Compared with matched individuals without the disease, people with advanced systemic mastocytosis (SM) and — to a lesser extent — nonadvanced SMNonadvanced SM In these subtypes of SM, mast cells accumulate in the body but do not usually cause severe organ damage. Nonadvanced SM includes the subtypes indolent SM and smoldering SM. experience elevated mortality risk, according to recent study. Findings were shared in a poster presentation at the 2026 American Academy of Allergy, Asthma and Immunology (AAAAI) Annual Meeting.
The results highlight the importance of correctly classifying SM subtype in clinical practice, as it may be a key predictor of long-term survival outcomes.
The study analyzed longitudinal electronic health record data from Kaiser Permanente Southern California and included 80 individuals with nonadvanced SM and 27 with advanced SMAdvanced SM In these subtypes of SM, mast cells begin to damage organs. Advanced SM includes the subtypes aggressive SM, mast cell leukemia and SM with an associated hematological neoplasm.. The authors confirmed all cases using the World Health Organization 2016 criteria.
What is the difference between advanced and nonadvanced SM?
Nonadvanced SM comprises the indolent SMIndolent SM A subtype of nonadvanced SM caused by the abnormal accumulation of mast cells in the bone marrow and other organs. Indolent SM accounts for around 90% of SM cases. and smoldering SMSmoldering SM A subtype of nonadvanced SM, with a higher mast cell burden than the indolent subtype. Smoldering SM has a higher likelihood of progressing to an advanced form. subtypes. Advanced SM includes aggressive SMAggressive SM A subtype of advanced SM marked by a high mast cell burden that leads to organ damage. Aggressive SM accounts for less than 10% of SM cases., SM with an associated hematologic neoplasm and mast cell leukemiaMast cell leukemia The most severe subtype of SM, caused by the rapid buildup of immature mast cells in the bone marrow and blood. Mast cell leukemia accounts for less than 5% of SM cases and has a poor prognosis..
Patients were age- and sex-matched with 749 controls with SM-like symptoms and 535 individuals with chronic spontaneous urticaria (chronic hives), a disease that shares symptoms with SM.
Overall, the five-year survival rates for nonadvanced and advanced SM were 91.6% and 38.6%, respectively. The ten-year survival rates were 71.9% in nonadvanced SM and 12.1% in advanced SM. The authors note, however, that ten-year survival in patients with nonadvanced SM is often greater than 90% in other major registries.
Read more about SM prognosis
Participants with advanced SM had a 58% higher absolute mortality risk at five years and a 78.7% higher absolute risk at ten years compared with patients with chronic spontaneous urticaria. The five- and ten-year risk differences compared with the symptom-similar cohort were 50.4% and 61.7%, respectively.
Mortality risk did not significantly differ between participants with nonadvanced SM and those with similar symptoms. However, nonadvanced disease was associated with an 18.8% greater absolute mortality risk at ten years compared with chronic spontaneous urticaria.
In adjusted statistical models, individuals with advanced SM had 30.50 times the risk of mortality at five years compared with patients with chronic spontaneous urticaria. Those with nonadvanced SM, on the other hand, had 3.70 times the adjusted risk of mortality.
“Future work should examine the impact of targeted therapies and disease surveillance on survival across the SM spectrum,” the authors concluded.
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