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Reliability and Validity of the Patient-Specific Functional Scale in Community-Dwelling Older Adults

Mathis, Ruth Ann PT, PhD1; Taylor, J. David PT, PhD2; Odom, Brian H. MSPT1; Lairamore, Chad PT, PhD3

Author Information
Journal of Geriatric Physical Therapy: July/September 2019 - Volume 42 - Issue 3 - p E67-E72
doi: 10.1519/JPT.0000000000000188
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Abstract

INTRODUCTION

The Patient-Specific Functional Scale (PSFS) is a questionnaire that was designed to be easily administered, applicable to a variety of conditions, and a means of measuring changes in physical function in response to an intervention. The PSFS allows the clinician to assess activities of physical function that are important to the patient. Administration of the PSFS enables an individual to report 3 to 5 functional activities that he or she either is unable to perform or has difficulty performing. The individual is instructed to rate the difficulty of each activity on a Likert scale of 0 to 10. A rating of “0” corresponds to being unable to perform the activity, while a rating of “10” corresponds to being able to perform the activity without difficulty.1

Previous studies indicate the PSFS is a reliable and valid assessment tool for physical function in orthopedic populations.1–4 In a systematic review,5 measurements for the PSFS have been reported to be reliable and valid for musculoskeletal lower extremity and spine dysfunction for the following conditions: knee dysfunction, acute low back pain, mechanical low back pain, cervical radiculopathy, and neck dysfunction. Furthermore, the PSFS has been found to be reliable and valid in upper extremity musculoskeletal dysfunction.6–9

Previous studies have also investigated the degree to which the PSFS compared to the International Classification of Functioning, Disability, and Health (ICF) domains.10–12 The result of these studies indicated that items of the PSFS could be mapped to the ICF domains and is an appropriate measure of the various aspects of physical function in people with musculoskeletal disorders.10–12 Although the results of these investigations support the content validity of the PSFS, data on the validity of the PSFS in various populations, when compared with measures of physical function with established validity, are lacking.

Although research indicates that the PSFS is an appropriate measure of physical function in patients with musculoskeletal conditions, a problem exists in that data on the reliability and validity of the PSFS in populations other than patients with musculoskeletal disorders are limited. Horn et al5 identified gaps in the current literature and concluded that supportive evidence for the reliability and validity of the PSFS was lacking in many cohorts. Out of 55 studies concerning the PSFS, 47 studies were performed in populations where the validity of the PSFS had not been previously reported.5 A broader application of the PSFS is important, considering older adults often demonstrate more than one chronic condition that is currently being managed and are therefore difficult to place in restrictive diagnostic categories.13 Establishing reliability and validity in a group of older, community-dwelling adults presenting with a variety of medical histories and comorbidities would allow the assessment tool to have a greater degree of generalization than is currently found in the evidence. The research hypothesis was that the PSFS is a reliable and valid outcome measure of physical function in older adults. The purpose of this study was to investigate the reliability and validity of the PSFS in community-dwelling older adults.

METHODS

Study Design

This multisite investigation was a reliability and validity study. Data were collected by 2 physical therapists trained on the study protocol. Administration of the outcome measures by the investigators was scripted. Participants completed 3 self-report questionnaires and 3 physical performance measures. Block randomization determined whether questionnaires or performance measures were administered first. Following the initial randomization, block randomization further determined the order in which the 3 questionnaires or 3 physical performance measures were administered. To investigate the reliability and validity of the PSFS, the 3 self-report questionnaires and 3 physical performance measures were administered during an initial session and then repeated in a subsequent session approximately 48 hours after the first session.

Setting and Participants

Community-dwelling older adults were recruited from the Central Arkansas geographic area, specifically Faulkner and White counties, through the use of study recruitment flyers, word of mouth, and electronic announcements. Data were collected at 3 locations: (1) Faulkner County Senior Wellness and Activity Center, (2) White County Senior Citizens Center, and (3) College Square independent living center. Participants were excluded from the study if any of the following criteria were met: (1) younger than 65 years, (2) acute or terminal illness, (3) myocardial infarction in the past 6 months, (4) unstable cardiovascular or metabolic disease, (5) neuromuscular or musculoskeletal disorders severely disrupting voluntary movement, (6) limb amputation, (7) upper or lower extremity fracture in the past 3 months, (8) cognitive impairment, or (9) inability to participate in this study due to a physical impairment. Participants signed a written, informed consent prior to their participation in the study. Following informed consent, participants filled out a detailed medical history and the following were assessed: blood pressure, heart rate, and lower extremity sensation. This study was approved by the Harding University and University of Central Arkansas Institutional Review Boards.

