Estimating Dutch reference scores for the PROMIS-29 Profile

Patient Reported Outcome Measures (PROMs) have become standard measurement tools in research to measure health and quality of life as perceived by the patient. In LifeLines the SF-36/RAND36 is being used to measure physical and mental health. These are important outcomes to study healthy aging from the perspective of the elderly. The SF-36 is widely used worldwide, however, a new system of PROMs has been developed, called PROMIS. It is expected PROMIS will become the gold standard world-wide for the measurement of patient-reported health. Specifically, the PROMIS-29 Profile will replace the widely used SF-36 because it measures similar concepts, but more reliably and in a better interpretable way. The aim of this study is therefore to compare the SF-36 to PROMIS and to obtain references score for the PROMIS-29 Profile, to facilitate migrating from the SF-36 to PROMIS. The study will also yield multi-dimensional self-reported health profiles of the LifeLines population and relevant subgroups. 
The Patient Reported Outcomes Measurement Information System (PROMIS®) has been developed over the past 10 years in the US with about 100 millions of dollars funding from the National Institutes of Health (NIH) [Cella 2007, Cella 2010]. Clinicians, researchers and psychmetricians have joined forces to collect, combine, and transform all existing PROMs into a new, state-of-the-art assessment system for measuring patient-reported health of adults and children that is more valid, reliable, and responsive than the existing PROMs [Fries 2011a, Fries 2011b, Magasi 2012]. 
PROMIS represents a revolution in the measurement of self-reported health because it combines two innovations in the development of patient-reported outcomes: first, PROMIS used state-of-the-art qualitative research and extensive patient input for developing the items. Second, PROMIS was developed using sophisticated psychometric methods, i.e. Item Response Theory (IRT). This resulted in the development of over 50 calibrated ‘item banks’, representing different aspects of physical, mental and social health. 
An item bank contains a (large) set of questions (called items) all representative of one construct (such as fatigue, anxiety, or pain). When item banks are analyzed using IRT, each item that fits the IRT model can be expressed in terms of its position on the measurement continuum. The location represents the difficulty of the items. Items can be selected from an item bank, picking items varying in difficulty, to make short forms (different short forms for each item bank are available). Since items in the bank are calibrated onto the same continuum, the scores obtained from the derived short forms are comparable to that from the complete bank. 
IRT calibrated item banks also have unique properties that enable Computer Adaptive Testing (CAT). With CAT people complete items at the computer. After a standard first item, the next items are selected from the item bank automatically by the computer, based on an individual’s response to previously administered items from the item bank. For example, if the question “are you able to walk 1 kilometer?” is answered by a person with “without difficulty”, the next question will concern a more difficult activity and the person will not be asked whether (s)he can get in and out of bed. The computer picks those items that are most informative for the respondent, and once the measurement precision of the estimate of the latent trait is low enough the assessment stops. The respondent commonly only needs only to answer a small subset of the entire item bank. This way, with a decreased questionnaire burden on participants, we will be able to measure as precisely as when administering the entire item bank. 
The Dutch-Flemish PROMIS National Center (www.dutchflemishpromis.nl) has translated 24 PROMIS item banks and 35 additional scales into Dutch-Flemish. Validation studies in different patient populations have been performed and CAT software is now available on a national level for research, clinical practice and other applications. 
It is expected that PROMIS instruments will replace widely used PROMs, especially generic PROMs, such as the EQ-5D and SF-36/RAND-36. One of the interesting PROMIS instruments is the PROMIS-29 Profile. The PROMIS-29 Profile is a collection of 4-item short forms measuring seven domains of self-reported health (depression, anxiety, physical function, pain interference, fatigue, sleep disturbance, and satisfaction with participation in social roles) along with a single item on pain intensity. It is expected that the PROMIS-29 Profile will replace the widely used SF-36 because it measures similar concepts, but more reliable and better interpretable. 
However, to facilitate the interpretation of PROMIS scores, reference scores from the Dutch general population are needed.  All PROMIS scores will be expressed as T-scores, in which scores have a mean of 50 and a standard deviation of 10 compared with the general population. Currently, only reference scores are only available for the US general population. Dutch reference scores are to be preferred over US reference scores. Because the Lifelines project offers the unique opportunity to obtain reference scores for the PROMIS-29 Profile from a representative sample of the Dutch general population, a proposal was approved (OV14_00227) to include the PROMIS-29 Profile in the follow-up questionnaire (follow-up questionnaire 2B). 
The data collected in Lifelines also offers the opportunity to perform a cross-walk (linking) study in which scores on the SF-36 are predicted from the PROMIS-29 Profile scores. In the US PROsettastone project some subscales of the SF-36 were already linked to PROMIS, but this was done on US data only [PROsettastone 2020]. These linking estimates have never been validated so we do not know whether they are applicable to the Dutch population. This linking will facilitate the migration from SF-36 to the PROMIS-29 Profile.