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* From the Division of Pulmonary and Critical Care Medicine, Department of Medicine (Drs. Chang, Curtis, and Raghu), and the Department of Health Services (Drs. Curtis and Patrick), University of Washington, Seattle, WA.
Correspondence to: Ganesh Raghu, MD, FCCP, Division of Pulmonary and Critical Care Medicine, University of Washington, Box 356522, Seattle, WA 98195; e-mail: graghu{at}washington.edu
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Abstract |
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 TOP Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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Design: Cross-sectional study.
Setting: Outpatient pulmonary clinic at a university referral center.
Patients: Fifty patients with interstitial disease such as idiopathic pulmonary fibrosis, sarcoidosis, hypersensitivity pneumonitis, and asbestosis.
Interventions: Patients were administered four quality-of-life questionnaires, the Medical Outcomes Study Short Form 36 (SF-36), the Quality of Well-being scale (QWB), the Chronic Respiratory Questionnaire (CRQ), and the St. George’s Respiratory Questionnaire (SGRQ). Patients concomitantly underwent pulmonary function testing and performed a 6-min walk.
Measurements and results: Validation of these instruments was based on testing an a priori hypothesis that worse quality-of-life scores should correlate with more severe physiologic impairment demonstrated by pulmonary function tests, exercise tolerance on the 6-min walk, and dyspnea scores. Our patients, on average, had a moderate degree of physiologic impairment and demonstrated moderately decreased quality-of-life scores. Scores from all four quality-of-life questionnaires correlated significantly with 6-min walk distance and dyspnea score. Scores from the SF-36, QWB, and SGRQ showed significant correlation with FVC, FEV1, and diffusing capacity as well. The SF-36 and SGRQ consistently showed the strongest correlation with physical impairment.
Conclusions: Our findings indicate that preexisting quality-of-life instruments can be applied to patients with interstitial lung disease and suggest that the SF-36 and the SGRQ, in particular, are sensitive tools for assessing quality of life in these patients. Future intervention studies of patients with interstitial lung disease should consider using these measures.
Key Words: health status indicators • lung diseases • interstitial • pulmonary fibrosis • quality of life
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Introduction |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Toxicity of therapy is one important reason why quality of life should be considered a separate therapeutic end point in addition to stability of disease by pulmonary function or radiographic severity. However, clinicians and researchers have primarily followed physiologic and radiographic end points because there are no validated methods for directly measuring quality of life in this patient population. Standardized and validated methods for assessing the health-related quality of life in interstitial lung disease are needed to better understand the effect of these diseases and to evaluate standard and new therapies.
Since the early 1980s, several standardized generic quality-of-life questionnaires have been developed to measure health-related quality of life for a variety of diseases.6 7 8 9 10 Subsequently, respiratory-specific quality-of-life questionnaires were created to specifically address the problems faced by patients with chronic pulmonary disease.11 Although many of these questionnaires were designed originally for patients with COPD, they have been applied to other patient populations as well.12 13 14 In this study we have evaluated whether these generic and respiratory-specific questionnaires could be used to assess quality of life in patients with interstitial lung diseases.
The validity of quality-of-life instruments is established primarily by demonstrating their content and construct validity. In applying well-established instruments to new patient populations, however, it is perhaps most important to demonstrate construct validity.15 The premise of construct validation is that quality-of-life instruments should provide unique information not provided by objective measurements but, nevertheless, should correlate to some degree with severity of disease measured by these physiologic variables. Our hypothesis a priori was that worse quality-of-life scores should moderately correlate with more severe physiologic impairment demonstrated by pulmonary function tests, 6-min-walk distance (6MWD), and dyspnea. Most of the instruments also have subscale scores, which are designed to evaluate specific aspects of quality of life. Subscale scores focusing on physical or functional impairment would be expected to correlate better with pulmonary function than subscale scores focusing on mental or emotional health. We have used construct validation as the basis of our evaluation of four health-related quality-of-life instruments for patients with interstitial lung disease.
