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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
J Rheumatol. Author manuscript; available in PMC 2010 June 3.
Published in final edited form as:
PMCID: PMC2880525


Sogol Amjadi, MS, Paul Maranian, MS, Harold E. Paulus, MD, Robert M. Kaplan, PhD, Veena Ranganath, MD, Daniel Furst, MD, Puja Khanna, MD, and Dinesh Khanna, MD, for the Western Consortium of Practicing Rheumatologists



New methodologies allow the scores for the Health Assessment Questionnaire-Disability Index (HAQ-DI) to be translated into preferences/ utility scores. We evaluated the construct validity of the HAQ-DI derived SF-6D score and assessed its responsiveness to change over 6- and 12-month follow-up periods in patients with early aggressive rheumatoid arthritis (RA).


Patients (N=277) participating in an RA observational study completed self-reported measures of symptoms and the HAQ-DI at baseline, 6 and 12 months. Total Sharp scores, C-reactive protein and erythrocyte sedimentation rate were assessed using clinical data. Construct validity was assessed by examining the association between SF-6D score and patient-reported and clinical measures using Spearman correlation coefficients. The responsiveness of SF-6D to change was assessed using patient and physician assessments of the disease as clinical anchors. The magnitude of responsiveness was calculated using SF-6D effect size (ES).


Mean SF-6D scores were 0.690, 0.720, and 0.723 at baseline, 6, and 12-month follow-up visits, respectively. Baseline patient-reported measures had moderate-to-high correlations with baseline SF-6D (r: 0.43 to 0.52); whereas clinical measures had negligible-to-low correlations with SF-6D (r: 0.001 to 0.32). ES was moderate for the groups that were deemed to have improved (ES: 0.63–0.75) but negligible-to-small for those who did not (ES: 0.13–0.46).


Our data supports the validity and responsiveness of the HAQ-DI derived SF-6D score in an early RA cohort. These results support the use of HAQ-DI derived SF-6D in RA cohorts and clinical trials lacking preference-based measures.

Key Indexing Terms: SF-6D, Health Assessment Questionnaire-Disability Index, health utility, preference-based measures, early rheumatoid arthritis, cost-effectiveness analysis


Rheumatoid arthritis (RA) is a chronic disorder that primarily involves the joints. Just as in many other chronic diseases, RA and/ or its treatment may have detrimental effects on health-related quality of life (HRQOL).

In general, there are two ways to assess HRQOL. These methods include health status and health utility (preference-based) assessments(1, 2). Health status measures describe a person’s ability to function in one or more domains (e.g., physical functioning and/ or mental well-being). Currently one of the most commonly used disease-specific health status instruments in RA is the Health Assessment Questionnaire-Disability Index (HAQ-DI)(3). It measures health status by assessing the patient’s ability to function physically, and includes questions that involve the function of both upper and lower extremities. HAQ-DI scores are associated with work productivity, disability, and mortality (4, 5) in RA.

Preference-based measures assess the value or desirability of a state of health against an external metric. They allow for the direct comparison of health status by integrating multiple pieces of information into a single summary number scaled between two anchor states, usually “dead” (0.0) and “perfect health” (1.0)(6). Preference-based measures are used as weights in calculating quality-adjusted life years (QALYs). QALYs take into account both quantity and quality of life (QOL) in a single metric, calculated as the arithmetic product of life expectancy and the QOL of the remaining life years. A year of perfect health is worth 1.0 QALY, a year of life in less than perfect health is worth less than 1.0 QALY, and being dead is worth 0.0 QALY. At a policy level, QALYs are incorporated into decision and cost-effectiveness (cost-utility) analyses of healthcare interventions(6). Preference-based measures are obtained either directly (via face-to-face interview with patients) or indirectly. Direct health utilities are usually ascertained via face-to-face interviews, with computer-assisted administration being the state of the art. The most common health utility measures are the standard gamble (SG), time tradeoff (TTO), and rating scale (RS)(6). Indirect health utilities such as EuroQol use population-assigned weights to calculate utility scores for particular health states from health status instruments. The ease of administration (self-administered) of these indirect measures enables them to be used in national surveys, and as the source of QOL weightings in economic evaluations.

