The second required co-criterion for systemic mast cell activation depends on documentation that mast cells are directly involved in the symptomatology. An increase in the serum level of tryptase, above baseline and within a narrow (generally accepted as one to two hour) window of time after a symptomatic episode, is proposed as the preferred method for providing evidence of mast cell involvement according to these criteria. 6, 28-30 The consensus article provides a method for calculating the required minimum rise in serum tryptase. 6 After a reaction, a level of serum tryptase that is a minimum of 20% above the basal serum tryptase level, plus 2 ng/ml, will meet the second criterion listed above for a mast cell activation event (see Tests for further information). Consensus members also agreed that when serum tryptase evaluation is not available or when the tryptase level does not rise sufficiently to meet the required increase for the co-criterion, other mediator tests could suffice. A rise in urinary n-methyl histamine, prostaglandin-D 2 , or its metabolite, 11β-prostaglandin-F 2α (24-hour urine test for any of the three), is considered an alternative for the co-criterion related to a requirement for a mast cell mediator level rise during a systemic mast cell activation event. 6
Mast cell tumors are an uncommon occurrence in horses . They usually occur as benign, solitary masses on the skin of the head, neck, trunk, and legs. Mineralization of the tumor is common.  In pigs and cattle , mast cell tumors are rare. They tend to be solitary and benign in pigs and multiple and malignant in cattle.  Mast cell tumors are found in the skin of cattle most commonly, but these may be metastases from tumors of the viscera .  Other sites in cattle include the spleen, muscle, gastrointestinal tract, omentum , and uterus. 
Tefferi et al. (2009) sequenced the TET2 gene (612839) in bone marrow cells from 42 patients who met the WHO criteria for systemic mastocytosis (SM) and identified 17 mutations in 12 patients (29%), including 13 frameshift, 2 nonsense, and 2 missense mutations. Of the mutation-positive patients, 2 had indolent SM, 2 had aggressive SM, and 8 had SM associated with a clonal non-mast cell-lineage hematopoietic disease. Tefferi et al. (2009) also analyzed for the D816V mutation in the KIT gene () and the V617F mutation in the JAK2 gene (), and found that 6 of the TET2-mutated patients also displayed D816V in KIT and that 1 TET2-mutated patient had V617F in JAK2. Tefferi et al. (2009) concluded that TET2 mutations are frequent in systemic mastocytosis and segregate with the D816V mutation in KIT.