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Materials and Methods |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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This case definition included patients with idiopathic pulmonary fibrosis, chronic sarcoidosis, chronic hypersensitivity pneumonitis, and asbestosis. Patients with idiopathic pulmonary fibrosis had histopathology consistent with either usual interstitial pneumonitis or nonspecific interstitial pneumonitis or met the clinical criteria of slowly progressive dyspnea of unknown cause and bibasilar infiltrates. Chronic sarcoidosis was diagnosed by the presence of noncaseating granulomas in all patients except one, who had typical radiographic features and a high CD4/CD8 ratio within a lymphocyte-predominant lavage fluid. The diagnosis of chronic hypersensitivity pneumonitis was based on the clinical history of exposure to a known offending antigen and confirmatory histopathology. Asbestosis was diagnosed primarily by clinical criteria including a history of heavy occupational exposure, although some patients had ferruginous bodies on tissue biopsy as well.
Seventy-seven eligible patients were identified, and 50 patients participated after providing written informed consent. The most common reason patients did not participate was the traveling distance to the study center (n = 10). Other reasons for nonenrollment were time conflicts with work (n = 5), inability to contact patients (n = 4), and patient disinterest (n = 5). Only three patients were too sick to participate or died before study enrollment. All study procedures were approved by the University of Washington Human Subjects Committee.
Study Design
The study was a cross-sectional comparison of scores from two
generic and two respiratory-specific health-related quality-of-life
instruments with physiologic data from pulmonary function tests and a
6MW test. Patients were asked to complete the four quality-of-life
questionnaires in a randomized order based on a random numbers table.
After completing each questionnaire, patients were asked to rate how
well the content of the questionnaire addressed quality-of-life issues
relevant to their disease on a 1 (worst) to 10 (best) scale.
Immediately after answering the questionnaires, the patients performed a modified 6MW test. The standard 6MW has been shown to be a reliable, valid, and safe method of assessing functional status in both patients with COPD and congestive heart failure.16 17 Our protocol for the 6MW was adjusted slightly for patient comfort and safety. Patients were allowed to stop and restart during the 6 min if necessary. Although 6MW tests are sometimes performed multiple times to account for a learning effect, only one test was performed because of many patients’ severe exercise limitation.18 In addition, patients were deliberately allowed to use supplemental oxygen at the same inspired concentration normally used with their daily activities. After completing the walk, patients rated the amount of dyspnea experienced during the test on a standard Borg scale.19
Patients underwent pulmonary function testing including spirometry, plethysmography, and diffusing capacity within 2 months of answering the questionnaires. Two months was believed to be the maximal length of time of stability in pulmonary function for this population. Two-thirds of the patients had pulmonary function tests within 1 week of the questionnaires, and half of the patients had tests on the same day. All tests were performed in accordance with published standards.20 21 22 FVC, FEV1, TLC, and diffusing capacity of the lung for carbon monoxide (DLCO) were expressed as percent of normal based on age, sex, race, and height-adjusted population standards.23 DLCO was adjusted for hemoglobin concentration.
Health-Related Quality-of-Life Questionnaires
Medical Outcomes Short Form 36:
The Medical Outcomes Short
Form 36 (SF-36) is a generic health-related quality-of-life instrument,
which has been used to assess quality of life in a variety of chronic
medical conditions including COPD and asthma.24
25
Its
validity, reproducibility, and responsiveness to changes over time have
been well demonstrated.8
It assesses eight dimensions of
quality of life: physical function, role limitation caused by physical
impairment, bodily pain, general health, vitality, social function,
role limitation caused by emotional impairment, and mental health. Each
dimension is scored separately on a 0 to 100 scale, in which higher
scores correspond to better quality of life. The SF-36 also
incorporates the subscale scores into two summary measures, the
Physical Component Score (PCS) and the Mental Component Score (MCS),
which provide a measure of the overall effect of physical and mental
impairment on quality of life. These summary scores are standardized to
responses from the general population, whose mean score is 50.
