DAPSA and cDAPSA Calculator for Psoriatic Arthritis
DAPSA and cDAPSA Calculator for Psoriatic Arthritis
Sixty-eight tender joints, sixty-six swollen, two patient scales and CRP in mg/dL, with the clinical version computed beside it — and the two have different cut-offs, one point apart at the upper two boundaries.
DAPSA
5 inputs → DAPSA and cDAPSATender joint count 5, swollen joint count 3, patient pain 4, patient global 6, CRP 8 mg/L
Five terms added, and the version without the laboratory
cDAPSA = the same without the CRP term
DAPSA: remission ≤ 4 · low ≤ 14 · moderate ≤ 28 · high > 28
cDAPSA: remission ≤ 4 · low ≤ 13 · moderate ≤ 27 · high > 27
- 68 tender and 66 swollen
- the full peripheral counts, not the DAS28’s 28. Psoriatic arthritis involves the distal interphalangeal joints and the feet, which the 28-joint set omits. The hips are assessed for tenderness and not for swelling, which is the whole reason the two counts differ by two
- CRP in mg/dL
- two independently published trial documents state the unit as mg/dL and the derivation paper does not state it at all. A CRP of 8 mg/L is 0.8 mg/dL; entering 8 as mg/dL adds 8 instead of 0.8, worth exactly 7.2 points, which crosses the low-to-moderate boundary from a wide range of starting scores. CRP unit converter
- the two patient scales
- pain and global assessment, each on a 10 cm visual analogue scale read in centimetres. Both ask about ARTHRITIS. There is no skin item, no enthesitis item, no dactylitis item and no axial item anywhere in the index
- why the cDAPSA cut-offs are one point lower
- the authors lowered the upper two ‘to account for the putative higher levels of CRP’, leaving the remission cut-off at 4 for both. So the two versions share one boundary and differ at the other two, and a patient can sit in different bands on them
- the rounding
- 4, 14 and 28 are rounded ‘for ease of use’ from unrounded values of about 4.2, 13.7 and 28.3, derived from 33 experts rating 30 patient profiles. A sensitivity analysis on majority rather than mean ratings put the middle cut-off at 16
Worked example
Tender joint count 5, swollen joint count 3, patient pain 4, patient global 6, CRP 8 mg/L
5 + 3 = 8 from the joint counts
4 + 6 = 10 from the two patient scales
8 + 10 = 18.0 — this is the cDAPSA, printed beside the result, and 18.0 is moderate disease activity on the cDAPSA scale (above 13, at or below 27)
CRP: 8 mg/L ÷ 10 = 0.8 mg/dL
18.0 + 0.8 = 18.8 — moderate disease activity on the DAPSA scale too (above 14, at or below 28)
THE UNIT ERROR: take tender 4, swollen 3, pain 3, global 3 and the same CRP of 8 mg/L. Correctly converted the total is 13.8, which is low disease activity. Entered as though 8 were mg/dL it is 21.0, which is moderate. The error is exactly 7.2 points
THE ONE-POINT SHIFT: tender 4, swollen 3, pain 3.5, global 3 gives a cDAPSA of 13.5. With a CRP of 4 mg/L the DAPSA is 13.9. 13.9 is low disease activity on the DAPSA (limit 14) and 13.5 is moderate on the cDAPSA (limit 13) — the smaller number in the worse band, which is exactly what two separately shifted cut-off sets do
DAPSA and cDAPSA cut-offs, and where they part company
| State | DAPSA | cDAPSA | Agree? |
|---|---|---|---|
| Remission | ≤ 4 | ≤ 4 | Yes |
| Low disease activity | > 4 to 14 | > 4 to 13 | No |
| Moderate disease activity | > 14 to 28 | > 13 to 27 | No |
| High disease activity | > 28 | > 27 | No |
What is in the DAPSA, and what psoriatic arthritis has that is not
| Domain | In the DAPSA? | Where it is measured |
|---|---|---|
| Peripheral joint count | Yes — 68 and 66 | this page |
| Patient pain and global | Yes — two of five terms | this page |
| Acute-phase reactant | Yes in DAPSA, no in cDAPSA | this page |
| Skin | No | PASI, and the PASI 75 and 90 targets |
| Enthesitis | No | a separate enthesitis index |
| Dactylitis | No | a separate digit count |
| Axial disease | No | ASDAS, on this site |
| Physical function | No | a disability index, which this site cannot supply |
A sum of five, a clinical version, and the four domains it does not see
The DAPSA descends from the DAREA, an index built for reactive arthritis, and it is deliberately the simplest composite in rheumatology: five numbers added with no weights and no transforms. Two of them are the full peripheral joint counts rather than the 28 the DAS28 uses, and the reason is the disease — psoriatic arthritis involves the distal interphalangeal joints and the feet, which the 28-joint set omits entirely, so a 28-joint index can miss a whole pattern of disease. The two counts differ by two because the hips can be assessed for tenderness but not reliably for swelling, which is why the tender count is 68 and the swollen count 66.
