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Table 3. Receiver Operating Characteristic Curve Analysis Thresholds From a Previous Study.9
Physical function
Pain interference
Area Under the Curve
95% Confidence Interval
P Value
0.77
0.57
0.64-0.90
0.42-0.72
<.01
.33
of the standard deviation of the change scores observed (ie,
the preoperative to postoperative changes in PROMIS PF
and PI scores).16
Two approaches were used to validate the proposed
thresholds that were previously published. Initially, ROC
analysis was completed on PROMIS PF and PI scores from
the current data set and compared with the previous analysis. This includes ROC analysis to determine the area under
the curve (AUC) for PROMIS PF and PI scores. The AUC
represents the percentage of patients who could be correctly
identified as achieving or failing to achieve MCID for all
possible thresholds. If the AUC was significant (ie, >0.5),
new thresholds were determined by selecting the thresholds
closest to 95% specificity for determining that the patient
did achieve an MCID improvement and 95% specificity for
concluding that the patient did not achieve a MCID
improvement. Once the thresholds were identified, likelihood ratios (the odds a patient would achieve or fail to
achieve MCID), pretest probabilities, and posttest probabilities were calculated. These are the probabilities that an
individual patient would be correctly identified preoperatively as meeting MCID or failing to meet MCID at the
postoperative time point, prior to application of the threshold (pretest probability) and after the threshold is applied
(posttest probability). To compare the effect of applying the
previous thresholds to the current data set, the thresholds
published (PROMIS PF, 29.7 and 42; PROMIS PI, 67.2 and
55) were applied to the present study data, and the same set
of variables were calculated for comparison.9 A subsequent
second approach evaluated the categorization of the new
data set using published thresholds. Using 2 × 2 tables,
thresholds from the previously published derivation sample
were used to categorize patients into 1 of 3 groups (rows)-
(1) expected to not achieve MCID, (2) expected to achieve
MCID, and (3) ambiguous as to MCID achievement9-and
this was compared with the known MCID improvement
(columns). Chi-square analysis was used to test whether the
proportion of patients placed in these 6 categories was due
to chance.
Results
There was significant improvement in PROMIS PF and PI
scores over time. The 2-way analysis of variance showed a
significant interaction of time and PROMIS scores (P < .001).
Neither age (P = .069) nor gender (P = .33) significantly
influenced the results. Pairwise comparisons for PROMIS
PI were significant (P < .001), with an average pre- to postoperative improvement of 7.0 (95% confidence interval
[CI], 4.9-9.1). Similarly, pairwise comparisons for PROMIS
PF were significant (P < .001), with average pre- to postoperative improvement of 6.0 (95% CI, 3.1-9.0).
ROC analysis showed that PROMIS PF and not PROMIS
PI was predictive of postoperative determination of meeting MCID (Table 3). The PROMIS PF AUC of 0.77 was
significant (P < .001). A PROMIS PF score of 23.8 or
lower yielded 94.1% specificity for identifying patients
who would achieve MCID improvement at follow-up. A
PROMIS PF score of 41.6 yielded 96.3% specificity for
identifying those patients who did not achieve MCID
improvement at follow-up. The likelihood ratios were 6.3
and 7.2 for ruling in achieving MCID improvement and not
achieving MCID improvement, respectively. As a consequence, the posttest probabilities were also high (83.3% for
achieving MCID improvement and 90% for failing to
achieve MCID improvement). Applying the threshold
scores from Ho et al9 resulted in similar values for specificity, likelihood ratios, and pretest and posttest probabilities
(Table 4). In contrast, the PROMIS PI AUC of 0.57 was not
significant (P = .33). Therefore, no similar analysis of
PROMIS PI was pursued. An unplanned, post hoc Pearson
correlation analysis of the preoperative PROMIS PI score
with the change from pre- to postoperative PROMIS PI
produced a low and nonsignificant correlation coefficient
(r = −0.25, P = .06).
The χ2 analysis of the 2 × 2 analysis of the categories on
the basis of MCID improvement was significant (PROMIS
PF χ2 = 18.9, P < .01). The proportion of patients placed in
each category is shown in Table 5. The total percentage of
patients classified as achieving or not achieving MCID
using the thresholds for this study was 45.8%. Applying the
threshold values from Ho et al9 to this data set, 39.3% of the
sample was categorized correctly (8 patients [13.1%]
achieving MCID and 16 patients [26.2%] failing to achieve
MCID). In contrast only 6.5% of the sample (4 patients)
was categorized incorrectly, and 54.2% (33 patients) could
not be classified.
Discussion
This validation study supports the use of preoperative
PROMIS PF scales as accurate global health PRO instruments while giving patients a voice in their health care.
These data in a geographically distinct location replicate the
Table of Contents for the Digital Edition of Foot & Ankle International - July 2018
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Foot & Ankle International - July 2018 - Intro
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