What is the Foot Function Index?

Clinicians and researchers have developed a number of tools to help them measure the impact of almost all health conditions and use that to help quantify the impact of that health condition on the person. This is used in clinical practice to measure progress in treatment and used in research to measure changes from treatments that are being researched.

The Foot Function Index (FFI) was developed in 1991 as a tool for clinicians and researchers to measure the impact of foot problems on function in terms of pain, disability and any activity restriction of those with the problem. It is a survey like tool that consisted of 23 questions. It was originally developed in the context of the problems of the feet that occur in rheumatoid arthritis, but is now widely used in clinical and research settings for a wide range of foot problems. The FFI questionnaire has 23 items that are divided into 3 subcategories on the basis of patient values in the 3 domains of pain, disability and activity limitation. The patient completes the questionnaire to score each question on a scale from 0 (no pain or difficulty) to 10 (worst pain imaginable or so difficult it requires help), that best describes their foot over the previous week. The overall score gives the Foot Function Index and there are also the score for each of the 3 sub-scales.

Of concern with all these types of tools that measure outcomes and related issues is that is the tool reliable and is it valid? Various studies on the test-retest reliability of the FFI total and subscale scores ranges have been reported it being from 0.87 to 0.69 which is considered good. It has been tested mostly on people with rheumatoid arthritis and the conclusion of that research is that it is a reliable tool to use in these patients. Reliability of any survey type tool is considered very important and is a way of measuring if the tool is repeatable, in that it gives as close as possible to the same result if it is used repeatably. Just as important as reliability is the concept that the Foot Function Index is valid. Validity refers to if the tool is measuring what it actually claims that it is measuring. One study that looked at this for the FFI showed a strong correlation between the FFI total and sub-scale scores and other clinical measures of foot symptoms which confirms that there is good criterion validity of the Foot Function Index. Another important characteristic of tools like this is that they are the responsive. This means that if the tool is used again after a period of time and there has been a change in the symptoms, is the tool sensitive enough and responsive enough to detect that change. The FFI has been rated positively for responsiveness.

Since its original development a revised version of the Foot Function Index has been created to improve the characteristics of the tool and make it more consistent with WHO guidelines. It has been translated into several languages where it has again been tested if it is reliable and valid in those languages. All the research so far has shown that it is.