¿Da igual cómo midamos la calidad de vida de nuestros pacientes?: un análisis de los cuestionarios SF-12 y SF-36 en población con insuficiencia renal
This contribution summarizes the communication awarded with the Award for the best communication. short oral at the XLIV Health Economics Conference (Madrid, June 18-20, 2025).
For a long time, clinical practice has relied almost exclusively on clinical indicators related to the disease: results of clinical examination, laboratory tests or diagnostic tests. All of them continue to be essential, but it is increasingly evident that they are not enough to really understand what impact diseases and treatments have on people’s lives. Two patients with similar clinical indicators can live very different realities, and these differences are not always detected with an analysis.
In this context, the health-related quality of life (HRQoL) has been gaining prominence as a central dimension in both research and clinical practice. It is not about replacing the classic indicators, but rather complementing them with the patient’s perspective: their energy level, their functional capacity, their emotional well-being, their social role. Measuring these aspects allows us to refine the clinical evaluation, improve doctor-patient communication and, in addition, provide key information for decision-making in the field of health management.
This is where validated quality of life questionnaires come into play, standardized tools that convert subjective perceptions into comparable and useful data. Among the best known are those provided by the University of Sheffield: the SF-36 and its abbreviated 12-question version, the SF-12, or the SF-6Dv2; or those provided by EuroQol: the EQ-5D-5L or its version with fewer responses, the EQ-5D-3L. All of them are widely used in clinical studies, in health services research and in economic evaluations, since they allow estimating the utility values that are used to calculate quality-adjusted life years (QALY), the reference metric in cost-utility analyses.
Source: Own elaboration.
But one question persists among many clinical and research teams: does it really matter whether you use one or the other? Is the shorter and more manageable SF-12 a perfect substitute for the SF-36? Or are we sacrificing relevant information when we opt for the short questionnaire? Recent research carried out in Spanish patients with kidney failure offers very interesting clues to answer these questions.
- Why is measuring HRQoL in kidney failure especially important?
Chronic kidney disease is a good example of why classic clinical indicators are not enough. Persistent fatigue, physical limitations, sleep disturbances, psychological impact. Many of these aspects weigh as much or more than some clinical outcomes when defining how the patient experiences their illness.
In this type of pathology, where the objective is not always to “cure”, but rather to improve well-being, measuring HRQoL is not an optional addition, but a necessity. It allows you to monitor progress, evaluate the real impact of different therapeutic strategies and detect problems that would otherwise remain invisible in the consultation.
Furthermore, from the point of view of health economics, chronic diseases concentrate a large part of health spending. Incorporating HRQoL measures makes it possible to go beyond the cost per event avoided and move towards analyzes that integrate quantity and quality of lifesomething essential when comparing interventions that do not always prolong survival, but can transform it.
- A Spanish study that tests the SF-12 and SF-36.
With this background, our research group (ECOLOGICAL HEALTH), conducted a study in 149 patients with advanced chronic kidney disease treated in a Nephrology service in Spain. All of them are administered the v2 versions of the SF-12 and the SF-36, along with the EQ-5D-5L, with two clear objectives:
- compare the psychometric properties of SF-12 and SF-36, and
- analyze their interchangeability when obtaining profits to carry out cost-utility analysis.
Psychometric properties—internal consistency, ceiling and floor effects, construct validity, and discriminatory power—are, in essence, the “quality test” of a questionnaire. They indicate to what extent it measures well what it aims to measure, whether it is able to differentiate between different health states and whether it avoids artificially grouping many patients into the highest or lowest level.
Source: Own elaboration.
- SF-36: longer, but also thinner
The results were quite consistent. Both instruments show adequate behavior to evaluate HRQoL in this population, but the SF-36 came out better in practically all indicators.
This questionnaire presented a lower ceiling effect, that is, fewer patients concentrated on the maximum level of health, something especially important in populations with chronic illness, where instruments sensitive to small changes are sought. It also showed greater internal consistency, which indicates that its dimensions more coherently capture the different aspects of health status. In addition, it obtained better results in convergent validity and discriminatory power, reflecting a greater ability to distinguish between patients with different clinical profiles.