Short Form-6D (SF-6D)(7) is an indirect preference-based measure that is derived from responses on the Medical Outcomes Short Form 36 (SF-36), a widely used generic health status instrument(8). Brazier et al.(7) developed the SF-6D which is based on eleven SF-36 items, by asking UK general population to report preferences for a sample of the SF-6D health states using a standard gamble technique. Although SF-36 has 8 domains, the SF-6D has reduced this to 6 domains (physical function, role limitation, social function, pain, mental health, and vitality). Based on econometric modeling of the observed preferences, they constructed a model for estimating mean preferences for all possible SF-6D health states. The scoring algorithm produces scores ranging from 0.29 to 1.00. Although clinical trials in RA often incorporate the SF-36 which can be used to calculate the SF-6D(9), they are usually limited to one or two years and do not represent the general RA population. On the other hand, observational studies in RA provide a unique perspective for assessing long-term outcomes such as joint replacement, cardiovascular morbidity, and mortality associated with RA(10). To assess the cost-effectiveness of interventions these problems necessitate (e.g., joint replacement, treatment of cardiovascular disease, etc), one needs a preference-based measure to assess QALYs. However, many long-term observational studies in RA have not included any preference-based measures. As stated by Bansback, “Because new programs and treatments in RA are competing alongside other disease areas for funding, it is important for the rheumatology community to be able to demonstrate the value of their interventions to policy makers(11) (Page 964).” Consequently, it is useful to find ways to convert the more traditional RA-related health status instruments (e.g., HAQ-DI) to preference-based measures (e.g., SF-6D).

Bansback et al.(11) recently developed several linear regression models to map the SF-6D from the HAQ-DI in RA patients from the UK and Canada. In the present study, we employed the model developed by Bansback et al. to estimate SF-6D scores from the HAQ-DI in U.S. subjects participating in an early RA observational cohort. The aims of this study were to evaluate convergent and divergent evidence for the construct validity of the HAQ-DI-derived SF-6D score. In addition, we assessed the responsiveness of the HAQ-DI derived SF-6D to changes in other patient reported measures such as patient global assessment over 6 and 12-month follow-up periods.


Patients and data

Patients included in this study are part of a long-term observational study involving the Western Consortium of Practicing Rheumatologists (CPR), which is a regional consortium of rheumatology practices in the western United States and Mexico(12, 13). The consortium physicians participating in this study were mainly from community and university practices in California, Idaho, New Mexico, Oregon, Utah, Colorado, Washington, Wyoming and Guadalajara, Mexico.

Since 1993, 323 patients have been enrolled into the study. Inclusion criteria for the CPR cohort included a diagnosis of early RA, no previous DMARD treatment, rheumatoid factor seropositive (rheumatoid factor titer ≥1: 80 or ≥40 IU), and ≥6 swollen joints and ≥9 tender joints. The consortium rheumatologists assessed patient disease status at study entry (baseline), 6 months, 1 year, and yearly thereafter. Using standard methods, detailed physician assessment included all of the core set outcomes measures required to calculate the disease activity score (DAS), including 28 tender and swollen joint counts and acute phase reactant measures, as well as 0–100 mm visual analog scales (VAS) for global, pain, fatigue, and arthritis severity assessments. In addition, study visits included radiographs of the hands, wrists and forefeet, and the total Sharp score was calculated(14). At each scheduled physician visit, blood specimens were collected for C-reactive protein (CRP); Erythrocyte Sedimentation Rate (ESR) was determined when clinically indicated, in rheumatologist’s office or local laboratory.

Patients were also asked to complete a detailed questionnaire at study entry and every 6 months thereafter for the duration of the study. The questionnaires evaluated changes in demographics, health, medication, pain and global VAS, the HAQ-DI, and the Center for Epidemiological Studies-Depression scale (CES-D).

Patient-reported measures

The HAQ-DI is a 20-item arthritis-targeted measure assessing upper and lower extremities functioning(15). The HAQ-DI score is computed by summing the highest item score in each of the 8 domains and dividing the sum by 8, yielding a score from 0 (no disability) to 3 (severe disability). The original HAQ-DI includes an additional grade of difficulty for patients using assistive/adaptive devices such as a cane or a walker.