Outcome Measures

The PSFS is a questionnaire aimed at recording patients' perceptions of their physical function limitations. Patients define their 3 main complaints (difficulties performing certain physical activities) and rate the difficulty of performance on an 11-point Likert scale (0 = unable to perform; 10 = no difficulties).1 As previously mentioned, the PSFS has been shown to be reliable and valid as an outcome measure of physical function in patients with musculoskeletal disorders.1–4

The Lower Extremity Functional Scale (LEFS) is a region-specific, patient-reported measure of physical function that can be used to assess functional status in patients with a variety of lower extremity disorders. The LEFS consists of 20 questions designed to assess the degree of difficulty of specific functional tasks. Each question is scored using a 5-point Likert-like scale, with “0” defined as “extreme difficulty” and “4” as “no difficulty,” and a total score ranging from 0 to 80, with higher scores representing higher levels of physical function.14 Previous research suggests that the LEFS is a reliable and valid measure of physical function in individuals with lower extremity musculoskeletal conditions and community-dwelling older adults.15,16

The Activity-specific Balance Confidence Scale (ABC) is a self-reported 16-item questionnaire that asks people to score their perceived level of balance confidence when performing common indoor and outdoor activities of daily living. Each item was scored on an 11-level rating scale, from 0% (no confidence) to 100% (full confidence in performing the activity without losing balance). The total ABC score is the mean sum of the individual item scores.17 Reliability and validity of the ABC in older adults have been previously reported.18,19

The Short Physical Performance Battery (SPPB) is composed of 3 timed components to assess balance, gait speed, and lower limb force. Each component is scored from 0 to 4 points, with individuals not able to perform a test scored at 0, and 4 representing the best category of performance. The final score is the sum of the 3 tests, ranging from a minimum of 0 to a maximum of 12 points.20 The SPPB has been found to be a reliable and valid assessment of physical function in community-dwelling older adults.21–23

The Berg Balance Scale (BBS) is a performance-based instrument originally developed by Berg et al24 for assessment of functional balance in older adults. The BBS assesses performance on 5 levels, from 0 (cannot perform) to 4 (normal performance), on 14 different tasks involving functional balance control, including transfer, turning, and stepping, giving a score between 0 (poor) and 56 (normal).24 Reliability and validity data of the BBS have been previously published and the BBS is considered a standard in the assessment of physical function in community-dwelling older adults.24–26

The Timed Up and Go (TUG) is a timed test that requires the participant to stand up from a chair with armrests, walk 3 m to a line on the floor, turn around, return to the chair, and sit down.27 The TUG has been shown to be valid and reliable in community-dwelling older adults and is also considered a standard for measuring physical function.26,28

Data Analysis

An a priori power analysis estimated that a total sample size of 29 participants would detect a correlation of 0.50, with a statistical power of at least 0.80 at an α level of .05. Means and standard deviations were calculated for age, body mass, and height to determine the participants' characteristics. Reliability of the PSFS and the remaining assessments were evaluated using the intraclass correlation coefficient (ICC[2,1]) at an α level of .05, the minimal detectable change using a 95% confidence interval (MDC95), and the standard error of the measurement (SEM). Portney and Watkins29 state that correlations ranging from 0.50 to 0.75 indicate a moderate to good relationship and correlation values above 0.75 are considered good to excellent.

The Pearson correlation coefficient (r) was used to assess validity of the PSFS by investigating the relationships between the PSFS and the ABC, LEFS, BBS, SPPB, and TUG assessments. Scatter plots with regression lines were used to visually illustrate statistically significant correlations between the PSFS and the other measures of physical function. For statistically significant correlations, Bland-Altman plots with 95% limits of agreement (LOA) were used to illustrate visual representations of measurement errors against true values by plotting the difference between the PSFS and comparison test values versus the mean of PSFS and comparison test scores. The 95% LOA was defined as 2 standard deviations above and below the mean of the difference scores. Because of differences in scoring of the outcome measures, transformation of the data to z scores was performed when Bland-Altman plots were constructed and 95% LOA were calculated so that the outcome measures could be compared on the same scale. To investigate validity, data from the initial assessments were analyzed. All statistical analyses were conducted using IBM SPSS 22 for Windows (SPSS, Inc, Chicago, Illinois).

