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Hereditary Alpha Tryptasemia, or HAT, has also been called familial tryptasemia and hypertryptasemia. Here are a few important numbers to remember (updated 3/2023).

HAT is caused by having 2+ copies of the TPSAB1 gene on the chromosome 16p13.3 (source). This website focuses on people experiencing HATS and its co-morbidities. While some studies have indicated a gene dose effect (ie, the more copies of the gene a patient has, the more symptomatic the patient is likely to be), another more recent study has contradicted that finding.

A person whose symptoms have manifested from HAT has Hereditary Alpha Tryptasemia Syndrome, or HATS. About 66% of patients with HAT are symptomatic, and thus have HATS (Kačar 2023).

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Fewer than 1000 people with HAT have been identified around the world--not all of whom experience HATS. However, the prevalence of HAT is considered to be between 3-5% of Caucasians. The same 2020 United Kingdom study mentioned above has indicated of 5% prevalence in their holistic population. In summary, neither HAT nor HATS is rare*; however, for whatever reasons, it is rarely diagnosed.

Using the above information (66% x 3-5%), 2 to 3% of the Caucasian population has HATS.

Kačar (2023) noted that there is a "moderate female preponderance of 64%."

If you suspect you may have Hereditary Alpha Tryptasemia Syndrome, see this article, "Confirming whether You Have HATS."


*"Rare" is a clinical and/or political term referring to a "low" percentage of a population (ie, defined as between <4 to <6% out of 10,000 people). It varies from country to country.

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