Fulk, George D. PT, PhD; Ludwig, Miriam OT, MS; Dunning, Kari PT, PhD; Golden, Sue PT, BS; Boyne, Pierce BS; West, Trent PT, BS
Gait speed is a measure that is commonly used to assess walking ability in people with stroke, both in the clinic with individual patients and in clinical trials to compare outcomes between groups. It has robust psychometric properties. Excellent test-retest reliability coefficients have been reported.1–4 Gait speed has strong construct validity. In people with stroke, gait speed is related to lower extremity (LE) strength,5,6 LE motor control,7 balance,8,9 functional mobility,7,9 gait endurance,10 energy expenditure,11,12 and disability.11 Gait speed can predict discharge destination13–15 and falls.16 Gait speed can be used to classify individuals poststroke into different functional walking categories,17 and transitions from between categories corresponds to improvements in social participation.18 Gait speed has been referred to as the gold standard19for measuring walking ability and as a “sixth vital sign.”20(pg2)
An important property of any outcome measure is its ability to measure change. If a patient's gait speed improves from 0.50 m/s to 0.61 m/s, is this an important amount of change? A reference value is needed to aid the clinician in determining whether this 0.11 m/s change is important. The ability of an outcome measure to evaluate change is termed its responsiveness.21 There are a variety of methods to determine the responsiveness of an outcome measure. Two methods have particular clinical value, the minimal detectable change (MDC) and the minimal clinically important difference (MCID).22 The MDC is the minimal amount of change in the outcome measure that is necessary to exceed error and variability in that measure.22,23 It is considered to be the smallest amount of change in an outcome measure that is necessary for the change to be considered a true change.
The MDC of gait speed has been reported by various authors.1–4,24,25 The reported MDC of gait speed for people with stroke ranges from 0.07 m/s to 0.36 m/s, depending on the time since stroke onset, need for physical assistance when walking, need for an assistive device when walking, and the walking surface. An important limitation of the MDC is that it estimates only the threshold for true change. This change may not be an important amount of change. Anchor-based methods are required to estimate important change in an outcome measure.22,26
The MCID is an estimate of the minimum change in an outcome measure that is considered to be clinically important.27–29 The MCID uses an anchor-based approach to determine important change; it examines the relationship between the outcome measure of interest and some independent criterion to determine clinically important change.22,26,30 Different external criteria can serve as the anchor, such as mortality, level of function on a commonly used measure, or discharge destination.22,30 A Global Rating of Change (GROC) scale27 is often used as the independent external anchor. The GROC asks patients or clinicians to rate on an ordinal scale the degree of change (worse, no change, or better) they have experienced over a certain time.27 The article by Beninato and Portney31 in this special issue provides more information related to the responsiveness of outcome measures.
There is only 1 study of which we are aware that reported estimates of the MCID of gait speed in people with stroke. Tilson and colleagues32 estimated the MCID of gait speed in people with stroke who were between 20 and 60 days poststroke. These authors used the Modified Rankin Scale32 (MRS) as the anchor for important change in disability. Participants who demonstrated a change on the MRS of 1 or greater were considered to have an important amount of change. By using receiver operating characteristics (ROC) curve and Classification and Regression Tree analysis, they estimated an MCID for gait speed of 0.16 m/s.32 A limitation of this finding is that a change on the MRS may not directly relate to a change in walking ability.33,34
The MCID of gait speed for estimating change in overall disability has been reported, but further research is needed to determine important change in gait speed that reflects improvement in walking ability, as perceived by patients and physical therapists. The purpose of this study was to estimate clinically important change in gait speed by using 2 different anchors for what is considered “important”: stroke survivors' perception of change and physical therapists' perception of change in walking ability measured with a GROC scale.
Participants were recruited from 2 outpatient physical therapy (PT) clinics: 1 in the Northeast and 1 in the Midwest United States. Inclusion criteria were undergoing outpatient PT after a first-time stroke; enrollment within 6 months poststroke; score of 2 or greater on the Functional Ambulation Categories35 (indicating the ability to walk atmost with the assistance of 1 person); and successful completion of the screening portions of the orientation, language, memory, and reasoning/judgment subscales of the Cognistat.36 Participants were excluded if they could not walk independently before the stroke. The institutional review boards at Clarkson University, Good Shepherd Rehabilitation Hospital, and the University of Cincinnati approved the study, and all participants provided informed consent.
Since the primary purpose of this study was to estimate important changes in gait speed, the intervention was not controlled. Physical therapy interventions were individualized by the participants' primary physical therapists on the basis of the body structure/function impairments, activity and social participation limitations, and goals of the participants.
