Normative values for the Bath Ankylosing Spondylitis Functional Index in the general population compared with ankylosing spondylitis patients in Morocco.

BACKGROUND: The Bath Ankylosing Spondylitis Functional Index (BASFI) has been commonly used in rheumatology to quantify functional disability in patients with ankylosing spondylitis (AS). Our aim was to evaluate the discriminating power of BASFI and determine the best cutoff score of this index in the general population compared with AS patients.

METHODS: A cross-sectional study that included 200 patients suffering from AS and 223 subjects from the general population matched for age and sex was carried-out. The discriminating power of the BASFI by strata of age was evaluated by the area under the Receiver Operating Characteristic curve and the best cutoff was determined by the Youden index.

RESULTS: The mean age of the general population was 39 ± 12 years. 76.7% of them were male. The median BASFI of the healthy subjects and patients was 0.2 and 4.5 (P < 0.001) respectively. The best cutoff of BASFI was 1.5 with a sensitivity of 86% and a specificity of 90%. In the age group of 18-29 years, the best cutoff of the BASFI was 0.9 with a sensitivity of 93% and a specificity of 94%. In the age group of 30-50 years, the best cutoff of the BASFI was 1.5 with a sensitivity of 84% and a specificity of 88%. For those over 50 years of age, the best cutoff of the BASFI was 2.5 with a sensitivity of 84% and a specificity of 97%.

CONCLUSIONS: This study suggests that the discriminating power of BASFI is considered good at any age. The best cutoff of this index increased as age increases as functional disability is associated in part with lifestyle choices and increases with age. The cutoff values of the BASFI that we have presented could be used as a reference benchmark for both clinical practice and research.