If you have axial spondyloarthritis you will probably meet the BASDAI at some point: on a clipboard in a waiting room, in a letter, or in a conversation about treatment. This is what it is, how the number is produced, and what it can and cannot tell you.
What BASDAI is
The Bath Ankylosing Spondylitis Disease Activity Index is a self-completed questionnaire developed at the Royal National Hospital for Rheumatic Diseases in Bath and published in 1994 (Garrett et al.). It was designed with input from people with AS, and it produces a single score from 0 to 10 describing how active the condition has felt over the past week. Higher means more active.
It is one of the most widely used measures of disease activity in AS and the wider axial spondyloarthritis spectrum, in clinics and in trials, and it is quick: the original validation reported 30 seconds to two minutes to complete (NASS, The Bath Indices).
The six questions
Each question refers to the past week and is answered on a 0–10 scale. The first five run from "none" to "very severe"; the sixth is a duration, from none to two hours or more, with one hour at the midpoint.
- How would you describe the overall level of fatigue or tiredness you have experienced?
- How would you describe the overall level of AS neck, back or hip pain you have had?
- How would you describe the overall level of pain or swelling in joints other than the neck, back or hips?
- How would you describe the overall level of discomfort you have had from any areas tender to touch or pressure?
- How would you describe the overall level of morning stiffness you have had from the time you wake up?
- How long does your morning stiffness last from the time you wake up?
Together they cover five symptoms: fatigue, spinal pain, peripheral joint pain, enthesitis (tenderness where tendons and ligaments attach to bone) and morning stiffness. Fatigue is there because research at Bath identified it as a major component of AS in its own right, not just a side effect of pain, and the index was developed with input from people with AS.
How the score is calculated
Morning stiffness has two questions, so to stop it counting double the two are averaged first. The arithmetic, as published by NASS, is:
- A. Add questions 1 to 4
Fatigue + spinal pain + other joint pain + tenderness.
- B. Average questions 5 and 6
(Stiffness severity + stiffness duration) ÷ 2.
- C. Add A and B, divide by 5
The result is the BASDAI, from 0 to 10.
The free BASDAI calculator does this arithmetic and shows each step.
What a score of 4 or more means
You will see the number 4 quoted often. In UK guidance, TNF-alpha inhibitors are an option for active ankylosing spondylitis or non-radiographic axSpA when the BASDAI is 4 or more and a spinal pain visual analogue scale score is 4 cm or more, and NSAIDs have not controlled the condition well enough or are not tolerated (NICE TA383). The same guidance says response should be assessed twelve weeks after starting, and defines it as a drop in BASDAI to 50% of the starting value or by 2 or more units, together with a fall of 2 cm or more in the spinal pain score. (An older appraisal, TA143, required the thresholds on two occasions twelve weeks apart; TA383 replaced it in 2016.)
That threshold is a decision rule for clinicians working within a specific guideline. It is not a line between "fine" and "not fine", and a score below it does not mean your symptoms do not matter.
What the score does not tell you
- It does not diagnose anything. BASDAI is used in people who already have a diagnosis of AS or axial SpA; a high score in someone without one means nothing on its own.
- It is self-reported, and it reflects how the past week felt. Two people with similar disease can score very differently, and the same person can score differently week to week for reasons unrelated to the condition.
- It does not measure damage or structural change. That is what imaging is for.
- It is not the only index. Your team may also use ASDAS (which adds a blood marker, CRP), BASFI for function, BASMI for spinal movement, and others. The Bath Indices booklet from NASS describes them.
Tracking BASDAI over time
A single score is a snapshot. Completed at regular intervals — monthly is common, or whenever your team asks — the scores become a trend, which is what a clinician can actually act on. It also helps to keep the underlying daily signals: if your BASDAI rose from 3 to 5, knowing whether that was fatigue or stiffness is the useful part. HAVYT charts both the BASDAI over time and the daily signals underneath it; see AS Mode.