Biphasic and protracted anaphylaxis to iodinated contrast media

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. 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. 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. 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.