In addition to completing the HAQ-DI, patients completed 4 visual analog scales as part of their patient questionnaires: patient global assessment of their arthritis (PGA), overall pain, overall fatigue, and overall arthritis severity; Patients were asked to indicate by placing a vertical mark on the line how fatigue, pain or arthritis interfered with their lives “during the past week”. Their rheumatologists also completed a physician global assessment. All scales ranged from 0 to 100mm where 0 indicated no symptoms and 100 represented very severe symptoms.

Predicting mean SF-6D using HAQ-DI

The SF-6D(7) derives preference-based scores from the SF-36 by using population-based utilities for SF-36 health states. The SF-6D revises the SF-36 into a 6-dimensional health state classification system: physical function, role limitations, social function, pain, mental health, and vitality; the general health scale items are not incorporated and 2 scales measuring role limitations due to physical and emotional problems are collapsed into a “role limitations” dimension. An SF-6D health state is defined by selecting 1 level from each dimension. A total of 18,000 health states are thus defined. The SF-6D is scored from 0.29 to 1.00 where 0.29 represents worst possible health and 1.00 is perfect health(7).

Bansback et al. developed several linear regression models to estimate the relationship between the HAQ-DI and SF-6D(11). They used two models to predict the SF-6D from HAQ-DI. Model 1 used the 8 HAQ-DI domain scores and treated them as continuous variables. In Model 2, the HAQ-DI domains were treated as ordinal variables. Both models displayed acceptable and very similar statistical fit. However, Model 1, by treating each domain score as a continuous variable, assumes the intervals between response levels are the same, which may not be completely valid. On the other hand, Model 2, by treating each level of the domain score as an ordinal variable, does not make this assumption and therefore is less restrictive(11). In our study we obtained similar results using both models. Predicted SF-6D under model 1 and model 2 were 0.675 and 0.690 at baseline, 0.718 and 0.720 at 6 months and 0.722 and 0.723 at 12 months. Since model 2 conforms better to an ordinal HAQ-DI scale, we calculated the results using model 2 as our prediction model.

Statistical analysis

Descriptive statistics for continuous variables are presented as means and standard deviations, and for categorical variables as proportions.

Construct validity

We examined the association between baseline SF-6D and other baseline patient-reported and clinical measures using Spearman correlation coefficients. We also assessed the association between change in SF-6D and change in the other patient-reported and clinical measures from baseline to 6 months and from baseline to 12 months. A correlation from 0.00–0.20 was interpreted as no correlation; 0.21–0.40 as low correlation; 0.41–0.60 as moderate correlation; 0.61–0.80 as marked correlation; and 0.81–1.00 as high correlation(16). Based on previous literature that showed moderate-to-high correlation between HRQOL and other patient-reported measures and low-to-negligible correlations between HRQOL and clinical measures(17, 18), we hypothesized that SF-6D scores would at least have moderate correlation (r > 0.40) with patient global assessment (PGA) , pain VAS, fatigue VAS, and low-to-negligible correlation (r < 0.40) with disease severity, physician global assessment, ESR, CRP, and Sharp score.

The ability of baseline SF-6D to discriminate baseline PGA, pain VAS, fatigue VAS, arthritis severity VAS and physician global assessment was assessed by classifying each of the visual analog scales into three categories: Mild (0.0–33.0), moderate (33.1–66.0), and severe (66.1–100.0)(19). Differences among mild, moderate and severe categories were evaluated for each variable using one-way ANOVA.

Responsiveness in change

We used PGA, patient-reported pain, fatigue and disease severity visual analog scales and the physician global assessment VAS as clinical anchors to assess the responsiveness to change(20). We divided our group into two categories: patients with improvement from baseline to 6 months and patients with no improvement from baseline to 6 months (the same was done at month 12) based on clinical anchors. Improvement was defined as a decrease in the VAS scores by greater or equal to 10 mm from baseline to 6-month follow-up and from baseline to 12-month follow-up. A cut-off of 10 mm on a 0–100 mm scale was based on previous published studies (2123) where a change of 10 mm on a 0–100mm scale is consistent with minimally important difference(20). In order to assess the responsiveness to change of SF-6D at 6 months, we estimated the SF-6D effect size (ES) by taking the change in mean SF-6D from baseline to 6 months and dividing the result by the standard deviation at baseline (SD= 0.06). The same was done to calculate the ES of SF-6D at 12 months. According to Cohen’s rule, an ES of 0.20–0.49 represents a small change, 0.50–0.79 represents a medium change, and 0.80 or higher represents a large change(24).

