Background: Multigene panel testing (MGPT) has identified TP53 pathogenic or likely pathogenic (P/LP) variants in patients with diverse phenotypes from no cancer to classic Li-Fraumeni syndrome (LFS). There is increasing recognition of variants at low allelic fraction (VAF) for TP53 in particular, which can be suggestive of post-zygotic mosaicism or aberrant clonal expansion (ACE), comprising clonal hematopoiesis of indeterminate potential (CHIP) or occult hematologic neoplasia. Distinguishing among these categories is essential because of widely different cancer risk and management implications for patients and their relatives. We report an evaluation of TP53 positive probands to determine germline versus somatic status from a cancer genetics clinic. Methods: We reviewed probands with TP53 P/LP variants by MGPT on blood (N = 83) or saliva (N = 1) samples from 2012-2019. Available VAFs were collected from commercial testing laboratories. Probands positive for a known familial variant, who met LFS testing criteria without indication of low VAF, or who carried the Brazil founder p.R337H variant were considered germline. For those with uncertain germline status, data was obtained from ancillary testing of family members, cultured skin fibroblasts, and other somatic benign or tumor tissues. TP53 variants were further categorized based on all available data. Results: Of the 84 probands, 28 (33%) had germline TP53 P/LP variants determined by above initial criteria; 18 (21%) were confirmed germline through ancillary testing. Seven (8%) individuals were classified as having constitutional mosaicism. In eleven (13%) individuals, the TP53 variants were consistent with ACE, in 9 (11%) with CHIP and in 2 (2%) with a hematologic malignancy (1 CLL, 1 NHL). Five (6%) cases could not be categorized despite ancillary testing. Fifteen (18%) probands declined any further workup. Conclusions: A TP53 P/LP variant found on peripheral blood or saliva MGPT does not always originate in the germline. In a clinical cancer genetics cohort, only 54% of patients had TP53 P/LP germline variants; these patients plus those with constitutional mosaicism (8%) require intensified surveillance. Assessment of VAF, family member testing, and analysis of TP53 in cultured fibroblasts or other tissue samples may distinguish germline and constitutional mosaicvariantsfrom the ACE spectrum. Expanding use of MGPT will increase this clinical challenge, which may motivate the modification of lab reports to include VAF and possible non-germline explanations. The findings of this study support a framework of multiple strategies to discern true constitutional status of a TP53 P/LP variant.