Evaluation of the minimal important difference for the feeling thermometer and the St. George's Respiratory Questionnaire in patients with chronic airflow obstruction
Introduction
Interpreting changes in health-related quality of life (HRQL) remains a challenge to both investigators and clinicians [1], [2], [3], [4], [5]. Ensuring widespread acceptance of HRQL as a patient-important outcome will require clear guidelines for establishing the significance of changes in HRQL [2].
One way of describing and interpreting changes in HRQL is through the minimal important difference (MID), the smallest difference in score in the domain of interest that patients perceive as important, either beneficial or harmful, and that would lead the clinician to consider a change in the patient's management [1]. Approaches to establishing the MID include distribution-based methods, reliance on experts (opinion-based methods), and approaches that rely on sequential hypothesis formation and testing (predictive or data driven) [6]. A particular form of the last approach (also characterized as anchor-based methods) relies on examining the associations between scores on the instrument that is under investigation and an anchor, typically an independent measure of HRQL that clinicians can easily interpret [1].
The Chronic Respiratory Questionnaire (CRQ), which measures HRQL in patients with chronic airflow limitation (CAL), has proven valid, responsive, and useful in the hands of many investigators, in many settings, in a number of countries [7], [8], [9], [10], [11], [12], [13], [14], [15]. A substantial body of evidence suggests that the MID of the CRQ, which uses seven-point Likert-type scales in four domains [16], is approximately 0.5 on the seven-point scale. Changes of 1.0 and 1.5 correspond, respectively, to moderate and large improvement or deterioration [17], [18], [19].
Another instrument investigators have used frequently in CAL patients is the St. Georges Respiratory Questionnaire (SGRQ) [20]. Jones et al. suggested a change of 4 in the total score, ranging from 0 to 100 as the MID for the SGRQ [21], with differences for moderate and large changes in HRQL of 8 and 12, respectively [3]. Little empiric evidence supporting the interpretability of the SGRQ has, however, emerged in the peer-reviewed literature.
In theory, if both instruments measure HRQL in similar domains and their scores correlate, one should be able to map changes in one instrument on to changes in the other. Hence, changes of 0.5 on the CRQ should correspond to changes of 4 on the SGRQ. Thus, if there is a sufficiently high correlation between these two instruments, one could confirm the MID of the SGRQ using the MID from the CRQ, for which there is greater empirical support.
The feeling thermometer (FT), a visual analog scale (VAS) shown as a thermometer, has recently become a focus of increasing investigation [22]. The FT provides preference scores on a scale from 0 (dead) to 100 (full health), and is simpler and generally more efficient than the standard gamble (SG), the gold standard instrument for utility measurement [22], although in one study respondents found the SG easier to complete than the FT [23]. Accumulating evidence suggests that the FT works well as an evaluative instrument in various groups, including patients with CAL [24], [25], [26], [27], [28], [29]. As with many other HRQL instruments, however, interpretability of the FT remains largely unexplored. We hypothesized that we can use the CRQ and the SGRQ to establish the MID for the FT in patients with CAL.
Thus, the purpose of this study was twofold. First, we compared the MID of the CRQ and the SGRQ. Second, having validated the MID of the SGRQ, we used both the CRQ and SGRQ to establish the MID for the FT.
Section snippets
Data and study design
The data for these analyses come from a study investigating measurement of HRQL in patients participating in respiratory rehabilitation programs at the University of Toronto and McMaster University in Hamilton, Ontario, Canada [29], [30]. Eligible patients included all inpatients and outpatients with CAL enrolled in the rehabilitation programs. We excluded patients with the following diagnoses: α1-antitrypsin deficiency, silicosis, sarcoidosis, asbestosis, lupus, or cancer, and those unable to
Baseline characteristics of study participants
Forty-six of the 130 patients initially enrolled did not complete the study. The reasons for not completing the study were: refusal to continue the interviewing process (n = 24), patients' statement “too sick” to complete the second interview (n = 7), failure to complete the rehabilitation program (n = 6), inability to contact the patient because they were out of the region (n = 4), and development of new severe symptomatic illness or cognitive impairment (n = 5). The patients who did not complete the
Discussion
Determining what constitutes important differences in HRQL scores is important for interpreting intervention effects on HRQL. We compared the MID for the SGRQ and CRQ and our results indicate an MID for the SGRQ that is within the range of the previously suggested value (approximately 3.1 compared to 4 on the 100-point scale) [21]. We also evaluated the MID for the FT, a preference-based HRQL instrument for patients with CAL, by comparison with the CRQ and the SGRQ. Based on linear regression
Acknowledgements
This work was supported by a grant from the Medical Research Council of Canada to Gordon H. Guyatt, and by a Buswell Fellowship to Holger J. Schünemann.
References (40)
- et al.
Clinical Significance Consensus Meeting G. Methods to explain the clinical significance of health status measures
Mayo Clinic Proc
(2002) - et al.
Clinical Significance Consensus Meeting G. Patient, clinician, and population perspectives on determining the clinical significance of quality-of-life scores
Mayo Clinic Proc
(2002) - et al.
Clinical Significance Consensus Meeting G. Assessing meaningful change in quality of life over time: a users' guide for clinicians
Mayo Clinic Proc
(2002) - et al.
Clinical Significance Consensus Meeting. The clinical significance of quality-of-life results: practical considerations for specific audiences
Mayo Clinic Proc
(2002) - et al.
Randomised controlled trial of respiratory rehabilitation
Lancet
(1994) - et al.
The long-term benefits of out-patient pulmonary rehabilitation on exercise endurance and quality of life
Chest
(1993) - et al.
Physiologic factors that determine the health-related quality of life in patients with COPD
Chest
(1996) - et al.
Further evidence supporting an SEM-based criterion for identifying meaningful intra-individual changes in health-related quality of life
J Clin Epidemiol
(1999) - et al.
Assessing the minimal important difference in symptoms: a comparison of two techniques
J Clin Epidemiol
(1996) - et al.
Measurement of health status. Ascertaining the minimal clinically important difference
Control Clin Trials
(1989)
The St George's Respiratory Questionnaire
Respir Med
Health-related quality of life and functional status of patients with rheumatoid arthritis randomly assigned to receive etanercept or placebo
Clin Ther
A randomized controlled trial to evaluate the effect of informing patients about their pretreatment responses to two respiratory questionnaires
Chest
Measuring functional status in chronic lung disease: conclusions from a randomized control trial
Respir Med
Prospective validation of clinically important changes in pain severity measured on a visual analog scale
Ann Emerg Med
Interpreting thresholds for a clinically significant change in health status in asthma and COPD
Eur Respir J
Foundations of the minimal clinically important difference for imaging
J Rheumatol
Reliability and validity of the chronic respiratory questionnaire (CRQ)
Thorax
Effects of a supervised home exercise program for patients with chronic obstructive pulmonary disease
Phys Ther
Randomised controlled trial of weightlifting exercise in patients with chronic airflow limitation
Thorax
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