RESULTS

A total of 34 potential participants were screened for the study and 3 were excluded: 2 for not meeting inclusion criteria and 1 for a schedule conflict. Thirty-one people (11 males, 20 females, mean age = 81.1 years [8.3]) participated in this study. The participants' mean body mass and height were 70.6 kg (15.0) and 164.5 cm (9.8), respectively. No adverse events occurred during this study.

Means and standard deviations for all variables are presented in the Table, along with reliability findings for the PSFS, ABC, LEFS, BBS, SPPB, and TUG assessments. The ICC[2,1] score for the reliability of the PSFS was 0.82. For the remaining assessments, the ICC[2,1] values ranged from 0.85 to 0.96. The MDC95 for the PSFS was 2.8 and the SEM was 1.0.

Table. - Reliability of Patient-Specific Functional Scale, Activity-Specific Balance Confidence Scale, Lower Extremity Functional Scale, Berg Balance Scale, Short Physical Performance Battery, and Timed Up and Go
Assessment Initial Test Mean (SD) Retest Mean (SD) ICC MDC95 SEM
PSFS 4.5 (2.4) 4.4 (2.4) 0.82 (0.67-0.91) 2.8 1.0
ABC 71.3 (19.5) 73.9 (14.3) 0.85 (0.73-0.92) 21.0 7.5
LEFS 49.9 (15.0) 50.2 (14.6) 0.91 (0.83-0.95) 12.4 4.5
BBS 45.6 (8.2) 47.1 (7.4) 0.87 (0.75-0.93) 8.2 2.9
SPPB 8.1 (2.7) 8.5 (2.6) 0.93 (0.86-0.96) 1.9 0.6
TUG 11.4 (4.7) 10.9 (4.6) 0.96 (0.93-0.98) 1.6 0.5
Abbreviations: ABC, Activity-specific Balance Confidence Scale; BBS, Berg Balance Scale; ICC, intraclass correlation coefficient (95% confidence interval); LEFS, Lower Extremity Functional Scale; MDC95, minimal detectable change based on the 95% confidence interval; PSFS, Patient-Specific Functional Scale; SEM, standard error of the measurement; SD, standard deviation; SPPB, Short Physical Performance Battery; TUG, Timed Up and Go.

Figure 1 presents correlation data between the PSFS and the other measures of physical function. Statistically significant correlations (P < .05) were found between the PSFS and 3 of the comparison measures (ABC, LEFS, and SPPB). Scatter plots with regression lines for PSFS scores versus scores for the ABC, LEFS, and SPPB are illustrated in Figure 1, indicating positive, linear relationships. The mean differences and 95% LOA in z scores between the PSFS and the ABC, LEFS, and SPPB were less than 0.001 (LOA =−1.6 to 1.6), less than 0.001 (LOA =−1.2 to 1.2), and less than 0.001 (LOA =−2.1 to 2.1), respectively. Because the mean differences in z scores were close to 0, reflecting the scaling of the outcomes between instruments, the discussion of agreement has been focused on the 95% LOA. Bland-Altman plots (Figure 2) illustrated the 29 of 31 data points were within the 95% LOA for the PSFS versus the ABC, 29 of 31 data points were within the 95% LOA for the PSFS versus the LEFS, and 31 of 31 data points were within the 95% LOA for the PSFS versus the SPPB.

F1
Figure 1.:
Scatter plots with regression lines for Patient-Specific Functional Scale scores versus Activity-specific Balance Confidence Scale, Lower Extremity Functional Scale, Short Physical Performance Battery, Berg Balance Scale, and Timed Up and Go scores.
F2
Figure 2.:
Bland-Altman plots for Patient-Specific Functional Scale scores versus Activity-specific Balance Confidence Scale, Lower Extremity Functional Scale, and Short Physical Performance Battery scores.