During the first and last week of outpatient PT, the participants' self-selected gait speed was measured by using a GAITRite (CIR Systems, Inc, Havertown, Pennsylvania). The GAITRite is an electronic walkway with embedded pressure sensors, which captures the geometry of footfalls as a person walks along the walkway. Computer software applications convert the raw footfall pattern data into temporal and spatial aspects of gait such as speed, cadence, step length, and percentage of gait cycle in single limb stance. The GAITRite is a valid and reliable method for assessing gait.38 Participants were instructed to walk across the 17′ × 3′ electronic walkway at their self-selected, comfortable pace by using whatever assistive device and/or orthotic that was prescribed by their physical therapist and, if necessary, with whatever amount of assistance/supervision that was required to ambulate safely. The use of assistive device and/or orthotic was not controlled from admission to discharge. One walking trail was recorded.
During the last week of outpatient PT, the participants also completed a GROC scale questionnaire.27 The GROC asked participants to rate their change in walking ability since they started outpatient PT on a 15 point ordinal scale with −7 indicating “a very great deal worse,” 0 indicating “no change,” and +7 indicating “a very great deal better” (Figure 1). During the last week of outpatient PT, the participant's primary physical therapists were also asked to rate the participant's change in walking ability since they started outpatient PT on the GROC scale. The participants and physical therapists were neither aware of the gait speed results when they completed the GROC nor of each other's GROC score. Fourteen physical therapists participated and rated their patient's walking performance. The median experience of the physical therapists was 5.5 years, with a range from 1 to 30 years.
Participants were dichotomized into 2 groups on the basis of GROC scores: those who answered between −4 and +4 were considered to have minimal to no change (stable/not improved) and those who answered +5 or more were considered to have important change (improved).39–44 This was done for both the participant GROC scores and the physical therapist GROC scores. Traditionally, a cutoff of +3 is used to determine minimal change, where participants who answer +4 or more are considered to have experienced minimally important change.27 A the cutoff of + 5 was chosen for 2 reasons: all people who were undergoing PT during this relatively early time frame after stroke were likely to experience some change in walking ability, and we were interested in estimating how much change in gait speed was considered important, not just minimally important. A score of +5, “a good deal better” may be reflective of more important improvement than “somewhat” or “moderately” better, which have been used to indicate “minimally important” improvement.22,39,40 Other researchers have indicated a need to identify changes in outcome measures that are more than just “minimally” important.29,45,46
An independent t test (continuous data) and χ2 test of independence (categorical data) were used to determine whether participants baseline characteristics (gait speed; age; gender; side of paresis; time from stroke to beginning outpatient PT; time from stroke to discharge from outpatient PT; number of PT visits; use of assistive device; amount of physical assistance required to ambulate; and temporal/spatial aspects of gait, eg, cadence, unaffected LE step length, affected LE step length, and percentage of gait cycle in stance on the affected LE) were different between groups (improved and stable/not improved).
An independent t test was used to examine the differences between change in gait speed and change in other temporal/distance measures of gait (cadence, unaffected LE step length, affected LE step length, and percentage of gait cycle in stance on the affected LE) between participants categorized into the improved group and those categorized into the stable/not-improved group. This was done to determine whether participants' and physical therapists perception of change in walking ability, as measured by the GROC scale, accurately reflected objective changes in gait characteristics. The relationship between the participant and physical therapist GROC and change in gait speed was explored by using a Spearman correlation coefficient to determine the relationship between the anchor (GROC) and the change score (change in gait speed from admission to discharge).
Receiver operating characteristic curves were constructed by plotting sensitivity values (true positive rate) on the y axis and1-specificity values (false positive rate) on the x axis for different changes in gait speed for distinguishing improved from stable/not-improved participants. Separate ROC curves were constructed for each of the 2 anchors: participant GROC and physical therapist GROC. The area under the curve (AUC) and 95% confidence intervals were obtained as a method for describing the ability of each measure to distinguish participants who improved from those who were stable/not improved.21,38–43 An AUC of 0.50 or less indicates that change in gait speed has no ability beyond chance to distinguish between participants who improved and those who were stable/not improved (as defined by the score on the GROC), while a value of 1.0 would indicate perfect ability to distinguish between improved and stable participants.
Clinically important changes in gait speed were estimated by identifying the point on the ROC curve nearest the upper left-hand corner, which is considered to be the best cutoff score for distinguishing improved from stable/not-improved participants.47 Sensitivity, specificity, positive likelihood ratio, and negative likelihood ratio were calculated for the estimated important change values. This was done for both the anchors (participant GROC and physical therapist GROC). Previous research has indicated that the use of an assistive device and the amount of assistance required to walk may have an impact on the MDC of gait speed in people with stroke.1 Because of this, the clinically important change in gait speed for participants who initially used an assistive device to ambulate and who could not walk independently was estimated by using the same methods described previously.
