The reference range is < 11.4 ng/mL.
There is only one commercially available method for measuring human serum tryptase levels (ImmunoCap Tryptase, Phadia Laboratory Systems, Uppsala, Sweden). According to a study cited by the manufacturers, the 95th percentile value for serum tryptase measured in 126 healthy individuals aged 12-61 years, was 11.4 ng/mL. The geometric mean is 3.8 ng/mL and the lower limit of detection is 1 ng/mL. 
The reference values for children aged 6 months to 18 years appear to be similar to adult values with a median range of 3.5 ng/mL as determined by a study on 197 children.  Infants younger than 6 months, however, have a higher median value of 6.1 ng/mL. 
Tryptase levels of 11.5 ng/mL or greater are indicative of either mast cell activation (as in anaphylaxis) or increased total mast cell levels (as in mastocytosis). The assay does not distinguish between α or β tryptase (both inactive monomeric and active tetrameric form). As such, determinations of tryptase levels are contingent on both the size and activation status of an individual’s mast cell population but is not informative of the specific contribution of either of these factors. 
False-positive and false-negative results
False-positive results may arise from the presence of heterophilic antibodies in the patient’s serum. Heterophilic antibodies, such as rheumatoid factor (RF) and human anti-mouse antibodies (HAMA), can bind to immune globulins of other species and thus interfere with the immunoassay since the tryptase immunoassay employs mouse-derived anti-tryptase. The presence of these heterophilic antibodies can thus lead to spurious elevations, especially in individuals who have received chimeric antibodies (such as Infliximab).  Most laboratories that use the serum tryptase immunoassay use a reagent with a suppressor for these heterophilic antibodies.
False-negative results typically result from sample collection later than 12 hours following onset of symptoms.
Collection and Panels
The currently available serum tryptase assay (ImmunoCap Tryptase, Phadia Laboratory Systems, Uppsala, Sweden) uses quantitative fluorescence to determine serum tryptase levels.  The assay does not distinguish between α or β tryptase (both inactive monomeric and active tetrameric form).
Venipuncture is used for specimen collection, and a red-top tube (preferable) or a serum gel separator tube is used. If anaphylaxis is suspected, sample collection should take place 15 minutes to 3 hours after onset of allergic signs and symptoms. Elevated levels can be detected 3-6 hours after the reaction and normalize in 12-14 hours. 
The serum or plasma (results are compatible with each other) is then separated with centrifugation or via clotting in a gel separator tube. At least 0.2-0.5 mL of serum is required, meaning that at least 0.5-1 mL of blood needs to be collected. Mild hemolysis, lipemia, and icterus in the sample are acceptable.
If blood collection is not possible, nasal lavage/wash sample is an alternative.  In this case, the sample needs to be diluted with the diluents included in the testing kit.
Specimen storage and transport
For shipping, the specimen may be kept at room temperature up to 2 days. The specimen can be stored at 2-8°C if it is to undergo assay within 5 days following collection. If longer periods are expected, the sample should be stored at -20 to -70°C. Analysis requires one day.
Tryptase is a trypsin-like proteinase that is found most abundantly in mast cells and basophils, with the former containing almost 300 times more tryptase.  As such, tryptase is specific to mast cell granules and can provide information about mast cell number, distribution, and activation depending on the clinical context.  Tryptase exerts sequential actions similar to histamine, namely as a vasoactive, pro-inflammatory, and chemotactic substance. 
The human tryptase gene is encoded on chromosome 16 and codes for four isoenzymes: alpha, beta, gamma, delta, and epsilon (Table 1). Of the four isoenzymes, beta tryptase is the predominant form stored in the mast cell granule where it is complexed as a tetramer stabilized by proteoglycans namely heparin. 
Table 1: Tryptase Types and Characteristics (Open Table in a new window)
|ϒ-tryptase||Monomer: membrane bound, unknown function|
|δ-tryptase||Monomer: short chain, unknown function|
The enzymatically active mature tetramer is secreted with mast cell activation. In contrast, the immature monomeric form of beta tryptase is continuously secreted from the unstimulated mast cell and constitutes an individual’s baseline serum level. 
Upon stimulation from immunoglobulin E (IgE) released by plasma cells in response to allergens, mast cells release contents of their secretory granules to elicit tissue specific responses. Histamine and tryptase are the predominant mediators of these responses and are detectable in serum in as little as 15 minutes in experimental models of anaphylaxis.
Tryptase, however, diffuses at a slower rate into tissues than histamine, presumably because of its association with a large proteoglycan complex. Serum levels of tryptase peek in 1-2 hours after allergen provocation and can remain elevated for up to 12-24 hours.  In contrast, histamine levels return to baseline in 15-30 minutes.  As such, the longer biological half-life of tryptase (2 hours) makes it an attractive biological marker for episodes of mast cell activation.
Anaphylaxis is a severe and potentially fatal systemic allergic reaction that occurs acutely after contact with an allergen.  It is classified under the Gell and Coombs hypersensitivity classification as a Type 1 immediate hypersensitivity reaction mediated by IgE release of histamine and other mediators, such as tryptase, through activated mast cells and basophils.
There is currently no laboratory test that can diagnose anaphylaxis in real time. However, measurement of mediators of the anaphylactic response can be measured to support the clinical diagnosis. Measurement of serum tryptase provides an ideal diagnostic tool as it can be detected within 15 minutes of the reaction and up to 3 hours thereafter. This is then compared to the patient’s baseline, which may be determined either prior to or at least 24 hours after the anaphylactic reaction. 
