Thresholds for interpreting the fragility index derived from sample of randomised controlled trials in cardiology: a meta-epidemiologic study

The fragility index (FI) was proposed as a simplified way to communicate robustness of statistically significant results and their susceptibility to a change of a handful number of events. While this index is intuitive, it is not anchored by a cut-off or a guide for interpretation. We identified cardiovascular trials published in six high impact journals from 2007 to 2021 (500 or more participants and a dichotomous statistically significant primary outcome). We estimated area under curve (AUC) to determine FI value that best predicts whether the treatment effect was precise, defined as adequately powered for a plausible relative risk reduction (RRR) of 25% or 30% or having a CI that is sufficiently narrow to exclude a risk reduction that is too small (close to the null, <0.05). The median FI of 201 included cardiovascular trials was 13 (range 1-172). FI exceeded the number of patients lost to follow-up in 46/201 (22.89%) trials. FI values of 19 and 22 predicted that trials would be precise (powered for RRR of 30% and 25%; respectively, combined with CI that excluded risk reduction <0.05). AUC for meeting these precision criteria was 0.90 (0.86-0.94). In conclusion, FI values that range 19-22 may meet various definitions of precision and can be used as a rule of thumb to suggest that a treatment effect is likely precise and less susceptible to random error. The number of patients lost to follow-up should be presented alongside FI to better illustrate fragility.