Background: Evidence-based antiemetic guidelines offer predominantly consistent recommendations for CINV prophylaxis. However, studies and surveys suggest that adherence to these recommendations is suboptimal. We explored potential inconsistencies between clinical practice and guideline-recommended treatment with a registry evaluating the effect of guideline-consistent CINV prophylaxis (GCCP) on patient outcomes. Methods: This was a prospective, non-interventional, observational, multicenter study designed to assess overall (0-120 h) complete response (CR: no emesis/no rescue use) rates in patients who received GCCP or guideline inconsistent CINV prophylaxis (GICP) using diaries for 5 days following chemotherapy. Cycle 1 results are presented in patients who received either 1) anthracycline/cyclosphosphamide (AC) highly emetogenic chemotherapy (HEC), non-AC HEC or carboplatin, with GCCP for all these groups consisting of prophylaxis with an NK₁ receptor antagonist (RA), 5-HT₃RA, and dexamethasone (DEX) prior to chemotherapy or 2) moderately emetogenic chemotherapy (MEC), with GCCP consisting of a 5-HT₃RA and DEX prior to chemotherapy as per MASCC 2016 guidelines. CR rates for cohorts deemed to be GCCP and GICP were compared using a chi-square test. Results: A total of 1,089 patients were part of the cycle 1 efficacy evaluation. Overall GCCP was 23% for all patients. CR rates were significantly higher in patients receiving GCCP versus GICP (Table). Conclusions: Consistent with prior studies, GCCP was very low. The primary endpoint of the study was achieved as there was a significant benefit of almost 10% improved prevention of CINV when administering GCCP. As per MASCC/ESMO guidelines such an absolute difference should be practice changing. Comprehensive multifaceted strategies are needed to achieve better adherence to antiemetic guidelines.

*Statistically significant difference (P < 0.05, chi-square test) between GCCP vs. GICP group