Quality of Well-Being Scale:
The Quality of Well-Being Scale
(QWB) is a general health outcome and health utility measure, which has
been validated in patients with a variety of medical conditions
including COPD.9
10
26
Its questions focus on symptoms and
medical problems, mobility, physical activity, and impact of illness on
activities of daily living, and it summarizes these into one score
ranging from 0 (death) to 1.0 (perfect health).
Chronic Respiratory Questionnaire:
The Chronic
Respiratory Questionnaire (CRQ) is a respiratory-specific
quality-of-life instrument originally developed for patients with COPD,
which has been validated in patients with asthma as
well.13
27
28
It assesses four aspects of quality of life:
dyspnea, fatigue, emotional function, and mastery. The questionnaire
has a unique format, which allows patients to rate the severity of
dyspnea associated with individually identified activities. Scores for
each of the four domains and the summary score have a range from 0 to
100, in which higher scores signify better health-related quality of
life.
St. George’s Respiratory Questionnaire:
The St. George’s
Respiratory Questionnaire (SGRQ) is a respiratory-specific instrument
developed for patients with COPD. Its validity, reproducibility, and
response to change over time have been demonstrated in patients with
COPD, asthma, and bronchiectasis.12
14
29
It has three
components: symptoms, which measures respiratory symptoms; activities,
which measures impairment of mobility or physical activity; and
impacts, which measures the psychosocial impact of disease. Scores for
these components and the summary score are on a 100-point scale, which
is opposite in direction to the other three instruments. Higher scores
correspond to worse health-related quality of life.
Statistical Analysis
Correlation between quality-of-life scores and pulmonary
function tests, 6MWD, and dyspnea score were evaluated using Pearson’s
coefficient. Quality-of-life scores were plotted against physiologic
measures on scatter plots to ensure a linear relationship between the
two (data not shown). Linear regression methods were then used to
determine whether physiologic measures remained independent predictors
of quality-of-life scores after considering patient demographics and
comorbid illness. Two-sample Student’s t tests were used to
determine whether quality-of-life scores differed between patients who
were dependent on supplemental oxygen and patients who were not.
Analysis of variance was used to assess whether quality-of-life scores
differed between patients by diagnosis. The criteria for statistical
significance was set at p < 0.05. The Friedman test, a nonparametric
test similar to analysis of variance for repeated measures, was used to
test for significant differences in patient ratings between the four
questionnaires.30
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Results |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Seventy-eight percent of the patients had at least one stable comorbid illness. Comorbid conditions included non–insulin-dependent diabetes mellitus without end-organ damage, stable coronary disease, or arthralgia attributed to degenerative joint disease.
Physiologic Characteristics
On average, the study group had moderately severe
restrictive lung disease indicated by their FVC and
FEV1 (Table 2
). There was a larger decrement in the mean DLCO. However,
the values for the FVC, FEV1, and
DLCO varied widely from values indicative of a severe
restrictive defect to values within normal limits (defined as 
80%
of predicted).
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88%.
Health-Related Quality of Life
The summary scores from each of the four quality-of-life
instruments are shown in Table 3
. The summary scores of the SF-36, the PCS and the MCS, were
standardized to scores from the general population, which conform to a
normal distribution with a mean of 50. The mean PCS for our patients
signified on average a moderately severe degree of impairment compared
with the general population. The larger decrease in the PCS compared
with the MCS suggested a more significant impairment in the physical
aspects of quality of life than the emotional aspects of health-related
quality of life. However, there was a wide range among individuals. The
extremely low minimal values observed indicated an extremely poor
health-related quality of life on both a physical and an emotional
level for some individuals. The mean QWB score was similarly decreased.
However, individual responses fell within a narrower range. Both the
total scores from the SGRQ and the CRQ indicated a moderate decrease in
quality of life because their medians were considerably below the
midpoint of the scales. Individual responses on both questionnaires,
but in particular the SGRQ, nearly covered the entire range of possible
scores.
