Background: As new Patient-Reported Outcomes Measurement Information System (PROMIS) instruments are incorporated into clinical practice, determining how large a change on these instruments represents a clinically relevant difference is important; the metric that describes this is the minimum clinically important difference (MCID). Prior research on MCIDs of the Neck Disability Index (NDI) and Oswestry Disability Index (ODI) has produced values ranging from 5 to 10 points, but these measures have not been presented in relation to MCID values of PROMIS instruments.
Questions/purposes: To establish a comprehensive repository of MCID values calculated both with distribution-based and anchor-based methods for four outcomes instruments in spine care, we asked: (1) What are the MCIDs of the PROMIS Physical Function (PF); (2) the PROMIS Pain Interference (PI); (3) the NDI; and (4) the ODI among spine patients?
Methods: We conducted a prospective study of previously tested diagnostic measures on 1945 consecutive patients with a reference standard applied. All patients aged 18 years and older visiting an orthopaedic spine clinic between October 2013 and January 2017 completed the PROMIS PF and PI, NDI, and ODI on tablet computers before their clinic visits. Patients were grouped by change level (self-report of meaningful change versus slight or no change) using an anchor question in comparison to baseline. Descriptive statistics, two anchor-based MCID values (mean change and receiver operating characteristic curve), and five distribution-based values (SD at 1/2 and 1/3 values and minimum detectable change [MDC] at 90%, 95%, and 99%) were analyzed four different times between 3 months and > 6 months of followup. A total of 1945 included patients with a wide range of spine conditions and varying treatments had a mean age of 58 years (SD = 15.5), were 51% (988 of 1945) male, 90% (1754 of 1945) self-identified as white, and 5% (94 of 1945) as Hispanic with 1% to 2% of patients refusing participation.
Results: The PROMIS PF mean change scores in the changed group (much worse, worse, improved, or much improved) ranged between 7 and 8 points. MCID values ranged from 3 to 23 points depending on the method of calculation with a median of 8. For the PROMIS PI, mean change scores ranged from 8 to 9 points and MCID values from 1 to 24 points with a median of 8. For the NDI, mean change scores ranged from 13 to 18 points and MCID values ranged from 6 to 43 points with a median of 18. For the ODI, mean change ranged from 17 to 19 points and MCID values ranged from 7 to 51 points with a median of 24. For each instrument, distribution-based SD yielded the smallest values, followed by anchor-based methods, with MDC yielding the largest MCID values.
Conclusions: This study uses a range of methods for determining MCIDs of the PROMIS PF and PI, NDI, and ODI from anchor-based to distribution-based methods. MCIDs do not have a static value for a given outcome measure, but have a range of values and are dependent on the method calculated. The lowest MCIDs identified for the NDI and ODI are consistent with prior studies, but those at the upper range are much higher. Anchor-based methods are thought to be most relevant in the clinical setting and are more easily understood by clinicians, whereas the distribution-based MCIDs are useful in understanding population breadth. Lower MCID values may be most appropriate for screening purposes or low-risk effects, and the median or higher MCID values should be used for high-risk effects or outcomes.
Level of evidence: Level I, diagnostic study.