Forty-four individuals with stroke participated in the study. On the basis of the participants' GROC scale, 11 participants (26.8%) were categorized as stable/not improved (GROC score ≤ +4) and 30 (73.2%) were categorized as improved (GROC score ≥ +5). The lowest GROC score reported by a participant was +2 (Table 1). In 3 participants, the participant GROC data were missing; data from these participants were not used for the analysis pertaining to the participants GROC but were used in the analysis pertaining to the physical therapists GROC as the anchor. On the basis of physical therapists GROC scale, 22 participants (51.2%) were categorized as stable/not improved (GROC score ≤ +4) and 21 (48.8%) were categorized as improved (GROC score ≥ +5). The lowest GROC score reported by a physical therapist was +1 (Table 1). The physical therapists GROC data for 1 participant were missing; data from this participant were not used for the analysis pertaining to the physical therapists' GROC but were used in the analysis pertaining to the participants' GROC as the anchor.
There were no significant differences in baseline characteristics (Table 2) and gait characteristics (Table 3) between the improved group and the stable/not-improved group (P > 0.05).
There was a significant difference in change in gait speed and other gait characteristics between participants who were categorized as stable/not improved versus improved on the basis of stroke survivors' GROC scores (Table 4). The mean change in gait speed for participants who were categorized as improved was 0.31 m/s, while mean change in gait speed for participants who were categorized as stable/not improved was 0.11 m/s; P < 0.05. There was also a significant difference in change in affected and unaffected step length and change in the percentage of gait cycle in single limb stance on the affected LE between improved and stable/not-improved participants. There were no significant differences in changes in gait speed (0.28 m/s vs. 0.22 m/s) or gait characteristics between participants that were categorized as stable/not improved vs. improved based on physical therapists GORC scores, p > 0.05 (Table 4).
As shown in Table 5, by using the participantsGROC score as the anchor, the AUC of the ROC curve was 0.80 (Figure 2). This resulted in an estimated clinically important change in gait speed of 0.175 m/s. Using the physical therapists GROC score as the anchor, the AUC of the ROC curve was 0.58 (Figure 3). This resulted in an estimated clinically important change in gait speed of 0.190 m/s.
The ROC curve results were similar for data obtained from participants who initially used an assistive device to walk and for data obtained from participants who could not walk independently (Table 6). Using the participants GROC score as the anchor for participants who used an assistive device to walk at baseline, the AUC of the ROC curve was 0.80. This resulted in an estimated clinically important change in gait speed of 0.175 m/s. Using the physical therapists' GROC score as the anchor for participants who used an assistive device to walk at baseline, the AUC of the ROC curve was 0.64. This resulted in an estimated clinically important change in gait speed of 0.190 m/s. Using the participants' GROC score as the anchor for participants who could not walk independently at baseline, the AUC of the ROC curve was 0.85. This resulted in an estimated clinically important change in gait speed of 0.130 m/s. Using the physical therapists' GROC score as the anchor for participants who could not walk independently at baseline, the AUC of the ROC curve was 0.71. This resulted in an estimated clinically important change in gait speed of 0.185 m/s.
The results of this study estimate that a change of 0.175 to 0.190 m/s in gait speed is clinically important in people with stroke who are undergoing outpatient PT during the subacute stage of recovery after stroke. The estimated clinically important change in gait speed is similar using either stroke survivor's perception of change or a physical therapist's perception of change in the walking ability. However, the AUC of the ROC curve, sensitivity, specificity, positive LR, and negative LR are all stronger based on stroke survivors' perception of change compared with physical therapists' perception of change. In addition, there was not a significant relationship between the physical therapists' GROC scores and change in gait speed, while there was a significant relationship for participants perception of change and change in gait speed. Using the important change value of 0.175 m/s in gait speed estimated from the participants' may be a more valid estimate of important change in gait speed.
These results can be used to help clinicians determine how much change in gait speed is necessary during the subacute stage of recovery after stroke to reflect a clinically meaningful improvement in walking ability. For example, if a patient's gait speed was 0.45 m/s at the initial examination, and on discharge from outpatient PT, it improved to 0.67 m/s, the treating physical therapist could conclude that this 0.22 m/s improvement was clinically meaningful. This amount of improvement exceeds the estimated clinically important change in gait speed of 0.175 m/s of this study.
In addition to reporting the statistical significance of differences in gait speed between 2 groups, researchers can use this criterion value for important change in gait speed to examine differences between intervention groups by comparing the proportion of subjects in the experimental group with that in the comparison group whose change in gait speed exceeded 0.175 m/s. These proportions can be used to determine the number needed to treat (NNT). The NNT is the number of patients who need to be treated with the experimental intervention before you can be sure that 1 patient improved, who would not have improved with the comparison intervention.48 Presenting results of research studies by using the NNT in addition to statistically comparing the differences between the groups in this manner has greater clinical utility than presenting only the statistical difference between groups.48 The lower the NNT, the greater the clinical impact of the experimental intervention compared with the comparison intervention.