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Validation of Health-Related Quality-of-Life Measures With
Physiologic Variables
Scores from all four of the quality-of-life instruments
showed statistically significant correlation with pulmonary function
test results (Table 4
). The PCS of the SF-36 and the QWB score were correlated with FVC,
FEV1, and DLCO. However, their
correlation with TLC was not statistically significant. The CRQ score
was only correlated with DLCO. Of the five quality-of-life
scores, the SGRQ total score was most strongly correlated with
pulmonary function. Its correlation with the FVC,
FEV1, and DLCO was stronger than its
correlation with TLC. The MCS of the SF-36 did not show a statistically
significant correlation with any of the pulmonary function tests.
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Long-term supplemental oxygen use was statistically associated with lower PCS, QWB, SGRQ, and CRQ scores (t = -2.774, -2.131, 4.718, and -3.355, respectively; p < 0.05). However, resting oxygen saturation, exertional oxygen saturation, and the presence of exercise-induced desaturation were not associated with quality-of-life scores from any of the instruments.
An additional method of validating the quality-of-life instruments was to assess the correlation between individual subscale scores and the physiologic data. Of the eight individual subscale scores of the SF-36, only "physical function" was statistically correlated with FVC% (r = 0.38, p < 0.01). Although the subscale scores of the SGRQ do not clearly delineate physical aspects of quality of life from emotional aspects, the activity score was expected to correlate with physiologic variables better than would the symptom score. The correlation coefficients for these subscale scores were consistent with our hypothesis: the FVC% was more strongly correlated with activity score (r = -0.39, p < 0.01) than the symptom score (r = -0.31, p < 0.05). Similarly, for the CRQ, the dyspnea score was significant correlated with the FVC% (r = 0.32, p < 0.05), whereas the fatigue score and the emotional score were not.
Demographic and physiologic variables were entered into multiple linear regression models to ascertain whether pulmonary function, walk distance, and dyspnea were independent predictors of quality-of-life scores. Even after considering age, oxygen use, and comorbidity, pulmonary function tests, 6MWD, and dyspnea remained significant independent predictors of the SF-36 PCS score and the SGRQ total score (data not shown).
There were no significant differences in the patients’ ratings of the
questionnaires in terms of the extent to which their content addressed
relevant quality-of-life issues (
2 = 0.98,
p = 0.81). The median ratings of the questionnaires (1 and 10 as the
worst and best ratings, respectively) were as follows: 9.0 (SF-36), 8.5
(QWB), 8.5 (CRQ), and 8.5 (SGRQ).
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Discussion |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Our data support the validity of using preexisting health-related quality-of-life questionnaires to assess the health-related quality of life of patients with interstitial lung disease. Our evaluation of the SF-36, QWB, SGRQ, and CRQ was based on construct validation. Scores from all four of the quality-of-life instruments correlated with severity of physical impairment reflected by pulmonary function, 6MWD, and dyspnea rating.
In addition to assessing whether these instruments were valid in this population, we were interested in determining whether some instruments were better than others. We based our judgment on the strength of the association between the quality-of-life scores and the physiologic measurements as well as the patient ratings of the questionnaire content. There were no significant differences in the patient ratings of the questionnaires. In comparing the two generic questionnaires, the PCS was consistently more strongly correlated with pulmonary function, 6MWD, and dyspnea rating than the QWB score. Likewise, between the two respiratory-specific instruments, the SGRQ total score was more strongly associated with all physiologic measurements than the CRQ score. Our findings suggested that the SF-36 and the SGRQ may be more-sensitive instruments for discriminating differences in quality of life between individuals with varying degrees of physical impairment caused by interstitial lung disease.
Observations from the study suggested possible reasons why the QWB and the CRQ may be less-sensitive quality-of-life instruments. Although our study population represented the full spectrum of disease severity, their QWB scores fell within a relatively narrow range. Because of the lesser variability in QWB scores, this generic questionnaire may be less successful in distinguishing between patients with interstitial lung disease. The few questions that addressed respiratory symptoms were answered in a similar manner by most patients, whereas much of the variability in the scores resulted from nonrespiratory symptoms and problems. The scores on the CRQ had more variability than the QWB but less variability than the SF-36 and the SGRQ. Although the CRQ is a respiratory-specific quality-of-life instrument, its individualized format may have accounted for why its scores were less consistently correlated with pulmonary function than the SGRQ. With the CRQ, patients rated the severity of their dyspnea based on symptoms during their own daily activities. In our study we observed that patients with the most severe disease frequently scaled down their activities to minimize dyspnea and therefore did not consistently report more dyspnea on the questionnaire. This explanation could have accounted for the attenuated difference in scores between patients with mild and severe physiologic impairment. The CRQ may be better suited for longitudinal follow-up of patients than comparisons between individuals.
