ASDAS-CRP and ASDAS-ESR Calculator
ASDAS-CRP and ASDAS-ESR Calculator
Both published ASDAS versions on the same four symptom scores, side by side, with the 2 mg/L CRP floor applied and the gap between them shown — the coefficients and the inflammatory term are different and the answers are not interchangeable.
ASDAS
6 inputs → both versionsBack pain 6, patient global 6, peripheral pain 4, morning stiffness duration 5, CRP 5 mg/L, ESR 45 mm/h
Two formulas, five coefficients each
ASDAS-ESR = 0.079·back pain + 0.113·patient global + 0.086·peripheral pain/swelling + 0.069·morning stiffness duration + 0.293·√ESR
- the four symptom items
- each on a 0–10 numerical rating scale. Entering a 0–100 mm visual analogue mark instead is the error this page refuses rather than absorbs: a patient global of 60 would add 6.6 points on its own, so the calculator returns nothing above 10
- CRP
- in mg/L, entering the formula as ln(CRP + 1). The +1 keeps the term finite at a CRP of zero, but the published convention goes further: a CRP below the limit of detection, or a high-sensitivity CRP below 2 mg/L, is replaced by the fixed value 2 mg/L — 0.2 mg/dL in ASAS’s own wording
- ESR
- in mm/h, entering as a square root rather than a logarithm — so there is no floor and no +1, because the term is simply absent at an ESR of 0
- which version this page reports
- the headline is ASDAS-CRP, candidate C of the four Lukas 2009 tested, and the ASDAS-ESR (candidate B) is printed beside it so the gap is visible. ASAS’s own page calls the CRP version the preferred one and records that since 2024 it is called simply “ASDAS” rather than “ASDAS-CRP”
Worked example
Back pain 6, patient global 6, peripheral pain 4, morning stiffness duration 5, CRP 5 mg/L, ESR 45 mm/h
ASDAS-CRP symptom items: 0.121 × 6 = 0.726, 0.110 × 6 = 0.660, 0.073 × 4 = 0.292, 0.058 × 5 = 0.290 — 1.968
CRP term: 5 mg/L is above the 2 mg/L floor, so 0.579 × ln(6) = 1.037. Total 3.01 — high disease activity, in the 2.1 to 3.5 band
ASDAS-ESR on the SAME four scores: 0.079 × 6 = 0.474, 0.113 × 6 = 0.678, 0.086 × 4 = 0.344, 0.069 × 5 = 0.345 — 1.841, plus 0.293 × √45 = 1.966, giving 3.81
The gap is 0.80 points and it crosses a cut-off: 3.01 is high disease activity and 3.81 is very high. One patient, one morning, two published states
Drop the CRP to 0 and the floor engages: the term becomes 0.579 × ln(3) = 0.636 and the ASDAS-CRP becomes 2.60, not 1.97 — which is what an unfloored implementation would print
The four candidate formulas Lukas 2009 derived, and which two survived
| Candidate | Inflammatory term | Clinical items | In use today? |
|---|---|---|---|
| ASDAS A | 0.383·ln(CRP+1) and 0.210·√ESR, both | back pain, patient global, stiffness duration | No |
| ASDAS B | 0.293·√ESR | patient global 0.113, peripheral 0.086, stiffness 0.069, axial pain 0.079 | Yes — this is ASDAS-ESR |
| ASDAS C | 0.579·ln(CRP+1) | back pain 0.121, patient global 0.110, peripheral 0.073, stiffness 0.058 | Yes — this is ASDAS-CRP |
| ASDAS D | 0.224·√ESR and 0.400·ln(CRP+1) | back pain 0.152, fatigue 0.078, stiffness duration 0.069 | No |
The published states, and what happens at the boundaries
| ASDAS | 2018 name | Machado 2011 name | Boundary as published |
|---|---|---|---|
| below 1.3 | Inactive disease | Inactive disease | strictly below 1.3 |
| 1.3 to below 2.1 | Low disease activity | Moderate disease activity | 1.3 or more, strictly below 2.1 |
| 2.1 to 3.5 | High disease activity | High disease activity | 2.1 or more, 3.5 or less |
| above 3.5 | Very high disease activity | Very high disease activity | strictly above 3.5 |
Two formulas, one patient, and the floor that stops the logarithm misbehaving
The ASDAS was built to replace a questionnaire with a composite. Lukas and colleagues started from the variables an ASAS Delphi exercise had selected, fitted four candidate formulas in the ISSAS cohort — more than 1,200 patients from ten countries enrolled by 145 rheumatologists, of whom 708 had complete data and were analysed — and cross-validated them in OASIS. Two were adopted. Against a patient global above 6 versus below 4 in OASIS the standardised mean differences were 2.22 for candidate C and 2.35 for candidate B, against 1.79 for the BASDAI. The paper is careful about those numbers and so is this page: that comparison rested on only 31 high-activity patients, which is why a further analysis with a looser cut-off was added.
