The fragility index is a measure of the robustness (or fragility) of the results from a clinical trial that uses dichotomous outcomes.2 It most often is used when results are statistically significant (P ≤ .05). The fragility index represents the minimum number of participants whose status needs to change from an “event” to a “nonevent” (or vice versa) so that the results switch from statistically significant to nonsignificant. The larger the fragility index, the more robust are the trial’s data. One can further evaluate the fragility index relative to the sample size by calculating the “fragility quotient” (fragility index divided by sample size), with a smaller fragility quotient indicating a more robust trial endpoint.
Currently, no specific fragility index value is accepted as “robust” (good) or “fragile” (bad). However, the results of a trial should be viewed with particular skepticism if the number of patients who are lost to follow-up is greater than the fragility index given that the unknown outcomes of these patients could alter the results. In a systematic survey of 40 spine RCTs, Evaniew et al5 provide a description of the fragility index among trials of spine surgery as follows:
- Median fragility index: 2 (interquartile range [IQR] 1-3)
- 75% of trials had a fragility index ≤ 3
- 20% of trials had a fragility index of zero
- 65% of trials had a fragility index ≤ the total number of participants lost to follow-up
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