This study used three anchor-based approaches and two distribution-based approaches to establish the MID for the urinary scales of the PFDI and PFIQ. The anchor-based approaches included one measure of the patient’s perspective (global rating of change) and two clinical measures of incontinence severity (ISI and number of incontinence episodes recorded on the bladder diary). Using anchor-based methods, the range of MID for the UDI was −22.4 to −6.4 points, for the UIQ was −16.5 to −4.6 points, and for the UDI-stress subscale was −17.0 to −6.5 points. These data were supported by two distribution-based estimates, effect size (1/2 SD) and 1 SEM.
When multiple approaches are used to determine the MID of a scale, a range of values rather than a single point estimate are expected, as was seen in this study. Clinically, a more narrow range or even a single point of MID for each urinary scale would be more helpful than the somewhat broad range determined by our calculations. It is recommended that a single MID value or narrow range be selected for a given scale by integrating the results from the multiple approaches in a systematic way, sometimes called triangulation.16, 21
Using this method, MID derived from anchor-based methods that reflect patient-rated and disease-specific variables are given the most weight.16
The global rating of change, the most commonly used anchor for MID determination, represents the best measure of the significance of change from an individual perspective.6
For the scales considered in this study, the MID estimates from the patient’s perspective are considerably smaller than when clinical criteria are used. When determining the MID, it is recommended that the patient’s perspective be given the most weight, although the clinician’s perspective should also be considered.16
Retrospective self-reports like a global rating of change are subject to recall bias and have a tendency to reflect a subject’s current health state more than a change from baseline.14
Therefore, when examining the MID estimates and 95% CIs from all anchor-based approaches and considering the potential for some recall bias on our global health rating, we propose MID thresholds of −11, −8, and −16 for the UDI, UDI-stress subscale and UIQ respectively. These values represent the MID determined by a clinically relevant anchor, a one level change in the ISI and are higher than the MID estimates obtained from the global rating of change. Thus, values that meet or exceed these thresholds represent the minimal scores that can be considered clinically relevant to both the patient and the clinician. We recognize, based on MID estimates of the global rating of change, that smaller scores may be clinically relevant to some patients, however it is unclear if these smaller scores would be relevant from a physician’s perspective. MID estimates obtained from the bladder diary represent more conservative estimates of clinically meaningful change than the ISI or global rating of change, but do not represent the least or minimum change clinically relevant to patient’s and clinicians in our opinion.
In MID determination, distribution-based methods can support and help interpret estimates from anchor-based methods and be used in situations where anchor-based approaches are unavailable.16
There are increasing data and growing consensus that an effect size of 0.5 (or change of ½ SD) is a conservative estimate that is likely to be clinically significant across different patient-reported questionnaires.16, 22
Sloan et al proposed that, in the absence of other information, ½ SD is a reasonable and scientifically supportable estimate of meaningful effect.22
The MID for the scales of the PFDI and PFIQ were previously estimated using this approach in a cohort of patients undergoing surgery for pelvic organ prolapse with and without SUI 4
Our study improves on this because our participants were not undergoing surgery (a treatment expected to show a large difference and thus overestimate the MID) 6
and all had urinary incontinence and therefore using measures that assess incontinence was more appropriate. Trials of non-surgical SUI therapies, such as ATLAS are particularly suited for MID determination because mild to moderate treatment effects are more common. Not surprisingly MID estimates corresponding to ½ SD were considerably less in our study than those derived from the surgical cohort (−30 versus − 20 points, and −49 versus − 29 points for the UDI and UIQ respectively).
Although the ½ SD approach provides scores that are certainly clinically significant and meaningful, they are not necessarily minimal. Consistent with other studies, MID estimates corresponding to ½ SD in our study represent the upper boundary of the range of MID estimates identified.16
There is also increasing evidence that a change in score equivalent to 1 SEM is a valid alternative for estimating MID in patient-reported health outcome measures.18
In this study, the 1 SEM criteria corresponded to MID estimates within the range of the anchor-based approaches for the UDI, UDI-stress subscale and UIQ.
As the method of scoring and range of possible scores differ between instruments, it is not meaningful to directly compare MID across different measures. However, one can compare the percentage of the total scale score represented by the MID (i.e. dividing the MID by the highest possible scale score).23
The range of anchor-based MID estimates seen in this study represents 2.1 to 7.5% change of the total score for the UDI, 1.6% to 4.3% for the UIQ and 4.6 to 16.5% for the UDI-Stress subscale. These are consistent with results of several other HRQOL questionnaires including the Medical Outcomes Study Short-form 36 (SF-36) (range 3 to 6%)24
, the Functional Assessment of Chronic Illness Therapy (FACIT) scales (3.7 to 12.5%)25
, the Incontinence Quality of Life (I-QOL) scale (2.5%)26
and Kings Health Questionnaire scales (5%).27
The strengths of this study include the use of multiple approaches to triangulate MID estimates following currently recommended guidelines, the use of validated and widely accepted SUI outcome measures as anchors, the large sample size, and the wide breadth of treatment response allowing an assessment of minimal change. A limitation is that we only determined MID for the urinary scales of the PFDI and PFIQ; MID estimates for the colorectal anal and prolapse scales will require further study using different patient populations. Until such estimates are available, a change of ½ SD or greater seems a reasonable conservative estimate for a clinically important change. An additional limitation of the study is that there were too few subjects who had a decline in urinary function to provide estimates for an MID for deterioration. The MID values proposed from this study represent MID for improvement only. It is also worth noting that MID estimates can vary across populations.16
In conclusion, MID can help researchers and clinicians understand whether HRQOL score differences between treatment groups or if changes within one group over time are clinically meaningful.23
From this study we recommend that a difference of 11, 8, and 16 points can be considered reasonable estimates of MID for the UDI, UDI-stress subscale and UIQ, respectively. Statistically significant improvements that meet or exceed these thresholds should be considered clinically important. However, some patients with changes in scores less than these estimates may perceive clinical important improvements. MID values should be confirmed based on accumulating evidence from multiple studies and, with increasing evidence, we will become more precise in the MID values for the urinary scales of the PFDI and PFIQ.