Patient-centred outcome measures
Dr Afsane Riazi, Neurological Outcome Measures Unit, National Hospital for Neurology and Neurosurgery, London
Way Ahead 2002;5(2):16-17
Patient-centred outcomes refers to the array of questionnaires, interview schedules and other related methods of assessing health, illness and benefits of health care interventions from the patient's perspective. The aim of this short review is to highlight:
- 1) the importance of patient-based outcome measures
- 2) criteria for selecting patient-based outcome measures and
- 3) ways of developing your own outcome measure.
Why are patient-based outcome measures important?
In contrast to physician-based outcomes (such as MRI and relapse rate in MS), patient-based outcomes are concerned with outcomes that are considered important to people as they focus on the quality of their health. However, there is some confusion in the literature as to what patient-based instruments actually measure. For example, there is little consistency of use or agreement in the meaning of terms such as 'quality of life', 'health-related quality of life', 'health status', 'functional status' and 'functional well-being'. Despite this lack of consistency in terminology, all patient-based instruments address some aspect of the patient's subjective experience of their health. They offer a unique perspective and may give different results to physician-based outcomes. Thus patient-based outcomes can be used to evaluate interventions and health care from the patients' perspective.
Broadly speaking, there are two types of patient-based outcome measures. Generic measures are those that are designed to be broadly applicable across different types and severity of disease, medical interventions, and demographic and cultural groups so as to permit comparisons across studies. The Medical Outcome Study Short Form-36 (1) is a well-known example. Disease-specific measures are designed to reflect clinically relevant issues for a specific disease. Recently, many disease-specific measures for MS have been developed. These include the Functional Assessment of MS (FAMS) (2), the MSQoL-54 (3), the Leeds MSQoL scale (4), the Guy's (now UK) Neurological Disability Scale (GNDS/UKNDS) (5), and the MS Impact Scale (MSIS-29) (6).
How do you select which patient-based outcome measure to use?
With an array of instruments all purporting to measure patient-based outcome, it is difficult to know which one to select for use in clinical research or practice. To aid in such selection, there are various criteria that can be examined. These criteria are based on psychometric theory, which is a scientifically rigorous field that is concerned with the science of assessing the measurement characteristics of instruments. In general, there are six psychometric properties that should be examined: data quality, scaling assumptions, acceptability, reliability, validity and responsiveness.
Indicators of data quality such as item non-response and missing scale scores, determine the extent to which an instrument can be used successfully in a clinical setting.
Scaling assumptions test whether items are correctly grouped into scales, and if the items can be summed without weighting or standardisation to produce a score.
Acceptability is concerned with the score distribution of the scale, and whether it represents the true distribution of the construct being measured in the sample.
The reliability of an instrument is defined as the extent to which it is free from random error. A reliable measure produces results that are accurate, consistent, stable over time and reproducible.
Validity can be broadly defined as the extent to which an instrument measures the concept it purports or is intended to measure.
Responsiveness is the ability of an instrument to measure clinically important change over time, and is essential when evaluating the relative benefits of different interventions
It is recommended that your choice of patient-based outcome measures be evidence-based; that is, how well the measure meets the above criteria. One method is to conduct a head-to-head comparison of various measures and selecting the measure with the best psychometric properties. Of course, if you are planning to use the measure in daily clinical practice, it is also important that the measure you have chosen is brief, user-friendly, practical to administer and cost effective.
How do you develop your own patient-based outcome measure?
Developing a patient-based outcome measure is not an easy task and should not be taken on lightly. However, if you feel that none of the available measures suit your needs, then you may consider developing your own measure in accordance with psychometric theory. It may also be a good idea to involve someone who has experience of instrument development. Firstly, you will need to define the construct you are proposing to measure and generate potential items from the appropriate sources. For example, when developing the MS Impact Scale6 we set out to measure the impact of MS from the person's perspective, and generated items from interviews with patients, expert opinion and literature review. These items are pre-tested on a small sample to assess how easily they can be understood and completed and appropriate alterations are made. Secondly, this version is used in a preliminary field test, the aim of which is to reduce the number of items and to develop scales. The instrument is administered to a large sample of patients and the results are analysed using standard psychometric techniques for item analysis. Based on these results, items are retained or discarded and grouped into subscales to produce a final version of the instrument. Thirdly, the psychometric properties (as described earlier) of this final version of the instrument are evaluated in a large independent sample in a second field test, to ensure that it is a rigorous measure. The fourth and final stage of instrument development is to ensure evidence-based use of the measure; this may include the development of a user-manual, head-to-head comparison with other scales, evaluation of the measure in diverse samples and cross-cultural validation.
In conclusion, patient-based outcome measures are increasingly recognised as being central in health-care evaluation as they offer patients' perspectives on the quality of their health. It is important that such instruments are scientifically rigorous to ensure that interventions for MS are accurately evaluated.
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- 2.Cella DF, Dineen K, Arnason B, Reder A, Webster KA, Karabatsos G, et al. Validation of the Functional Assessment of Multiple Sclerosis quality of life instrument. Neurology 1996;47:129-139.
- 3.Vickrey BG, Hays RD, Harooni R, Myers LW, Ellison GW. A health-related quality of life measure for multiple sclerosis. Quality of Life Research 1995;4:187-206.
- 4.Ford HL, Gerry E, Tennant A, Whalley D, Haigh R, Johnson MH. Developing a disease-specific quality of life measure for people with multiple sclerosis. Clinical Rehabilitation 2001;15(3):247-258.
- 5.Sharrack B, Hughes RAC. The Guy's Neurological Disability Scale (GNDS): a new disability measure for multiple sclerosis. Multiple Sclerosis 1999;5:223-233.
- 6.Hobart JC, Lamping DL, Fitzpatrick A, Thompson AJ. The Multiple Sclerosis Impact Scale (MSIS-29): a new patient-based outcome measure. Brain 2001;124:962-973.