Background: Pathology laboratories are required to immediately report results which indicate a patient is at critical risk, but there is little consensus about what values are deemed critical. The aim of this review was to systematically review the literature on alert thresholds for common chemistry and hematology tests in adults and to provide an explicit and ranked source of this evidence. Methods: The literature search covered the period of 1995-2014. Evidence sources were critically appraised and ranked using the 1999 Stockholm hierarchy for analytical performance specifications in laboratory medicine modified for establishing decision limits. Results: The 30 most frequently reported laboratory tests with alert thresholds are presented with evidence rankings. Similar thresholds were reported in North America, Europe and Asia. Seventy percent of papers reported thresholds set by individual institutions, while 18% contained thresholds from surveys of laboratories or clinicians. Forty-six percent of the papers referred to 1 or both of the 2 American laboratory surveys from the early 1990s. "Starter sets" of alert thresholds were recommended by 6 professional bodies, 3 of which were collaborations between pathologists and clinicians. None of the 9 outcome studies identified dealt with confounding factors. Conclusions: Recommendations by professional bodies based on outdated surveys of the former state of the art or consensus are currently the best sources of evidence for laboratories to build their alert list. Well-designed outcome studies and greater collaboration between clinicians and the laboratory are needed to identify the most appropriate alert thresholds that signify actionable, critical or significant risk to patient well-being.