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Objectives: To investigate the prevalence of biphasic and protracted anaphylaxis to iodinated contrast media (ICM), their risk factors and practical observation duration for detecting biphasic reaction.Methods: 145 patients with ICM anaphylaxis from January 2005-February 2016 were retrospectively categorised into uniphasic, biphasic (anaphylaxis recurrence within 72 h after resolution of initial anaphylaxis) and protracted (anaphylaxis >5 h) reaction groups. Multivariate regression analyses of potential risk factors were performed. We calculated negative predictive value (NPV) for biphasic reactions and additional person-hours required to detect one case during post-anaphylaxis observation.Results: Fifteen patients had biphasic reactions with secondary reactions with similar or milder severity and six had protracted reactions. Most significant risk factors were anaphylaxis duration >40 min for biphasic reactions (odds ratio (OR), 8.65 [95 % CI, 1.05-70.71]; P=0.044), and additional epinephrine administration within 1 h of initial dosing for protracted reactions (OR, 102.0 [95 % CI, 3.40-3057.25]; P=0.008). A 6-h post-anaphylaxis observation produced NPV of 96.4 %, while requiring a minimum of 65.5 additional person-hours to detect one additional case.Conclusion: Biphasic and protracted ICM anaphylaxis developed in 10.3 % and 4.1 %, respectively, with revealing risk factors. Six hours could be practical for post-anaphylaxis observation to detect biphasic reaction.Key Points: • Incidence of biphasic anaphylaxis in iodinated contrast media anaphylaxis is 10.3 %. • Incidence of protracted anaphylaxis in iodinated contrast media anaphylaxis is 4.1 %. • Initial anaphylaxis >40 min can predict for biphasic anaphylaxis development. • A 6-h post-anaphylactic observation in ICM-related anaphylaxis seems practical. [ABSTRACT FROM AUTHOR]