Plasma zinc concentration (PZC) is a recommended biomarker to assess zinc status and the risk of zinc deficiency in populations. However, the relation between PZC and clinical signs of zinc deficiency remains uncertain. These analyses were conducted to evaluate the relation between PZC and clinical signs of zinc deficiency and to determine a cutoff for PZC below which individuals would have an increased likelihood of having clinical signs associated with zinc deficiency. Electronic bibliographic searches were conducted of literature indexed in PubMed, Embase, CINAHL Plus, and EBSCO and related to experimental zinc depletion studies in adults and case reports in children and adults (ages <1 mo-43 y) with acrodermatitis enteropathica (AE). Data extracted included demographic characteristics, PZCs, and the presence or absence of clinical signs likely associated with zinc deficiency (e.g., dermatitis, diarrhea). Mean PZC was significantly lower among adults consuming severely zinc-restricted diets (<1 mg Zn/d) who developed clinical signs compared with those who remained asymptomatic (36.0 ± 16.8 vs. 67.9 ± 13.3 μg/dL, P < 0.034). Likewise, patients with AE had a lower mean PZC when symptomatic compared with post-treatment PZC when they were asymptomatic (38.2 ± 20.7 vs. 102 ± 34.7 μg/dL, P < 0.01). Among individuals with restricted dietary zinc intake, PZC predicted clinical signs with 82% sensitivity and 92% specificity when using a cutoff of 50 μg/dL. Among individuals with AE, PZC predicted clinical signs with 80% sensitivity and 89% specificity when applying a cutoff of 50 μg/dL. These analyses demonstrate a clear relation between PZC and the presence of clinical signs associated with zinc deficiency among presumably healthy individuals undergoing periods of dietary zinc restriction, as well as in individuals with AE, further validating the usefulness of PZC as a biomarker of severe zinc deficiency.