Disease status, in terms of disease activity, disease progression and prognosis is difficult to define in ankylosing spondylitis (AS). No gold standard exists. Therefore, the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), a self-administered instrument, has been developed as a new approach to defining disease activity in patients with AS.

The index, designed by a multidisciplinary team with input from patients, consists of six 10 cm horizontal visual analog scales to measure severity of fatigue, spinal and peripheral joint pain, localized tenderness and morning stiffness (both qualitative and quantitative). The final BASDAI score has a range of 0 to 10. The index was distributed to a cross section of patients, including inpatients receiving 3 weeks of intensive physiotherapy treatment and hospital outpatients. BASDAI was completed by a total of 154 patients. Validation of the new instrument was achieved through analysis of user friendliness, reliability (consistency), score distribution and sensitivity to change. Comparisons were made with a previous Bath disease activity index (DAI) and the Newcastle Enthesis Index.

The BASDAI was found by patients to be quick and simple to complete (mean: 67 s). Test-retest reliability was good (r = 0.93; p < 0.001), as was the distribution of scores across the scale (score range: 0.5-10; mean: 4.31). BASDAI was sensitive to change, reflecting a 16% (mean) improvement in inpatient scores after 3 weeks of treatment. It is superior to the DAI in terms of construct and content validity and to the Enthesis Index in all aspects.

In summary, BASDAI is user friendly, reliability, sensitive to change and reflects the entire spectrum of disease. It is a comprehensive self-administered instrument for assessing disease activity in AS.