Simplified Disease Activity Index (SDAI) Calculator
Simplified Disease Activity Index (SDAI) Calculator
The CDAI plus CRP in mg/dL — and mg/dL is where implementations go wrong. Both totals are shown, so the fifth term is visible, and the SDAI is also the ACR/EULAR index-based definition of remission.
SDAI
5 inputs → CDAI plus CRPTender joint count 6, swollen joint count 4, patient global 4.5, evaluator global 3.5, CRP 12 mg/L
The CDAI plus one term, in the unit that catches people
Remission ≤ 3.3 · low ≤ 11 · moderate ≤ 26 · high > 26
- CRP, in mg/dL
- and this is the trap. Most laboratories report mg/L, and the two differ tenfold. A CRP of 10 mg/L is 1.0 mg/dL; entering 10 as though it were mg/dL adds 10 instead of 1 and moves the total by exactly 9.0 — enough to shift a patient from moderate to high activity, or out of remission. The unit select exists for that reason. CRP unit converter
- the other four terms
- identical to the CDAI, with equal weight and no transform. SDAI minus CDAI is exactly the CRP term, which is why both totals are printed
- why the cut-offs differ from the CDAI’s
- 3.3, 11 and 26 against 2.8, 10 and 22. The SDAI’s sit higher because it carries a fifth term that can only add. The sets are not interchangeable
- SDAI ≤ 3.3 as a remission definition
- this threshold is also the ACR/EULAR index-based definition of remission for clinical trials, published with the Boolean definition in 2011 and validated by the 2022 revision. It and the Boolean definition classify different patients: see Boolean and index-based remission criteria
- range
- 0 to 86 as scaled here, taking the CRP input to 10 mg/dL. The index has no true ceiling because CRP has none
Worked example
Tender joint count 6, swollen joint count 4, patient global 4.5, evaluator global 3.5, CRP 12 mg/L
6 + 4 + 4.5 + 3.5 = 18.0 — this is the CDAI, and it is printed beside the result
CRP: 12 mg/L ÷ 10 = 1.2 mg/dL
18.0 + 1.2 = 19.2 — moderate disease activity, because 19.2 falls above 11 and at or below 26
SDAI − CDAI = 19.2 − 18.0 = 1.2, which is exactly the CRP term. That identity is the point of having both indices
THE UNIT ERROR, numerically: take a cleaner patient with tender 6, swollen 4, globals 4 and 3, and a CRP of 10 mg/L. Correctly converted the CRP adds 1.0 and the SDAI is 18.0, which is moderate activity. Entered as though 10 were mg/dL it adds 10 and the SDAI is 27.0, which is high activity
The error is exactly 9.0 points at a CRP of 10 mg/L, and it scales: at 40 mg/L it is 36 points
At the other end of the scale the CRP term is what decides remission. Tender 1, swollen 1, globals 0.5 and 0.3 give a CDAI of 2.8 — CDAI remission — and an SDAI of 3.3 with a CRP of 5 mg/L, which is SDAI remission. Raise the CRP to 10 mg/L and the SDAI becomes 3.8: out of index-based remission on a CRP of 1.0 mg/dL
What the CRP term is worth, in both units
| CRP as reported (mg/L) | In mg/dL | Points added to the SDAI | Points added if entered as mg/dL by mistake |
|---|---|---|---|
| 1 | 0.1 | 0.1 | 1.0 |
| 5 | 0.5 | 0.5 | 5.0 |
| 10 | 1.0 | 1.0 | 10.0 |
| 40 | 4.0 | 4.0 | 40.0 |
| 100 | 10.0 | 10.0 | 100.0 |
The same four clinical items, three different verdicts
| Clinical items | CDAI | CDAI state | CRP | SDAI | SDAI state |
|---|---|---|---|---|---|
| TJC 1, SJC 1, globals 0.4 and 0.4 | 2.8 | Remission | 2 mg/L | 3.0 | Remission |
| TJC 1, SJC 1, globals 0.5 and 0.4 | 2.9 | Low activity | 2 mg/L | 3.1 | Remission |
| TJC 1, SJC 1, globals 0.4 and 0.3 | 2.7 | Remission | 9 mg/L | 3.6 | Low activity |
One extra term, the unit that breaks it, and the remission definition hiding inside
The SDAI is the CDAI with CRP added, and the pair was built to answer a specific question: how much does the acute-phase reactant actually contribute to a composite index? Aletaha and Smolen’s answer was that it adds little across most of the range, which is why the CDAI is usable at all. Look at the arithmetic and the reason is obvious: a CRP of 50 mg/L is 5 mg/dL and contributes 5 points to a scale on which the joint counts alone can supply 56. In a patient with moderate or high disease activity the CRP term is noise against the joint counts. Near remission it is the opposite — the whole remission band is 3.3 points wide, so 1 mg/dL of CRP is nearly a third of it, and the ACR/EULAR Boolean remission criteria put their CRP limit at exactly that 1 mg/dL.