Our results are similar to the MDC findings for gait speed reported by other authors for subjects with a similar time since stroke onset. In a sample of people 2.8 months poststroke, Hill and colleagues2 reported a 95% confidence interval for true change in gait speed of 0.08 to 0.16 m/s. In people 3.7 months poststroke, Stephens and Goldie4 reported a 95% confidence interval related to the error of measuring change of −0.10 to 0.12 m/s on a tiled walking surface and −0.13 to 0.17 m/s on a carpeted walking surface. The estimated clinically important change in gait speed of 0.175 m/s found in this study is slightly greater. This is encouraging, as an important change in an outcome measure should exceed the amount of error and variability in a measure.29,30
Our finding of an estimated clinically important change in gait speed of 0.175 to 0.190 m/s is also similar to the MCID of gait speed of 0.16 m/s, reported by Tilson and colleagues.32 This slight difference in values is likely due to the difference in the anchor defining important change; stroke survivor and physical therapist report of important change in walking ability versus improvement in overall disability, as measured by the MRS. It is encouraging that despite these differences, the estimated important change in gait speed was very close between the 2 studies; a difference of only 0.015 m/s was noted.
It appears that people with stroke were better able than physical therapists to judge change in their walking ability as it related to gait speed. There were differences in change in some gait characteristics between the improved and stable groups based on participants' GROC scores, but there were not differences in change in gait characteristics between the improved and stable groups based on physical therapists' GROC scores. This could be a result of physical therapists having preconceived criteria for important change in gait based on experience and due to the fact that they see the patient in a relatively closed environment for a relatively short period of time. People with stroke are basing their criteria for important change on how changes in walking ability are impacting their daily lives. The physical therapist may not realize these changes as he/she sees the patient for only a limited amount of time in the environment of the clinic. Physical therapists may be basing their criteria for important change on other components of gait such as quality of movement, and this may not be reflected in the change in gait speed.
There are limitations of using a GROC scale to determine the threshold for important change in outcome measures. Participants may have difficulty recalling their initial status when reporting on change in ability on the GROC scale.29,30 They may instead report on their current status. However, in this sample of people with stroke, it appeared that they were able to accurately report on change in walking ability on the GROC to some degree. There were significant differences in change in some of the gait characteristics between stroke survivors who were categorized as improved compared with those who were categorized as stable/not improved based on their GROC scores. In addition, the GROC is a global scale. It asked participants to rate their overall improvement in walking ability; however, gait speed is just one aspect of walking ability. Some participants with stroke and therapists perceived that their walking ability improved, but this was not reflected in a change in gait speed. Other aspects of gait may have improved such as requiring less assistance or a more normal walking pattern.
Another limitation of this study is the relatively small sample size used to estimate the important change in gait speed, although other studies that have used a GROC scale for the anchor of important change have used similar sample sizes.38,49 The estimated important change values reported here should be used only with individuals with similar characteristics. Most of the participants in this study had a mild to moderate stroke, as evidenced by the fact that most subjects would be classified as limited community ambulators on the basis of their gait speed.17 We did not control for the type of assistive device used when measuring gait speed. At both the initial and discharge gait speed measurement participants walked with the assistive device they were using at that time. Any potential change in assistive device to one that was less-supportive may have impacted their gait speed. However, in clinical practice, physical therapists often prescribe less-supportive assistive devices as patients improve. Of the 19 participants in this study who changed from an assistive device to either no assistive device or a less-supportive one, only 3 had a decrease in gait speed at discharge.
Estimated clinically important change values are likely to vary depending on the baseline scores of the outcome measure.29,30,50 Individuals with an initial low score on an outcome measure may need only a relatively small amount of change for it to be important. If we considera patient who has an initial gait speed of 0.15 m/s and improves to 0.30 m/s, this change represents a doubling of the patient's gait speed and may be important, but it does not exceed the estimated important change threshold of 0.175 m/s. This criterion score for important change in gait speed should only be used with patients that have characteristics that are similar to the participants of this study.
A change of 0.175 m/s is estimated to be a clinically important change in walking ability in people undergoing outpatient PT after a stroke. This criterion score can be used with individual patients to interpret changes in gait speed or by clinical researchers to compare the effectiveness of different interventions designed to improve walking ability in individuals with stroke. The estimated important change exceeded error/variability in the measurement of gait speed2,4 and is similar to a reported value of the MCID32 that used improvement in disability as the anchor for important change in gait speed.
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Stroke; Gait; Walking; Outcome Measures; Responsiveness