In order to calculate quality-adjusted life-years (QALY) we plotted the mean SF-6D at baseline, 6 months and 12 months. The mean QALY was calculated by estimating the area under the path for each individual patient who had data available at baseline, 6-month, and 12-month visits (N= 177). The area under the path is equal to the sum of the areas under consecutive SF-6D measurements and the area under SF-6D measurements is obtained by multiplying the duration of the SF-6D in months by the average score of SF-6D. We used the following formula to assess average QALY: [ (0.5 * (SF-6D at baseline + SF-6D at 6 month) * 6 )+ (0.5 * (SF-6D at 6 months + SF-6D at 12 months) * 6) ] / 12 (25). We assumed that the SF-6D changes between measurements at baseline, 6 months and 12 months were smooth and gradual.

We also assessed the proportion of subjects with floor and ceiling effects (percentages of respondents scoring at the lowest and highest possible scale level).

Computations were achieved using the statistical software package SAS System Release 8.2 (SAS Institute Inc., Cary, NC, USA).


Two-hundred and seventy seven patients had data available to compute the HAQ-DI and the SF-6D at baseline and formed the study sample. The subjects were mainly Caucasian (79.4%) and female (76.9%) with a mean ± standard deviation (SD) age of 51.1 ± 13.2 years and the mean disease duration of 8.6 ± 10.2 months; 85% of patients had disease duration ≤ 12 months. The PGA, physician global assessment (0–100mm) and DAS were 42.2 ± 23.5, 49.3 ± 21.5, and 6.0 ± 1.1 respectively, representing moderate-severe disease (Table 1). There were no floor and ceiling effects observed for SF-6D score at baseline.

Table 1
Baseline Characteristics

The HAQ-DI scores were 1.18 ± 0.70 at baseline, 0.78 ± 0.65 at 6 months and 0.72 ± 0.68 at 12 months. The corresponding mean SF-6D scores were 0.690 ± 0.056 (n= 277), 0.720 ± 0.053 (n= 206), and 0.723 ± 0.057 (n= 211), at baseline, 6, and 12-month follow-up visits, respectively. The distribution of SF-6D scores at baseline, 6 and 12 months are shown in Figure 1. Because we captured data every 6 months, we were able to calculate average QALYs over a period of 12 months. The mean (SD) QALY during the first 12 months was 0.72 ± 0.05 (Figure 2).

Figure 1Figure 1
Histogram illustrating the distribution of SF-6D scores (0.29–1.00) at baseline, 6 and 12-month follow-up visits.
Figure 2
Mean QALY over 12 months: The area under the curve represents the mean QALY over 12 months of follow up.

Construct Validity

Table 2 reports the Spearman correlation between HAQ-DI derived SF-6D scores and several patient-reported and clinical measures. As expected, clinical measures such as ESR and total sharp score had low to negligible correlations with SF-6D (r= 0.001 for the Sharp score and r= −0.14 for ESR ). Among clinical measures, CRP had the highest correlation with SF-6D at baseline (r= −0.31). Baseline patient-reported measures such as PGA and CESD had at least moderate correlations with SF-6D (r > 0.40) with PGA having the highest correlation with SF-6D (r= −0.52).

Table 2
Construct Validity: correlation between SF-6D, disease related visual analog scales and other clinical measures.

Discriminant validity of the SF-6D

The SF-6D scores were able to discriminate between mild, moderate, and severe PGA, pain VAS, fatigue VAS, and arthritis severity VAS with F-test p-values of < 0.0001 for the overall comparisons (Figure 3). In addition, the SF-6D scores were discriminative of mild and moderate, mild and severe, and moderate and severe scores for each of the VAS assessments (p-value < 0.01), with the exception of moderate versus severe fatigue VAS scores (p-value= 0.06).

Figure 3
Discrimination of the SF-6D among mild, moderate and severe patient global assessment, pain, fatigue, arthritis severity and physician global assessment visual analog scales.