The cut-offs came later and from a different kind of study. Schoels and colleagues showed 30 patient profiles, chosen to span a broad range of activity, to 44 invited international experts, 33 of whom responded, and took the boundaries from how those experts classified them. The published values of 4, 14 and 28 are rounded ‘for ease of use’ from unrounded figures of about 4.2, 13.7 and 28.3, and the paper records a disagreement within its own analysis: a sensitivity analysis using majority rather than mean expert ratings put the middle cut-off at 16, and the main analysis kept 14. The clinical version drops the CRP and shifts the upper two boundaries down by one point each ‘to account for the putative higher levels of CRP’, which leaves the two versions agreeing on remission at 4 and disagreeing everywhere above it. A cDAPSA of 13.5 and a DAPSA of 13.9 on the same patient fall in different named states, with the smaller number in the worse band.
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. The derivation paper does not state the CRP unit at all; two independently published trial documents do, and both say mg/dL — one of them writing the formula out as ‘DAREA/DAPSA = SJC66 + TJC68 + PtGA + PAIN + CRP’ with ‘CRP (in mg/dL)’ and the two patient scales in centimetres. At a CRP of 8 mg/L the mistake is worth 7.2 points, which is enough to cross the low-to-moderate boundary from a wide range of starting scores. The arithmetic also explains why the clinical version works: below about 10 mg/L the CRP term is worth less than a single tender joint.
What the DAPSA does not measure is the rest of the disease. There is no skin item, no enthesitis item, no dactylitis item, no axial item and no measure of physical function, so a patient with quiet peripheral joints and extensive plaques scores low here and is not well. The skin is measured with PASI and tracked against the PASI 75 and PASI 90 response targets; axial involvement with ASDAS-CRP and ASDAS-ESR. The composite indices that cover every domain were not built here because their components include licensed instruments — the minimal disease activity definition requires a disability index and an enthesitis index, and one composite includes a licensed quality-of-life summary — so they are named rather than computed. Methotrexate monitoring sits with the methotrexate unit converter.
Frequently asked questions
What are the DAPSA and cDAPSA cut-offs?
DAPSA: remission 4 or less, low disease activity above 4 to 14, moderate above 14 to 28, high above 28. cDAPSA: remission 4 or less, low above 4 to 13, moderate above 13 to 27, high above 27. The upper two were lowered by one point each to allow for the CRP the clinical version does not carry, so the two agree only on remission.
Why are the joint counts 68 and 66 rather than 28?
Because psoriatic arthritis involves the distal interphalangeal joints and the feet, which the 28-joint set omits. The two counts differ by two because the hips are assessed for tenderness but not reliably for swelling, so they are in the tender count of 68 and out of the swollen count of 66.
Is the DAPSA’s CRP in mg/dL or mg/L?
mg/dL. The derivation paper does not state the unit; two independently published trial documents do, and both say mg/dL. A CRP of 8 mg/L is 0.8 mg/dL, and entering 8 as though it were mg/dL adds 8 instead of 0.8 — exactly 7.2 points too many.
How reliable are the cut-off values?
They come from 33 of 44 invited international experts rating 30 patient profiles, and the published figures of 4, 14 and 28 are rounded from about 4.2, 13.7 and 28.3. The paper’s own sensitivity analysis, using majority rather than mean ratings, put the middle boundary at 16. They are expert-consensus boundaries on a continuous score, not biological thresholds.
Related calculators
References
- Schoels MM, Aletaha D, Alasti F, Smolen JS. Disease activity in psoriatic arthritis (PsA): defining remission and treatment success using the DAPSA score. Ann Rheum Dis. 2016;75(5):811–8.
- Guselkumab and golimumab combination therapy in active psoriatic arthritis — statistical analysis plan, NCT05071664. ClinicalTrials.gov document archive. (States the DAPSA and cDAPSA components, the CRP unit as mg/dL and both full cut-off sets.)
- PF-06700841 in active psoriatic arthritis — statistical analysis plan, NCT03963401. ClinicalTrials.gov document archive. (States the formula as DAREA/DAPSA = SJC66 + TJC68 + PtGA + PAIN + CRP, with the two patient scales in cm and CRP in mg/dL.)
- 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.
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/