Objective
To determine the feasibility, acceptability, discriminative validity, responsiveness, and minimal important difference (MID) of the SF-6D for people with spinal cord injury (SCI).
Methods
A total of 305 people with SCI completed the SF-36 health status questionnaire at baseline and at subsequent occurrence of a urinary tract infection (UTI) or 6-month follow-up. Normative SF-36 data were obtained from the Australian Bureau of Statistics. SF-36 scores were transformed to SF-6D utility values using Brazier's algorithm. We used UTI as the external criterion of clinically important change to determine responsiveness and two categories of the SF-36 transition question (“somewhat worse” and “somewhat better”) as the external criterion to determine the MID. Derived SF-12 responsiveness was also assessed.
Results
The mean SF-6D values were: 0.68 (SD 0.21, n = 305) all patients; 0.66 (SD 0.19, n = 167) tetraplegia; 0.72 (SD 0.26, n = 138) paraplegia; 0.57 (SD 0.15, n = 138) with UTI. The Australian normative SF-6D mean value was 0.80 (SD 0.14, n = 18,005). The SF-6D was able to discriminate between SCI and the Australian normative sample (effect size [ES] = 0.86), tetraplegia–paraplegia (ES = 0.23), and it was responsive to UTI (ES = 0.86 SF-36 variant, ES = 0.92 SF-12 variant). The MID for respondents who reported being somewhat worse or somewhat better at follow-up was 0.03 (SD 0.17, n = 108/305), while the MID for only those who were somewhat worse was 0.10 (SD 0.14, n = 58).
Conclusions
The content of the SF-6D is more appropriate than that of the SF-36 for this physically impaired population. The SF-6D has discriminative power and is responsive to clinically important change because of UTI. The MID is consistent with published estimates for other disease groups.
Preference-based measures of health allow the relative
value of health states to be compared, both within and
across diseases [1]. A fundamental concept underpinning
this is health utility [2], a measure of preference
for health outcomes. Combined with survival data,
utilities can be used to estimate quality-adjusted lifeyears
(QALYs). Utilities and QALYs are used in costutility
analyses to assess the relative value of health
interventions, across a range of purposes (preventive,
diagnostic, curative, palliative), types (programs, services,
technologies, pharmaceuticals), and populations
(within and across diseased, disabled, and healthy
populations). Preference-based measures are therefore
useful and important outcome measures for policymakers,
both locally and internationally.
The SF-6D, a relatively new utility measure, is particularly
attractive as it is calculated from the SF-36, a
health status measure commonly used to assess the
impact of disease and disability, including spinal injury
[3]. In common with other multiattribute utility instruments
such as EQ-5D (Euroqol) and Health Utilities
Index (HUI) [4], it allows those experiencing the health
states to contribute directly to utility scores. A particular
advantage of the SF-36 and SF-6D is that they
economize on data collection, yielding measures of
both health status and utility. Since the SF-6D methodology
was published 5 years ago, it has rapidly
become a popular method of utility estimation. A
recent systematic review of the use of heath status
measurement instruments to calculate QALYs foundthat, despite its contemporary origin, the SF-6D
accounted for 5% of the instruments used [5].
The SF-6D is a utility measure based on a sixdimensional
health state classification. It is derived
from a subset of 11 SF-36 questions covering the
dimensions of Physical Functioning, Role Limitation,
Social Functioning, Pain, Mental Health, and Vitality.
It allows a possible 18,000 health states to be defined.
A survey (involving SF-6D, ranking, and standard
gamble) of 249 health states defined by the SF-6D was
valued by a representative sample of the UK general
public (n = 611). Econometric methods were then used
to determine a model for predicting the standard
gamble scores generated by the valuation survey [6].
Brazier et al. have shown that the SF-6D is a viable
alternative preference measure [6]. It can be derived
from either the SF-36 [6] or the shorter SF-12 [7]. It
has been suggested that the SF-6D may be more sensitive
than the EQ-5D, especially for mild–moderate
health issues [6]. Limitations and outstanding issues
with the SF-6D include whether it compromises the
richness of the original SF-36 [6] and whether it is less
sensitive when used in poorer health states [6,8,9]. It is
therefore important that additional validation studies
are performed in different populations and settings.
The present article describes such a validation in an
Australian population with spinal cord injury (SCI),
most of whom were living in a general community
setting.
The minimal important difference (MID) allows clinicians
to determine whether a change observed on a
self-reported health rating scale is meaningful or
trivial. It has been defined as the smallest difference in
score that the patient perceives as beneficial [10]. For
our purposes, in the absence of significant side effects
or cost barriers, this would lead to a change in clinical
decision-making.
This article provides the first validation and MID
values for the SF-6D in the SCI population. We assess
the acceptability and appropriateness of the SF-6D for
application in SCI, evaluate its discriminative ability,
and determine its responsiveness to clinically important
change. The external criterion used to define clinically
important change is the occurrence of a urinary
tract infection (UTI), a common comorbidity in this
patient population group, with a reported incidence of
1.82 episodes per annum [11,12].
The SF-6D can reliably discriminate not only the gross
differences between persons with SCI and a normative
population group, but also the smaller and more
clinically relevant differences between patients with
paraplegia and tetraplegia. The SF-12 and SF-36 variants
of the SF-6D are both responsive to the additional
disease burden of UTI in this patient group. The MID
is consistent with published estimates for other disease
groups. The content of the SF-6D makes it more
appropriate than the SF-36 for use in this physically
impaired population.