SF-6D responsiveness to change

The magnitude of ES was larger for the group that was deemed to have improved (ES > 0.50); patients who improved had an ES of moderate magnitude compared to negligible-to-low magnitude for patients who did not improve. The largest SF-6D ES was observed for the change in PGA (ES= 0.75) and pain VAS (ES= 0.75) in patients who improved at 12 months (Table 3).

Table 3
Responsiveness of SF-6D to change in patient global, pain, fatigue and arthritis severity and physician global visual analog scales (VAS: 0–100mm).


Measuring health-related quality of life (HRQOL) in patients with rheumatoid arthritis (RA) makes it possible to distinguish between the effectiveness of different therapies. The main use of preference measures is to guide decision-making (1, 26). For example, preference measures can serve as “quality-adjustment factors” for calculating quality-adjusted life years (QALYs) in decision and cost-effectiveness analyses(2). QALY has the potential to impact public policy and resource allocations, as it is an effective way to compare therapeutic interventions within the disease, and even across illnesses. Due to lack of time and resources, few studies have administered preference-based measures (27) Consequently, linear regression models have been developed to estimate the preference-based values using other HRQOL measures in other chronic diseases (9, 28, 29).

Recently, Bansback et al.(11) developed models of the relationship between HAQ-DI and SF-6D using various regression analyses. Their results suggested that the models are helpful in utilizing existing valuation data by offering a method for researchers who need preference scores, but have not used a preference-based measure in their study.

In the present study we assessed the construct validity and responsiveness to change of HAQ-DI derived SF-6D scores in a population of patients with early RA. Our patients had a mean SF-6D of 0.69 at baseline, which is very similar to Bansback’s results (UK: 0.62, Canada 0.68). The small difference between the SF-6D scores is explained by differences in the HAQ-DI scores. Our patients had a lower HAQ-DI than the UK subjects at baseline (1.18 vs. 1.41), which resulted in a higher SF-6D (closer to perfect health). This may be due to early disease duration of our cohort; UK disease duration was not provided in the manuscript.

SF-6D scores had both convergent and divergent construct validity (Table 2); SF-6D scores had moderate association with HRQOL measures and no to low association with clinical measures. Marra et al. utilized the data from the Canadian RA population used to develop HAQ-DI derived SF-6D in a different manuscript(30) and assessed construct validity of indirect preference-based measures (including SF-6D) and RA-related variables. Similar to our results, they found moderate to high correlations between baseline SF-6D and baseline patient-reported outcomes (pain and patient global VAS’s). However, they also found moderate correlations between baseline SF-6D score and baseline tender and swollen joint counts (r= 0.47 to 0.53), while we found negligible-to-small correlations between SF-6D and clinical measures such as tender and swollen joint counts, ESR, and radiographic damage. The differences may be related to the estimation of SF-6D; Marra et al. estimated SF-6D directly from SF-36 using the formula from Brazier et al.(7) whereas we derived SF-6D from the HAQ-DI using the model described by Bansback et al.(11). In addition, we found lower correlations over time, compared to baseline. This is to be expected, as change scores inflate error variance thereby attenuating the correlations(31)

The SF-6D scores at baseline were able to discriminate between mild, moderate and severe PGA, pain VAS, fatigue VAS, arthritis severity VAS, and physician global assessment VAS with the exception of moderate versus severe fatigue VAS (P-value: 0.059). A similar finding was seen in another analysis which assessed SF-6D in scleroderma (17); the SF-6D scores at baseline were able to differentiate between mild, moderate, and severe patient global assessment.

The ability of HRQOL instruments to detect clinically important changes is crucial to their usefulness in determining the effectiveness of different therapies(32). The magnitude of responsiveness as measured by these instruments is useful in assessing treatment efficiency and in estimating sample size for future study designs(33). In our study, SF-6D scores were able to discriminate between patients who were deemed to have improved and those who did not improve. SF-6D scores had a larger magnitude of ES for the improved group (both from baseline to 6 months and from baseline to 12 months with ES ranging from 0.62 to 0.74) compared to the group with no improvement (ES ranging from 0.13 to 0.45). Overall, the SF-6D had the largest magnitude of change to PGA and pain VAS at 12 months. Previous studies have found that the minimally important difference – the smallest difference in scores that patients perceive as beneficial (34) – in SF-6D for different arthritides ranges from 0.030-0.037(17, 30, 35). In this study, the mean differences in SF-6D scores at 6 and 12 months were 0.030 and 0.033, respectively; these are minimally important differences and are thus clinically meaningful. In addition, SF-6D scores increased over 12-month period, suggesting that treatment of early RA in our cohort resulted in higher preferences by RA patients for their current health states.