DISCUSSION

The ICC data from this study suggest that the PSFS is a reliable measure of physical function in older adults. The ICC 95% confidence interval of the PSFS indicates that clinicians can be 95% confident that the ICC for the population of older adults is between 0.67 and 0.91. The MDC findings infer that a change of less than 2.8 points using the PSFS is likely due to measurement error, but a change of 2.8 points or greater is possibly a true change in physical function.

The validity of the PSFS as a measure of physical function was investigated by comparing the PSFS to various other assessments. The Pearson correlation coefficient showed statistically significant associations between the PSFS and the ABC, LEFS, and SPPB. Scatter plots with regression lines also illustrated such relationships. A comparison of mean differences, 95% LOA, and Bland-Altman plots suggests that considerable agreement existed between the PSFS and the ABC, LEFS, and SPPB.

The agreement between the PSFS and the ABC, LEFS, and SPPB could be explained by similarities in measuring the same physical function constructs. The PSFS is a questionnaire that asks a patient to list any specific physical function limitations and rate the level of difficulty, with each limitation using a Likert scale of 0 to 10. The physical function limitations reported by the study participants on the PSFS may have been closely related to the items of the ABC, LEFS, and SPPB. When the data of the PSFS, ABC, LEFS, and SPPB were transformed into z scores so that the outcomes of all measures could be compared on the same scale, a considerable degree of agreement was found. The greatest agreement (narrowest 95% LOA) was between the PSFS and the LEFS. Because the measurement scales are different, the clinical use of the instruments interchangeably may be problematic. However, the agreement between the PSFS and the other previously validated measures supports the use of the PSFS for measuring physical function in community-dwelling older adults.

The findings of this study address 2 needs presented in the research literature: (1) to address the gap identified by Horn et al5 to broaden the reliability and validity of the PSFS to cohorts other than those with musculoskeletal dysfunction and (2) to address the need identified by Nicholas et al30 to establish outcome measures, which can be applied to a variety of conditions represented in a given demographic. The findings of our study are similar to the findings of other researchers who have investigated the reliability and validity of the PSFS in specific musculoskeletal populations.1–4 Cleland et al2 found the PSFS to be reliable in patients with cervical radiculopathy, with an ICC = 0.82 and an MDC of 2.1. Likewise, the ICC and MDC values for the PSFS in our study were 0.82 and 2.8, respectively. Abbott et al31 reported a correlation of 0.43 (P < .001) between the PSFS and the LEFS in patients with lower extremity musculoskeletal disorders. In our investigation, the correlation between the PSFS and the LEFS was higher (0.81, P < .001). The difference in correlation values between Abbott et al31 and our study may be explained by differences in study participants.

Loss of physical function and mobility is a major determinant in whether or not an older adult is able to remain living within their community or constrained to reside in a facility where assistance is provided, whether that be a group home, assisted living, or long-term care center.32 Loss of physical function is associated with physical disability, reduced quality of life, and mortality.32,33 Establishing reliability and validity of the PSFS in the older, community-dwelling adult is significant, as the questionnaire could be used clinically to identify loss of physical function, specific to the individual, which may have previously gone undetected.

A limitation of this study is that participants were not blinded to questionnaire scores, as this was not possible. A 48-hour time lapse was implemented between the initial and subsequent sessions to minimize recall of the participant. Another limitation is that the study included community-dwelling older adults with a variety of conditions and therefore the findings may not be applicable to specific diagnoses. However, the lack of diagnosis homogeneity is also a strength, as it broadens the generalizability of the results to older adults. Evidence broadening the generalization of the PSFS lends greater legitimacy to this self-report measure, which has already been widely employed. Finally, data were not collected on the responsiveness to change of the PSFS. Future studies should investigate responsiveness to change of the PSFS in this population. Furthermore, future studies would benefit from a larger sample size and a participant pool with equal distribution of males and females.

CONCLUSION

The results of this study have provided evidence that the PSFS is a reliable and valid questionnaire to assess physical function in community-dwelling older adults. The PSFS is significantly associated with the ABC, LEFS, and SPPB, and the findings of this investigation infer that considerable agreement exists between the PSFS and the other measures of physical function. Furthermore, a change of 2.8 or greater on the PSFS indicates a true change in physical function in community-dwelling older adults.

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Keywords:

community-dwelling older adults; Patient-Specific Functional Scale; reliability and validity

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