The correlation between the quality-of-life scores and pulmonary function tests and 6MW tests in this population with interstitial lung disease were similar to those described in patients with COPD and asthma. Several investigators found significant correlation of similar magnitude between the SGRQ total score and subscale scores and FEV1 and FVC in patients with COPD (r = -0.28 to -0.51).12 33 Similarly, they found a statistically significant association between quality-of-life scores and 6MWD (r = -0.26 to -0.59).12 The SF-36 has been tested less extensively in COPD, but the correlation found by Jones and colleagues34 is similar to our findings. Other investigators have identified strong correlation between CRQ scores and the 6MW, but the correlation between CRQ scores and pulmonary function tests have been less consistent in other studies as well.13 27 35 Kaplan and colleagues10 also found a significant correlation between the QWB score and FVC and FEV1 in patients with COPD.
In our study, we found a significant correlation between health-related quality of life and the need for supplemental oxygen but no association between quality of life and oxygenation at rest or during exercise. The association between health-related quality-of-life scores and oxygenation reported by other investigators has been inconsistent. Although Jones and colleagues12 found no significant correlation between scores from the SGRQ and resting oxygen saturation in patients with COPD, Okubadejo et al36 did find a statistically significant association between scores on this questionnaire and PaO2. Our results are not necessarily contradictory to the findings of Okubadejo and colleagues.36 In their study, none of the patients were using long-term oxygen therapy although it was initiated in some based on the low arterial oxygen noted. Impaired oxygenation may not necessarily be associated with poorer quality of life provided that patients can use long-term supplemental oxygen. Additionally, the amount of oxygen individual patients use may vary with their different daily activities and may not be the same as the average amount of oxygen use reported and consequently used during our 6MW tests.
One potential limitation of our study is that the patient population was a referral population, which may differ from patients with interstitial lung disease in the community. Our patients had, on average, moderately severe restrictive lung disease by physiologic criteria supporting a possible referral bias. However, the severity of illness varied from severe physiologic defects to within normal limits, and therefore our sample provides a reasonable representation of the complete spectrum of disease.
Another limitation of our study is the possibility that we did not accurately assess the patients’ maximal exercise tolerance because the 6MW was only performed once. The potential inaccuracy of the 6MWD is unlikely to jeopardize the validity of our results, because the association between the 6MWD and the quality-of-life scores was highly statistically significant (p < 0.01). However, inaccuracy in measuring the 6MWD may have affected our estimate of the magnitude of the correlation.
In summary, we have demonstrated that a general measure, the SF-36, and a respiratory-specific measure, the SGRQ, are sensitive instruments for evaluating health-related quality of life in patients with interstitial lung disease. These findings are an important first step to the application of standardized quality-of-life measurements in clinical research. Our cross sectional study has shown that the SF-36 and the SGRQ can be used to examine the health-related quality of life of this patient population and how it varies based on severity of disease. Future studies are needed to address the ability of these instruments to assess change over time. The outcomes of these studies, in combination with our results, will allow selection of specific quality-of-life instruments as end points in clinical trials to evaluate the efficacy of new therapies.
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Acknowledgements |
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Footnotes |
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Supported in part by the Respiratory Research Training grant, No. 5 T32 HL07287. Additional project support was provided by the Firland Sheltered Workshop Foundation, a nonprofit agency with a mission of support for projects relating to care of individuals with tuberculosis and other chronic respiratory problems.
Received for publication February 22, 1999. Accepted for publication June 8, 1999.
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