Our study is not without limitations. First, our results are applicable for informing health policy decisions and not for individual preferences as we used mean scores of the SF-6D. Second, Bansback et al.’s, Canadian study population had moderate disease and a mean duration of 13.98 ± 11.64 years at baseline, whereas our patients had more aggressive disease (DAS: 6.0± 1.1) with 84% having baseline disease duration of ≤ 12 months. Thus, the validity of these models needs to be assessed in patients with milder RA and shorter disease duration. Another limitation is that Bansback et al.’s models are somewhat limited. The Bansback group created several translation models, but each model had an R2 for predicting SF-6D values of only about 0.50. Although an R2 of 0.50 is respectable, it still means that only about half of the variance in SF-6D scores can be known on the basis of HAQ-DI responses. Even though translations are attractive, investigators may still be better advised to select utility based outcome measures such as EuroQol and Quality of Well Being scales when measuring outcomes for cost-effectiveness analyses (36). Lastly, we only assessed the construct validity of the HAQ-DI derived SF-6D and did not assess the criterion validity as SF-36 was not administered in this observational study. A more accurate measure of validity would be to assess the criterion validity which requires the instrument in question (HAQ-DI derived SF-6D in this case) to correlate with an instrument that is considered the “gold standard” (Observed SF-6D in this case).

The results of our study provide support for the validity of HAQ-DI derived SF-6D scores in patients with early RA over a period of 12 months. In addition, the results of our study show that SF-6D scores are responsive to changes in HRQOL measures. In conclusion, our study supports the use of the HAQ-DI-derived SF-6D in RA observational cohorts where no preference-based measure has been obtained.


The Western Consortium of Practicing Rheumatologists:

Robert Shapiro, Maria W. Greenwald, H. Walter Emori, Fredrica E. Smith, Craig W. Wiesenhutter, Charles Boniske, Max Lundberg, Anne MacGuire, Jeffry Carlin, Robert Ettlinger, Michael H. Weisman, Elizabeth Tindall, Karen Kolba, George Krick, Melvin Britton, Rudy Greene, Ghislaine Bernard Medina, Raymond T. Mirise, Daniel E. Furst, Kenneth B. Wiesner, Robert F. Willkens, Kenneth Wilske, Karen Basin, Robert Gerber, Gerald Schoepflin, Marcia J. Sparling, George Young, Philip J. Mease, Ina Oppliger, Douglas Roberts, J. Javier Orozco Alcala, John Seaman, Martin Berry, Ken J. Bulpitt, Grant Cannon, Gregory Gardner, Allen Sawitzke, Andrew Lun Wong, Daniel O. Clegg, Timothy Spiegel, Wayne Jack Wallis, Mark Wener, Robert Fox

Supported by NIH Award: P.P. Khanna was supported by a National Institutes of Health Award (T32 AR 053463) and D. Khanna was supported by NIAMS K23 AR053858-01A1

Contributor Information

Sogol Amjadi, Division of Rheumatology, UCLA School of Medicine, Los Angeles, CA.

Paul Maranian, Division of Rheumatology, UCLA School of Medicine, Los Angeles, CA.

Harold E. Paulus, Division of Rheumatology, UCLA School of Medicine, Los Angeles, CA.

Robert M. Kaplan, Department of Health Services, UCLA School of Public Health.

Veena Ranganath, Division of Rheumatology, UCLA School of Medicine, Los Angeles, CA.

Daniel Furst, Division of Rheumatology, UCLA School of Medicine, Los Angeles, CA.

Puja Khanna, Division of Rheumatology, UCLA School of Medicine, Los Angeles, CA.

Dinesh Khanna, Division of Rheumatology, UCLA School of Medicine, Los Angeles, CA. Department of Health Services, UCLA School of Public Health.

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