Source: Own elaboration.
Simply put, the SF-36 provides a more detailed and clear picture of the patient’s quality of life. It not only tells if it is “better” or “worse”, but also helps to understand in what specific dimensions the changes occur.
- And what happens when we move from the clinic to the economy?
From a clinical point of view, these differences are already relevant, but they become even more important when we enter the field of economic evaluation.
Utilities that are used in the calculation of QALYs can be derived from SF-12 and SF-36. Utilities are indicators – summary of HRQoL – whose values can range between 0 and 1 (in some cases, even negative values), where 0 is the worst HRQoL (commonly associated with the state of death) and 1, the best. In this study, the mean values were very similar: 0.676 for SF-12 and 0.669 for SF-36. Furthermore, the level of agreement was high (agreement weight coefficient of 0.817), which at first glance could suggest that both instruments are practically equivalent.
However, the finer analysis revealed a crucial nuance: there were 20% inconsistencies between both questionnaires. That is, in one in five patients, the estimated utility differed enough to change their relative position or potentially influence the results of a cost-utility analysis.
Source: Own elaboration.
This has important implications. When comparing treatments, programs or health technologies, small differences in utilities can tip the balance in favor of one intervention or another. Therefore, the choice of instrument is not neutral. Although SF-12 and SF-36 are correlated, they are not perfectly interchangeable.
- What can a clinician extract from all this?
At this point, it is worth asking: what does all this contribute to daily clinical practice?
First, a clear message: Measuring HRQoL with validated instruments provides relevant and useful information that is not obtained through other means.. It allows us to objectify perceptions, follow the patient’s evolution, detect problem areas and evaluate the real impact of our clinical decisions. In this context, our research group always recommends completing an HRQoL questionnaire specific to the pathology that the patient presents; and another generic based on preferences such as the SF-12, the SF-36 or the EQ-5D.
Second, that not all questionnaires are the same. And this not only applies to the SF-12 and SF-36, the subject of this study. Other studypublished in The European Journal of Health Economics by our research group (ECOLOGICAL HEALTH), revealed that the utilities derived from SF-36 and those derived from EQ-5D-3L are also not interchangeable. And the same thing happens in another study that we have pending publication that analyzes the EQ-5D-3L and the EQ-5D-5L.
Third, that the brevity also has value. The study itself concludes that, although it is advisable to use the SF-36 whenever possible due to its greater psychometric solidity, the SF-12 is a perfectly feasible alternative when time is limited. In saturated consultations, units with a high care load or projects where HRQoL is not the main outcome, having a short and validated instrument can make the difference between measuring… or not measuring. For this reason, the EQ-5D or the SF-6Dv2, recently developed, although not yet validated for Spain, are a good option for short questionnaires with 5 and 6 questions, respectively.
- From research to consultation
One of the great challenges is to transfer these instruments outside the strictly research field. Integrating them into clinical practice requires organizational changes, training and, above all, the conviction that they add value.
But the advantages are clear. Incorporating HRQoL questionnaires allows:
- Enrich the clinical history with systematic patient information,
- Monitor evolution more completely,
- Improve clinical communication,
- Early identify functional or emotional problems, and
- Generate useful data for both clinical management and economic evaluation.
In chronic pathologies such as kidney failure, where treatment accompanies the patient for years, these instruments can become allies to guide decisions, prioritize resources and evaluate programs.
- Beyond the numbers
Sometimes, talking about utilities and QALYs may sound excessively technical or distant from clinical practice, but it is worth remembering that behind each decimal there is something deeply human: the ability to walk without fatigue, to sleep well, to maintain social relationships, to maintain autonomy.
Measuring HRQoL is not bureaucracy, it is a way of systematically listening to the patient and incorporating their experience into decision-making.
Ultimately, if we want to move towards more person-centered medicine and more efficient healthcare systems, we need to measure what really matters. And HRQoL, without a doubt, matters.
