PsA and SSc are chronic, often disabling diseases with a detrimental impact on HRQoL [27, 28]. Assessing health values (utilities) - ideally from the general public - is an essential element for economic evaluations of healthcare interventions in these and other diseases.
There are two different approaches to obtaining utilities from the general public. First, patients with a particular disease can fill out a health state classification instrument that uses population-assigned weights to calculate utility scores for particular health states. A variety of measures are available for this purpose, including the EQ-5 D, the QWB-SA, the Health Utilities Index, and the SF-6 D [16, 29–32]. The SF-6 D, which is derived from the SF-36 Health Survey, is a health state classification instrument that uses population weights assessed in the U.K. Using data from two different studies, we analyzed SF-6 D scores in patients with either limited or diffuse SSc of varying severity . The mean (SD) SF-6 D scores in the two studies were 0.61 (0.12) and 0.64 (0.13) on a scale ranging from 0.29 to 1.00. Neither study assessed the severity of patients' SSc.
The second method is to ask people from the general public directly to value health states common to a particular disease. The advantage of this method over the health state classification measurement method is that specific aspects of the disease can be described in various ways (e.g., with pictures or videos) beyond simple brief written descriptions available in a generic health status measure [33, 34]. To obtain community utilities for PsA and SSc, we interviewed 218 participants in a mid-size city in the U.S. The proportion of Caucasians (66%) in our sample is representative of the 2005 U.S. census and the proportion of African-Americans (28%) is representative of the city in which the study took place. The health status of our participants, as captured by the SF-12, was similar to that of the U.S. general population [16, 35].
The utility approach explicitly acknowledges that preferences are used to express the relative importance of various health outcomes . Understanding the concepts of the SG and TTO may be difficult for some subjects. To assess that, we asked our participants about their understanding of the health value assessment exercise; 91% rated it as clear or very clear. In addition, 98% of participants were able to empathize with the persons described in the PsA and SSc health states. Both of these findings lend confidence to our results. Furthermore, the health utility scores for mild, moderate and severe PsA and SSc support the construct validity of the utility measures in that more severe health states were assigned lower utilities than were less severe health states. In addition, as described in the literature previously, TTO and SG scores were generally relatively higher than RS scores, as the RS does not involve tradeoffs against an external metric such as time or risk of death. Our findings are consistent with previously published data that suggest that utility values derived using the SG are higher than those using the TTO for more severe health states, whereas the reverse may be true for less severe health states [36–38].
Several of our findings warrant particular attention. First, subjects assigned similar disutility to mild SSc and PsA health states, but moderate and severe PsA was assigned a greater disutility (lower utility) than moderate and severe SSc with or without lung involvement. This finding may be due to the public's perception that having thickened skin (from SSc) is more acceptable than having erythematous, pruritic scaly skin lesions (from PsA). Alternatively, it is possible that participants did not fully understand the full spectrum of differences between the two diseases, especially as related to mortality. This also may be reflective of the relatively young population of respondents, or their relatively low education and/or socioeconomic status.
Our data corroborate previous research showing that the general public assigns greater disutility to hypothetical health states in most, but not all, circumstances than do patients experiencing those health states . When health utilities were assessed in 107 patients with SSc of varying severity, the mean RS, TTO, and SG scores were 64.3, 0.76, and 0.74, respectively , scores that fell in the least severe SSc categories in our study. Health utilities have also been assessed in patients with psoriasis but without arthritis . In those patients, the RS, TTO, and SG correlated inversely with extent of skin involvement. Specifically, the median RS score was 0.76 for patients having less than 10% of their skin surface involved vs. 0.34 for those having more than 30% skin involvement. Corresponding median TTO and SG values were 0.99 for both measures (< 10% of skin involved) vs. 0.75 for both measures (> 30% of skin involved). Although patient-derived utilities are valuable for decision making involving individual patients, for cost-effectiveness analyses, the U.S. Public Health Service Panel on Cost-Effectiveness in Health and Medicine has recommended using utilities assessed from the general public . Although the general public tends to underestimate utilities of patients with a given condition, the Panel reasoned that community utilities for hypothetical health states represent the public's interest better [40, 41]. Also, members of the general public are potential future patients .
We had hypothesized that the general public would assign a lower utility to diffuse SSc, which manifests as greater skin thickening compared with limited SSc, for otherwise similar health states. Surprisingly, this was not the case. In other words, to the general public, the extent of skin thickening does not significantly affect the value of SSc health states. This finding may due to the way the health states were described or to limited power to detect differences in utilities for limited vs. diffuse SSc; alternatively, when assigning utilities, subjects may have focused more on ability to perform avocational and day-to-day activities rather than the extent of skin involvement. Health state classification systems by necessity are limited in the number of attributes they cover. Because we based our health state descriptions on the QWB-SA, we did not include additional clinical manifestations of PsA and SSc. It is possible that had we described additional aspects of severe SSc (e.g., finger contractures, painful ulcers, and painful calcinosis), then the utility for severe SSc might have been lower, perhaps even lower than the utility for PsA. Capturing those manifestations may have elucidated differences in utilities related to extraarticular features of PsA and to extent of involvement of SSc, but also may have generated too many health states for the subjects to be able to process. We also did not include prognostic information in describing the health states for diffuse vs. limited disease and in describing the associated lung disease. Prognostic information in the form of life expectancy is already captured in calculating QALYs; thus, the convention is to exclude prognostic information from the health state description per se so as to avoid double counting . Still, although diffuse SSc is more severe than limited SSc, in a previous study of patients with SSc we found that SG scores were actually higher among patients with the diffuse subtype (mean score 0.79 vs. 0.69 for patients with limited SSc) and that TTO scores were similar (0.76 for diffuse SSc and 0.77 for limited SSc) in the two groups .
Our study had several limitations. Participants were not selected randomly - rather, they were a convenience sample of respondents to newspaper ads and posters in one city. Thus, it is unlikely that the sample is truly representative of the U.S. population, especially given our low proportion of Hispanic patients. Nationally, Hispanics represent 14% of the U.S. population . Although health utilities generally don't differ by ethnicity, further research is necessary . Second, we sought to recruit subjects who had not experienced symptoms of inflammatory arthritis. By excluding patients with SSc, PsA, and other inflammatory arthritides from the utility assessment exercise, the results may be slightly non-representative of the general population. We believe that any such bias is minimal, given that with a sample size of 218, one would only expect to have 3-4 patients with these conditions [46, 47].