Interpretation of the four risk factors for postoperative nausea and vomiting in the Apfel simplified risk score: an analysis of published studies.

Purpose: The Apfel simplified risk score, developed in 1999, is the most widely used tool for risk stratification of postoperative nausea and vomiting (PONV). It includes four risk factors: female sex, non-smoking status, history of PONV or motion sickness, and use of postoperative opioids. Nevertheless, there is considerable heterogeneity in the definition and application of these risk factors in PONV research.