The utility of the test thus also extends to distinguish anaphylaxis from other forms of systemic shock, which are not mast cell mediated, such as cardiogenic shock, septic shock, neurogenic shock or obstructive shock.
Mastocytosis is a hematologic group of disorders characterized by accumulation of pathologic mast cells in different tissues such as the skin (urticarial pigmentosa), bone marrow, gastrointestinal tract, liver, spleen and lymph nodes.  The skin is the most common presenting organ involved and when mastocytosis is diffusely present in any extracutaneous tissue, it is called systemic mastocytosis. A set of diagnostic criteria was established by the World Health Organization (WHO) and includes determination of serum baseline tryptase as a criterion (Table 2). Baseline levels of serum tryptase reflect a patient’s mast cell burden as they detect the constitutively secreted inactive and monomeric forms of α and β tryptase.
Table 2: WHO Diagnostic Criteria for Systemic Mastocytosis (Open Table in a new window)
The presence of either 1 major and at least 1 minor or 3 minor criteria provides a definitive diagnosis of systemic mastocytosis.
* does not apply to patients with an associated hematologic clonal non–mast-cell lineage disease
Of note, a basal tryptase level less than 20 ng/mL does not rule out the presence of mastocytosis. A recent study revealed that in indolent systemic mastocytosis, a subgroup of systemic mastocytosis in which bone marrow demonstrates abnormal collections of mast cells but not other end evidence of end organ damage, 5-7% of these patients will have serum tryptase less than 11.4 ng/mL.  Conversely, baseline levels slightly more than 20 ng/mL can be seen in other disease states, such as myeloid neoplasms and renal disease. As such, although elevated baseline serum tryptase more than 20 ng/mL strongly suggests the presence of mastocytosis, it should not be used as the sole criterion to diagnose mastocytosis. 
Baseline serum tryptase levels can give prognostic information in patients with systemic mastocytosis, in whom stable levels over time convey a good prognosis and rising levels indicate disease progression and poor prognosis. 
A progressive increase in serum tryptase in patients with systemic mastocytosis can also herald the transition to more aggressive forms such as mast cell leukemia. In these patients, serum tryptase levels are markedly elevated (often >500 ng/mL) and may increase rapidly (>100 ng/mL in a few weeks). 
Hypereosinophilic syndrome (HES) is a heterogeneous group of hematologic disorders classically defined by Chusid et al with the following criteria: 1) persistent eosinophilia (>1500 eosinophils/mm3 for at least 6 months) that is otherwise unexplained by a comprehensive evaluation 2) evidence of eosinophil related organ damage. 
Recent advances and expert consensus opinion have attempted to address limitations of these rigid definitions and recognize that tissue eosinophilia can exist in the absence of blood eosinophilia. Moreover, eosinophilia can be broadly defined as >1500 eosinophils/mm3 on two occasions at least one month apart. (Table 3). 
Table 3 (Open Table in a new window)
1. Hypereosinophilia—absolute eosinophil count >1,500 cells/μL for ≥1 mo, checked on ≥2 occasions.
Alternatively, tissue hypereosinophilia can be identified in addition to an elevated absolute eosinophil count with tissue hypereosinophilia, defined as:
i. Eosinophils >20% of nucleated cells in bone marrow
ii. Extensive tissue infiltration of target organ by histologic analysis
iii. Histologic evidence of eosinophil degranulation in a target tissue in the absence of eosinophils in that target tissue
|2. Evidence of eosinophil-mediated target organ damage|
|3. Exclusion of all other potential causes of hypereosinophilia|
Central to the evolving understanding of HES is the recent identification of subgroups with clonal populations of eosinophils or lymphocytes suggesting that HES can be divided into 2 major groups: those with a primary disorder of myelopoiesis and those with secondary cytokine mediated eosinophilia from a clonal population of lymphocytes. [21, 22]
In the myeloproliferative form of HES, the clonal molecular defect implicated is a deletion on chromosome 4q12 resulting in a fusion gene between Fip1-like (FIP1L1) and platelet-derived growth factor receptor-α (PDGFRA), leading to the production of a FIP1L1-PDGFRA fusion kinase. 
Since serum tryptase has been found to be elevated in some myeloproliferative disorders, primarily because of increased mast cell volume, the myeloproliferative variant of HES has also been shown to manifest elevated serum tryptase levels. 
However, there is considerable overlap between tryptase levels in myeloproliferative HES and systemic mastocytosis, and, as such, serum tryptase is not sensitive or specific enough to replace invasive molecular or flow procedures in the diagnosis of myeloproliferative HES.
Determination of serum tryptase level, in conjunction with other tests may provide additional information on HES. Specifically, a group of researchers found elevated tryptase levels were more common in HES patients with the FIP1L1-PDGFRA fusion gene, than those who did not. 
Serum tryptase may provide prognostic and predictive information in patients with HES. A study on a cohort of HES patients showed that patients with elevated serum tryptase had worse prognosis as they were more likely to develop fibroproliferative end organ damage and accelerated rates of death within 5 years. In contrast, all patients with normal serum tryptase levels were alive at 5 years. Moreover, the study also demonstrated that all patients with HES and elevated serum tryptase levels had a clinical and hematologic response when treated with Imatinib. This suggests that serum tryptase can be a marker for Imatinib responsiveness in HES patients. 
Several conditions have also been associated with increased baseline serum tryptase levels.