We welcome the comments on our systematic review on factors associated with the Work Ability Index (WAI) with regard to the practical implications of the WAI instrument. After reading the review, the author of the e-letter concludes that ‘the WAI should be used with caution outside samples of people with musculoskeletal disorders and that more robust psychometric data be produced in other groups’. The intended message of the review is that the WAI is a useful tool in the field of employability of (older) workers, and that various work-related and individual factors play a role in someone’s workability. Our review was restricted to those factors that are not incorporated in the work ability index itself and, thus, we have not investigated health complaints, such as musculoskeletal disorders, as determinants and also refrained from analyzing in which occupational populations the use of the work ability index may be warranted. We agree with Nicholas Glozier that the work ability concept is complex and needs further study. Since the work ability index is a summary score over different dimensions, caution is required in drawing conclusions on a decreased WAI score without further diagnosis of reasons for a decreased WAI and underlying mechanisms. Previous studies have shown that the WAI is a useful tool among workers in physically demanding professions[1] as well as among mentally demanding professions[2] to analyze factors at population level that influence employability of workers and, thus, may be considered for addressing in interventions.

[1] Alavinia SM, van Duivenbooden C, Burdorf A. Influence of work- related factors and individual characteristics on work ability among Dutch construction workers. Scand J Work Environ Health. 2007 Oct;33(5):351-7.

[2] van den Berg TI, Alavinia SM, Bredt FJ, Lindeboom D, Elders LA, Burdorf A. The influence of psychosocial factors at work and life style on health and work ability among professional workers. Int Arch Occup Environ Health. 2008 Aug;81(8):1029-36.

The systematic review of factors associated with the Work Ability Index raises significant questions about this measure. The paper repeats the assertion that "the bases for work ability are health and functional capacity, but work ability is also determined by professional knowledge and competence (skills), values, attitudes, and motivation, and work itself." Certainly clinicians are constantly asked to assess the health and functional components of an individual's work related problems and help with this aspect of assessment is welcome. However, as table 6 succinctly demonstrates, the WAI shows consistent null associations with three of the four health and functional capacity constructs - cardiorespiratory fitness, "mental performance" and poor balance. Whilst some of these may reflect type 2 errors in the initial studies from the ORs quoted in the table this is concerning if the WAI is to be used across a range of health conditions. The results of intervention studies, which appear to be detected as positive only in those interventions with physical function components, may reflect a lack of responsiveness of the instrument to other interventions targetting improvements in, for example, mental health. In summary this review suggests that the WAI should be used with caution outside samples of people with musculoskeletal disorders and that more robust psychometric data be produced in other groups.