Health utilities are cardinal values in identifying an individual’s preference for different health outcomes. Health utilities are transformed to Quality Adjusted Life Years (QALYs) to enable comparisons between health interventions in cost-utility analysis in Pharmacoeconomics. Health utilities are measured using various utility ‘instruments’, which can be either generic (that is, can be applied to all diseases) or disease-specific.[1] Utility instruments are most of the times well-crafted questionnaires designed to capture and score the utility and thereby calculate the overall quality of life. Since the comparison of utility gains are often used in healthcare policy setting, it is essential that the instruments used to measure the utilities are rigorously validated.

Many of the popular and repeatedly validated utility instruments such as EQ-5D (EuroQol 5-dimension domain) and HUI (Health Utilities Index) are ‘generic’ and can be applied to most health conditions. Despite their popularity, the fact remains that these generic instruments are unable to capture some disease-specific factors that might determine the quality of life in patients suffering from the disease. For example, in a disease like psoriasis, the quality of life is determined to a large extent by disease-specific factors such as itching and embarrassment, which are not captured by EQ-5D.[2] To rectify this inadequacy, the existing generic measures are modified by adding disease-specific dimensions as ‘bolt-ons’, with an aim to comprehensively describe and value health.

Bolt-ons are dimensions or measures added to the parent instrument/Questionnaire in order to compensate/improve the inadequacy faced while applying it for a specific group of population or a particular health state.[3] The use of Bolt-ons ensures that not only the validity of the original utility instrument is retained, but also improves the sensitivity of the instrument in the therapeutic area where it is deficient. The wording and development of Bolt-ons in research are selected based on their statistically significant values and consistency by exploring various studies. This also depends on the topic of interest by the researcher. There is no public or patient involvement in the development of Bolt-on descriptions.[4]

For example, a ‘cognitive function’ bolt-on was added to the EQ-5D, and the resulting ‘EQ-6D’ was found to be more sensitive in capturing the overall health status of the population.[5] Likewise, two additional dimensions, namely ‘skin irritation’ and ‘self-confidence’ were bolted on to the EQ-5D so that the resulting questionnaire (the EQ-PSO bolt-on questionnaire) was more suitable to assess utilities in patients with psoriasis.[2] An enhanced version of EQ-5D with a vision bolt-on was found to be more discriminative while assessing quality of life in patients with visual problems than the original version of EQ-5D.[6] Next, while assessing the quality of life loss due to respiratory problems, a team from the Netherlands explored the impact of adding two ‘respiratory’ bolt-ons (‘limitations in physical activity due to shortness of breath’ and ‘breathing problems’) to EQ-5D-5L, and concluded that both the bolt-ons, especially the ‘ later, was associated with an improved discrimination of declining quality of life, when compared to the original instrument.[7] However, not all bolt-ons can be expected to enhance the parent utility measure. For example, adding a ‘sleep’ bolt-on to EQ-5D did not improve the validity or sensitivity of EQ-5D in calculating the quality of life, probably because the existing items in EQ-5D themselves captured the impact of sleep on overall quality of life.[8]

Though the Bolt-on dimensions enhance the health care utility values, there are some challenging issues while designing the Bolt-on descriptors and its implementation. This is because the population recruited is usually heterogenous. The terminology used should be subjected to Qualitative interviews prior to its use in research. However, additional information on the bolt-on descriptors can be obtained via quantitative research. Bolt-ons may not be necessary when the intervention involved removal of the intervention (E.g. from spectacles to laser eye surgery). Also, the impact of the Bolt-on dimensions depends on the severity states of EQ-5D.

The research on implementation of bolt-on dimensions is still in the infant stage which necessitates research in larger group of population to identify whether more complex models are necessary. The impact of addition of the bolt-on dimension should be rigorously explored in comparison with the original instrument. The validity and sensitivity of the modified measure should be evaluated in heterogeneous populations using appropriate psychometric testing to establish the bolt-on instrument as a viable tool for research. Care should be taken that the addition of the bolt-on is not compromising the validity of the instrument, and at the same time, the bolt-on is not providing spurious information.

With the rapid mushrooming of health utilities, developing new instruments entail careful validation and psychometric testing with notable gaps involving considerable time and resources. Bolt-on dimensions bridge the gap by being synergistic to the existing utility instruments and making them more responsive rather than just being additive.

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  1. What are health utilities? 2nd ed. Newmarket: Hayward Medical Communications; 2014 [cited 26 August 2020]. Available from:
  2. Swinburn P, Lloyd A, Boye K, Edson-Heredia E, Bowman L, Janssen B. Development of a Disease-Specific Version of the EQ-5D-5L for Use in Patients Suffering from Psoriasis: Lessons Learned from a Feasibility Study in the UK. Value in Health. 2013;16(8):1156-1162.
  3. Longworth L, Yang Y, Young T, et al. Use of generic and condition-specific measures of health-related quality of life in NICE decision-making: a systematic review, statistical modelling and survey. Southampton (UK): NIHR Journals Library; 2014 Feb. (Health Technology Assessment, No. 18.9.) Chapter 4, Developing ‘bolt-on’ items to EQ-5D.Available from:
  4. Stevens K. How Well Do the Generic Multi-attribute Utility Instruments Incorporate Patient and Public Views Into Their Descriptive Systems?. The Patient – Patient-Centered Outcomes Research. 2015;9(1):5-13
  5. Hoeymans N, van Lindert H, Westert GP. The health status of the Dutch population as assessed by the EQ-6D. Qual Life Res. 2005;14(3):655-663. doi:10.1007/s11136-004-1214-z
  6. Luo N, Wang X, Ang M, et al. A Vision “Bolt-On” Item Could Increase the Discriminatory Power of the EQ-5D Index Score. Value Health. 2015;18(8):1037-1042.
  7. Hoogendoorn M, Oppe M, Boland MRS, Goossens LMA, Stolk EA, Rutten-van Molken MPMH. Exploring the Impact of Adding a Respiratory Dimension to the EQ-5D-5L. Med Decis Making. 2019;39(4):393-404.
  8. Yang Y, Brazier J, Tsuchiya A. Effect of adding a sleep dimension to the EQ-5D descriptive system: a “bolt-on” experiment. Med Decis Making. 2014;34(1):42-53.

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