What the derivation did not settle is which version to use, and the two are not interchangeable. The coefficients differ, the inflammatory term differs in shape — a logarithm against a square root — and the relative weighting of the symptom items is reordered: back pain is the heaviest clinical item on the CRP version and the lightest on the ESR version. The DESIR analysis found “good agreement between ASDAS-hsCRP and ASDAS-ESR; however, formulae are not interchangeable”. On the default patient here the two differ by 0.80 and land in different published states. Pick one version per patient and keep to it. The two formulas are not interchangeable. The 2022 ASAS-EULAR recommendations say ASDAS “preferably calculated using CRP, is a well-balanced index without redundancy across its items”, which is as close to an instruction as the literature gets.
The CRP term is where implementations fail. A conventional assay reports ‘below 5 mg/L’ rather than a number, and ln(0 + 1) is zero — so a naive calculator silently credits an undetectable CRP with no inflammatory contribution at all. ASAS’s own page instructs that such a value, and any high-sensitivity result below 2 mg/L, is replaced with “the fixed value of 2 mg/l (0.2 mg/dl)”. The convention came from 257 patients in the DESIR cohort, where using half the detection threshold was tested instead and described as not data-driven. This page applies the floor and prints the floored value beside the result, so nothing is silent; it is worth 0.636 of a point. The ESR version needs no floor, because √0 is 0.
Two things this page deliberately does not do. It does not compute the BASDAI, although three of the four symptom items here are BASDAI questions, because the BASDAI’s licensing position could not be established: an independent implementation marks its intellectual property unresolved and ships no item text, its custodian is a commercial licensor, and the published reports of its terms contradict each other. This page reproduces a formula and its published cut-offs, not a questionnaire. And it does not carry the BASDAI’s averaging rule, which is the commonest source of a wrong ASDAS: the BASDAI averages its two morning-stiffness questions before taking a mean, and the ASDAS takes the duration question alone. For change over time use ASDAS improvement and worsening; the rheumatoid equivalents are DAS28-ESR, DAS28-CRP, CDAI and SDAI.
Frequently asked questions
Do ASDAS-CRP and ASDAS-ESR give the same answer?
No. The published position is that the formulae are not interchangeable even though agreement is described as good. On this page’s default patient they differ by 0.80 of a point and fall into different published states, and the three state cut-offs are the same numbers for both.
What do I enter when the CRP is reported as below the limit of detection?
Enter 0 or 2; this page gives the same answer either way. The published convention, from an analysis of 257 patients in the DESIR cohort and stated on ASAS’s own instrument page, is that a CRP below the limit of detection or a high-sensitivity CRP below 2 mg/L is replaced by the fixed value of 2 mg/L (0.2 mg/dL). The floor is applied here before the logarithm and the value used is printed beside the result.
Is the morning stiffness item the severity or the duration?
The duration, and one question rather than an average. In trials it is BASDAI question 6 alone. The BASDAI itself averages its severity and duration questions to a single value before taking a mean of five, and carrying that rule into the ASDAS is the commonest way to produce a wrong ASDAS.
Related calculators
References
- Lukas C, Landewé R, Sieper J, et al. Development of an ASAS-endorsed disease activity score (ASDAS) in patients with ankylosing spondylitis. Ann Rheum Dis. 2009;68(1):18–24.
- Machado P, Landewé R, Lie E, et al. Ankylosing Spondylitis Disease Activity Score (ASDAS): defining cut-off values for disease activity states and improvement scores. Ann Rheum Dis. 2011;70(1):47–53.
- Machado PM, Landewé R, van der Heijde D; Assessment of SpondyloArthritis international Society (ASAS). Ankylosing Spondylitis Disease Activity Score (ASDAS): 2018 update of the nomenclature for disease activity states. Ann Rheum Dis. 2018;77(10):1539–40.
- Machado P, Navarro-Compán V, Landewé R, van Gaalen FA, Roux C, van der Heijde D. Calculating the ankylosing spondylitis disease activity score if the conventional C-reactive protein level is below the limit of detection or if high-sensitivity C-reactive protein is used: an analysis in the DESIR cohort. Arthritis Rheumatol. 2015. doi:10.1002/art.38921.
- Assessment of SpondyloArthritis international Society. ASDAS calculator. asas-group.org. (ASAS’s own instrument page: both formulas in rounded form as text, the state and improvement cut-offs, the instruction that a CRP below the limit of detection or below 2 mg/L is replaced by “the fixed value of 2 mg/l (0.2 mg/dl)”, and the note that since 2024 the CRP-based index is called simply ASDAS.)
- Ramiro S, Nikiphorou E, Sepriano A, et al. ASAS-EULAR recommendations for the management of axial spondyloarthritis: 2022 update. Ann Rheum Dis. 2023;82(1):19–34.
- AScalate: Treat-to-target in Axial Spondyloarthritis — statistical analysis plan, NCT03906136. ClinicalTrials.gov document archive. (The only document read that states both the ASDAS-CRP and the ASDAS-ESR coefficients to three decimal places.)
- CamCOPS documentation. Ankylosing Spondylitis Disease Activity Score (ASDAS) and Bath Ankylosing Spondylitis Disease Activity Index (BASDAI). (Independent open-source implementation notes, read for the intellectual-property position on each instrument.)
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/