white cells, low red cells, low platelets).
• Enlarged spleen.
• Increased triglycerides (fat) or decreased fibrinogen (protein necessary for clotting) in the blood.
• Increased ferritin (protein that stores iron) in the blood.
• Abnormal bone marrow test with evidence of Haemophagocytosis (ingestion of red or white cells by histiocytes) but not malignancy or other cause.
• Abnormally high CD25 (also known as sIL2ra) in the blood indicating abnormally increased T-cell activation.

The test for low or absent natural killer cell (NK) function has been found useful in making a clinical diagnosis of HLH. This abnormality is found in many patients with FHL, as well as in many cases of secondary disease but rarely in the X-linked forms.

However, it is just one piece of information and should not be used to determine the diagnosis of HLH as primary or secondary. NK function cannot be determined before birth, and it may not be reliably studied until a child is at least 6 weeks of age. FHL is suspected if siblings have been diagnosed with HLH, if symptoms intensify during treatment for HLH, or if symptoms return after therapy has been stopped.

Since it is difficult to tell the difference between secondary HLH and FHL, any case of HLH should be considered for genetic testing to confirm the diagnosis. Since 1999, at least seven defective genes have been identified. Autosomal recessive: PRF1 (perforin), MUNC13-4, STX11 (Syntaxin), STXBP2, and RAB27A. X-linked: SH2D1A, BIRC4.

There are some FHL patients (approximately 30%) with no identified gene defect, so normal genetic test results do not necessarily rule out the diagnosis of FHL. Genetic testing is usually done on blood, although other kinds of tissue samples can be used. Once the genetic cause is known, the parents can quickly be tested to confirm that they are carriers for that specific genetic type of FHL. Other siblings can also be easily tested, even before birth, once the genetic cause of the disorder in the family is known. Even in the event of death, salvaged tissue can be tested to determine if siblings are at risk.

Please be advised that all the information you read here is not a replacement for the advice you will get from your consultant and their team.

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