This formula takes CRP in mg/dL, not mg/L. A laboratory reporting 4 mg/L is reporting 0.4 mg/dL, and entering the SI figure as though it were conventional multiplies that term by ten. This is not a theoretical worry. A trial protocol read for this page states the SDAI’s CRP input range as 0.1 to 10.0 mg/dL, which is 1 to 100 mg/L — the numbers overlap the range a laboratory reports in SI units, so nothing about an entry of 12 looks wrong. The unit select here takes a mg/L entry and divides by ten, prints the mg/dL figure that actually entered the sum, and prints the mg/L equivalent beside it so the reader can check against the report in their hand. At a CRP of 10 mg/L the mistake is worth exactly 9 points; at 40 mg/L it is 36, which is more than the whole distance from remission to high disease activity.
An SDAI of 3.3 or less is not only the bottom activity band. It is the ACR/EULAR index-based definition of remission in rheumatoid arthritis, published in 2011 beside the Boolean definition and validated rather than altered by the 2022 revision. The 2011 committee tested candidates for their ability to predict a good outcome in the second year of follow-up — stable radiographs together with a stable disability index of 0.5 or less — and found that the DAS28 definitions predicted radiographic outcome poorly while the SDAI, CDAI and Boolean candidates performed similarly. In the 2022 validation, across four randomised biologic trials and 2,048 participants, the positive likelihood ratio for that combined outcome was roughly 3.8 to 4.3 for the SDAI, the CDAI and the revised Boolean definition alike.
What those definitions do not do is agree on which patients are in remission. The index-based definition sums five terms and lets them trade off against one another; the Boolean definition tests four thresholds that must all hold at once and does not test the evaluator global at all. In the 2022 validation, concordance between the original Boolean definition and the SDAI was 74% in established rheumatoid arthritis, rising to 94% once the patient-global threshold was raised from 1 to 2. Boolean and index-based remission criteria computes all four definitions on one patient and shows which disagree. For the weighted alternative see DAS28-ESR and DAS28-CRP; for response rather than state, EULAR DAS28 response criteria and ACR20, ACR50 and ACR70.
Frequently asked questions
Is the SDAI’s CRP in mg/dL or mg/L?
mg/dL, and this is where implementations go wrong because most laboratories report mg/L. A CRP of 10 mg/L is 1.0 mg/dL. Entering 10 as though it were mg/dL adds 10 instead of 1 and moves the total by exactly 9.0 points — enough to shift a patient from moderate to high disease activity. This page carries a unit select and prints the mg/dL figure that entered the sum.
What are the SDAI cut-offs?
Remission 3.3 or less, low disease activity above 3.3 up to 11, moderate above 11 up to 26, high above 26. The CDAI’s are 2.8, 10 and 22. The SDAI’s sit higher because it carries a fifth term that can only add, and the two sets are not interchangeable.
Is SDAI 3.3 the same thing as ACR/EULAR remission?
It is the index-based half of it. The ACR/EULAR criteria offer two definitions: an index-based one, SDAI of 3.3 or less, and a Boolean one requiring tender joints, swollen joints, CRP in mg/dL and patient global each at or below their limits simultaneously. The two classify different patients, and concordance in established disease was 74% under the original Boolean definition.
How much does the CRP actually change the answer?
Little in the middle of the range and a great deal near remission. A CRP of 50 mg/L adds 5 points to a scale where the joint counts alone supply up to 56. But the remission band is only 3.3 points wide, so 1 mg/dL is nearly a third of it — which is both why the CDAI exists and why the Boolean remission criteria set their CRP limit at 1 mg/dL.
Related calculators
References
- Aletaha D, Smolen J. The Simplified Disease Activity Index (SDAI) and the Clinical Disease Activity Index (CDAI): a review of their usefulness and validity in rheumatoid arthritis. Clin Exp Rheumatol. 2005;23(5 Suppl 39):S100–8.
- Felson DT, Smolen JS, Wells G, et al. American College of Rheumatology/European League Against Rheumatism provisional definition of remission in rheumatoid arthritis for clinical trials. Ann Rheum Dis. 2011;70(3):404–13.
- Studenic P, Aletaha D, de Wit M, et al. American College of Rheumatology/EULAR remission criteria for rheumatoid arthritis: 2022 revision. Ann Rheum Dis. 2023;82(1):74–80.
- MSB11456 in participants with moderately to severely active rheumatoid arthritis — statistical analysis plan, NCT04512001. ClinicalTrials.gov document archive. (States the SDAI and CDAI definitions, the CRP unit as mg/dL, both full cut-off sets and the ACR response definition.)
- A study of LY3462817 in participants with rheumatoid arthritis — protocol, NCT04634253. ClinicalTrials.gov document archive. (Independently corroborates SDAI ≤ 3.3 and ≤ 11, CDAI ≤ 2.8, and states the SDAI CRP input range as 0.1 to 10.0 mg/dL.)
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/