Serum Tryptase

Tryptase is a serine protease stored in mast cell granules. Total serum tryptase (alpha and beta forms) is measured by fluorescence enzyme immunoassay (ImmunoCAP) in ng/mL (micrograms/L). Baseline tryptase reflects mast cell burden and is raised in mastocytosis and hereditary alpha tryptasaemia, whereas an acute rise over baseline marks mast cell degranulation in anaphylaxis.

Two uses are distinct: an acute rise to confirm anaphylaxis (and sometimes post-mortem diagnosis), and a baseline measurement to screen for clonal mast cell disease in patients with severe venom or unexplained anaphylaxis.

Anaphylaxis affects 1.6 to 5% of people during life. Tryptase peaks 1 to 2 hours after onset (starting at 15 to 30 minutes) and returns to baseline within 6 to 12 hours, with half life of about 2 hours. It is raised in over 90% of perioperative and venom anaphylaxis and in 60 to 70% of drug anaphylaxis, but is often normal in food anaphylaxis (only about 20 to 40% have a rise).

Systemic mastocytosis has a prevalence of about 1 in 10 000, and baseline tryptase over 20 ng/mL is a minor diagnostic criterion. Hereditary alpha tryptasaemia (extra copies of TPSAB1 alpha tryptase gene) affects about 4 to 6% of the population, with baseline tryptase of 8 to 20 ng/mL, and associates with more severe anaphylaxis and mast cell symptoms.

Indications. Suspected anaphylaxis or severe allergic reaction (during or after, to document mast cell activation), especially perioperative anaphylaxis and reactions to drugs or venom. Unexplained flushing, syncope, hypotension or recurrent anaphylaxis.

Suspected mastocytosis (urticaria pigmentosa, Darier sign, unexplained osteoporosis, anaphylaxis to venom with hypotension, gastrointestinal symptoms and flushing). Before venom immunotherapy (baseline). Sudden unexpected death with suspected anaphylaxis (post-mortem sample). Suspected mast cell activation syndrome. Not for food allergy diagnosis or as a general allergy test.

Specimen. Serum 1 mL (separate within 2 hours). For suspected anaphylaxis, three timed samples: as soon as possible after the start of treatment (within 15 to 30 minutes), 1 to 2 hours after onset (the peak), and a baseline sample at least 24 hours after full recovery (or a stored sample). Post-mortem sample from femoral blood within 15 hours. Stable at 2 to 8 °C for days and longer frozen.

Reference values (ImmunoCAP).

• Baseline total tryptase: under 11.4 ng/mL (micrograms/L) in most healthy adults (median about 5).
• Borderline baseline: 8 to 11.4 ng/mL (consider hereditary alpha tryptasaemia).
• Elevated baseline: over 11.4 ng/mL. Over 20 ng/mL suggests systemic mastocytosis (a minor criterion).
• Acute rise: the consensus formula for mast cell activation is peak tryptase greater than 1.2 times baseline plus 2 ng/mL.

Interpretation.

• Acute tryptase over 1.2 x baseline + 2 ng/mL in a compatible clinical context: confirms mast cell activation and supports anaphylaxis. A value of 11.4 ng/mL or more during a reaction is also suggestive when no baseline is available.
• Normal tryptase does not exclude anaphylaxis, particularly in food triggered reactions and in children, where the mediator may be histamine and other products.
• Persistently raised baseline (over 11.4 ng/mL): look for mastocytosis (bone marrow biopsy with KIT D816V mutation analysis, CD25 and CD2 expression), hereditary alpha tryptasaemia (TPSAB1 copy number), myeloid neoplasms (chronic eosinophilic leukaemia, myelodysplasia, acute myeloid leukaemia, hypereosinophilic syndrome), chronic kidney disease (reduced clearance) and parasitic infection.
• Baseline over 20 ng/mL with anaphylaxis to hymenoptera venom or other trigger: high likelihood of clonal mast cell disease, with bone marrow evaluation and lifelong venom immunotherapy if venom allergic.
• Post-mortem tryptase over 10 to 45 ng/mL (femoral blood) supports fatal anaphylaxis but can also be raised in other causes of death (sudden infant death, trauma, resuscitation, heroin overdose).
• Tryptase falls rapidly, so the timing of sampling is critical. A sample drawn after 6 hours may be normal.
• Mast cell activation syndrome is diagnosed with episodes of symptoms in at least two organ systems, a rise in tryptase of 20% + 2 ng/mL, and response to mast cell mediator directed treatment.

Critical values and findings needing immediate action.

• Suspected anaphylaxis (airway swelling, wheeze, hypotension, collapse): intramuscular adrenaline immediately, oxygen, intravenous fluids and airway support. Tryptase sampling never delays treatment.
• Tryptase raised during or after perioperative reaction: allergy referral for investigation of neuromuscular blockers, antibiotics, chlorhexidine and latex, and alert for future anaesthesia.
• Baseline tryptase over 20 ng/mL with unexplained flushing, hypotension, osteoporosis or skin lesions: haematology referral for bone marrow assessment.
• Venom anaphylaxis with raised baseline tryptase: venom immunotherapy (lifelong in mastocytosis) and an adrenaline auto injector.
• Patient given an adrenaline auto injector with raised baseline tryptase: education on avoidance of triggers (alcohol, NSAIDs, heat, opioids).

Interferences and pitfalls.

• Timing: samples taken too early (before 15 minutes) or too late (after 6 to 12 hours) miss the peak. Three timed samples give the best information, and the baseline is compared with the acute value.
• A normal acute tryptase does not rule out anaphylaxis. Sensitivity is lowest for food induced reactions and in reactions without hypotension.
• Baseline tryptase rises with age and in chronic kidney disease, and falls in children (children's range is lower). Heterophile antibodies, haemolysed samples and some assays can give false results.
• Mature tryptase (beta) rises in anaphylaxis, whereas total tryptase includes pro forms that are raised in baseline conditions such as mastocytosis, so assays that measure only mature tryptase are used in some laboratories.
• Raised baseline due to hereditary alpha tryptasaemia is benign in many individuals, and a lack of KIT mutation or other criteria excludes mastocytosis.
• Some drugs and conditions raise tryptase without anaphylaxis (opioids, severe infection, chronic urticaria in some, malignancy, haematological disease).
• Specific IgE testing and skin tests are done 4 to 6 weeks after the reaction to find the trigger, and tryptase does not identify it.
• Serial tryptase is not a treatment response marker, except in mastocytosis under cytoreductive therapy.

Special groups.

• Children: baseline tryptase is lower, mastocytosis mostly cutaneous with spontaneous regression, and acute tryptase is less often raised.
• Pregnancy: baseline is stable, and anaphylaxis in pregnancy is an emergency for mother and foetus.
• Older adults: venom anaphylaxis with cardiovascular collapse, with clonal mast cell disease in a significant proportion.
• Patients on beta blockers or ACE inhibitors: more severe anaphylaxis and reduced response to adrenaline (glucagon is used).
• Regions: access to tryptase assay is limited, and diagnosis is often clinical. Snake and scorpion envenoming can mimic anaphylaxis.

Follow up tests. Baseline serum tryptase, allergen specific IgE and skin prick tests 4 to 6 weeks after the reaction, drug provocation testing under supervision, venom specific IgE, KIT D816V mutation in blood or bone marrow, TPSAB1 copy number analysis, bone marrow aspirate and trephine with mast cell immunophenotype, full blood count with eosinophils, bone density scan,

abdominal ultrasound for hepatosplenomegaly, 24-hour urinary methylhistamine and prostaglandin metabolites, and serum histamine in specialist laboratories.