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Diffuse unpatterned alopecia (DUPA) is a clinicopathologic phenotype of global follicular miniaturization with loss of donor dominance, rendering follicular unit excision/transplantation (FUE/FUT) unsafe. This review integrates clinical mapping, multi-zone trichoscopy, standardized trichometry/phototrichogram with AI-assisted counts, and horizontal-section histopathology to operationalize donor-area safety. Discriminants that consistently separate DUPA from diffuse effluvium and other overlap phenotypes include hair shaft diameter diversity (HSDD) ≥20% across vertex, parietal, and occipital zones with increased vellus proportion and single-hair follicular units, and phototrichogram evidence of miniaturization disproportionate to telogen shift. An occipital terminal:vellus ratio <4:1 substantiates an Unsafe Donor Area (UDA). We outline a pragmatic workflow history and five-zone photography; vertex, parietal, occipital trichoscopy objective counts in predefined fields; and low-threshold occipital biopsy when metrics are discordant to reduce misclassification and avert iatrogenic transplantation. Where UDA is confirmed, management should be medical and AGA directed with mapped fields and a 4 to 6-month reassessment; surgical restoration is contraindicated. Translational avenues include non-invasive tape-strip transcriptomics and sebum lipidomics to quantify perifollicular microinflammation and Wnt/PGD2-axis signaling, alongside clinic-ready AI for rapid screening and longitudinal response tracking. Consensus cut-offs for HSDD and occipital T:V ratio, harmonized capture fields, and an externally validated, clinic-usable risk score are immediate priorities to standardize DUPA adjudication and improve patient safet y .