How this instrument works
BASDAI asks six questions, each answered on a 0-to-10 scale: fatigue, spinal pain, joint pain or swelling, localized tenderness, the severity of morning stiffness, and the duration of that stiffness. It would be easy to just average all six equally, but the scale's authors deliberately didn't do that, and the arithmetic is the single most distinctive thing about this index.
Morning stiffness gets two separate questions — severity and duration — while every other symptom domain gets only one. Left alone, that would let stiffness quietly count for two-fifths of the total instead of its intended one-fifth. So the two stiffness questions are first averaged together into a single combined sub-score, and only then is that sub-score averaged alongside the other four full-weight questions. The formula reads: BASDAI = (Q1 + Q2 + Q3 + Q4 + (Q5 + Q6)/2) / 5. Working it through keeps all five underlying symptom domains — fatigue, spinal pain, joint symptoms, tenderness, and stiffness — carrying equal weight in the final figure, rather than letting stiffness dominate simply because it was asked about twice.
A BASDAI of 4 or higher is the standard threshold used to call disease 'active,' and international treatment recommendations commonly use that same cutoff as an eligibility bar for starting biologic therapy after conventional treatment has failed. It's worth noting this particular threshold was set by consensus rather than derived from a formal statistical cut-point analysis — a real limitation the rheumatology literature itself has pointed out, even as the number remains the standard in practice.
- Rate Fatigue, Spinal pain, Joint pain/swelling, and Localized tenderness each on a 0 (none) to 10 (worst imaginable) scale, based on how the person has felt over roughly the past week.
- Rate Morning stiffness severity and Morning stiffness duration separately, also on 0 to 10 — these two feed into the same combined stiffness sub-score rather than counting as two independent domains.
- Read the BASDAI output; the working block shows the four full-weight questions added to the averaged stiffness pair, then divided by five.
- Compare the figure to the 4-point threshold commonly used to define active disease, and track it over repeat visits — a single reading matters less than whether the trend is rising or falling under treatment.
Worked example — well above the active-disease threshold
A person rates fatigue at 6, spinal pain at 7, joint pain/swelling at 5, and localized tenderness at 6. Their morning stiffness runs an 8 for severity and a 6 for duration. First average the stiffness pair: (8 + 6) / 2 = 7. Add that to the other four: 6 + 7 + 5 + 6 + 7 = 31. Divide by five: 31 / 5 = 6.2 — well above the 4-point cutoff commonly used to call this active disease.
Compare a much milder week: fatigue 1, spinal pain 1, joint symptoms 0, tenderness 1, stiffness severity 1, stiffness duration 0. The stiffness average is (1 + 0) / 2 = 0.5. Adding the rest: 1 + 1 + 0 + 1 + 0.5 = 3.5, divided by five gives 0.7 — comfortably under the threshold, describing quiet disease rather than an active flare.
Questions
Why are the two morning-stiffness questions averaged together instead of counted separately?
Because leaving them as two independent full-weight questions would let morning stiffness count for two of six questions — roughly 40% of the total — while every other symptom domain, like fatigue or spinal pain, only got one-sixth. Averaging severity and duration into a single combined figure first, then folding that average in alongside the other four questions, restores the intended balance: five symptom domains, each carrying equal weight.
What BASDAI score counts as active disease?
A total of 4 or above is the conventional threshold, and it's the figure most international treatment guidelines reference when discussing whether someone might be a candidate for biologic therapy after standard anti-inflammatory treatment hasn't worked well enough. It's a widely used consensus number rather than one derived from a rigorously validated statistical cut-point, which is worth knowing if a result sits close to the line.
Can BASDAI alone decide whether someone starts a biologic medication?
No — it's typically one criterion among several in formal eligibility rules, alongside things like inflammatory markers, imaging findings, and how the person has responded to at least two different anti-inflammatory drugs already. A rheumatologist weighs BASDAI together with the full clinical picture rather than acting on the number by itself.
How far back should someone think when answering the six questions?
The standard version asks about symptoms over roughly the past week, so answers should reflect a typical recent stretch rather than the single worst day or an unusually good one. Averaging across several recent days in your own head before answering each question tends to give a more representative figure than picking the most extreme moment.
Where does the BASDAI scale come from?
Garrett and colleagues at the Royal National Hospital for Rheumatic Diseases in Bath, England, published it in 1994, building six visual-analog questions into a single composite figure meant to track disease activity in ankylosing spondylitis over time, in a way simpler outcome measures hadn't managed cleanly before.
References
- Garrett S et al., 1994, J Rheumatol — original BASDAI validation (PubMed)
- van der Heijde D et al., 2017, Ann Rheum Dis — 2016 ASAS-EULAR management update (PubMed)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.