Serum tryptase is a biomarker used to aid in the identification of certain myeloid neoplasms, most notably systemic mastocytosis, where baseline (BST) levels >20 ng/mL are a minor criterion for diagnosis. Whereas clonal myeloid neoplasms are rare, the common cause for elevated BST is the genetic trait hereditary alpha-tryptasemia (HαT) caused by increased germline TPSAB1 copy number. To date, the precise structural variation and mechanism(s) underlying elevated BST in HαT and the general clinical utility of tryptase genotyping, remain undefined. Through cloning, long-read sequencing, and assembling of the human tryptase locus from an individual with HαT, and validating our findings in vitro and in silico, we demonstrate that BST elevations arise from over-expression of replicated TPSAB1 loci encoding canonical α-tryptase protein due to co-inheritance of a linked over-active promoter element. Modeling BST levels based upon TPSAB1 replication number we generate new individualized clinical reference values for the upper limit of 'normal'. Using this personalized laboratory medicine approach, we demonstrate the clinical utility of tryptase genotyping, finding that in the absence of HαT, BST levels >11.4 ng/mL frequently identify indolent clonal mast cell disease. Moreover, substantial BST elevations (e.g., >100 ng/mL) which would ordinarily prompt bone marrow biopsy, can result from TPSAB1 replications alone and thus be within 'normal' limits for certain individuals with HαT.