Composite scores
These have been developed and validated to describe disease activity using a single parameter. Most of these have been validated in RA, but some have been applied to psoriatic arthritis.
Disease Activity Score (DAS) There is a version that includes assessment of 44 joints and the ESR but the 28 joint version has been validated in RA and a CRP version has been developed (DAS28-CRP), which tends to be the most commonly used version. The inputs are tender joint count, swollen joint count, CRP and patient global disease activity in cm out of 10 in the past week, 0 being lowest and 10 being highest possible disease activity. The joints used in the DAS28 are shown in figure 1. A complex formula involving square roots and natural logs combines the data into a single figure. Fortunately there is no shortage of on-line calculators. The reliability, validity and responsiveness of this score are all excellent.
The DAS28 definitions of disease activity are given in table 1 (below).
Figure 1. DAS28 joints
Assignment Julia has 4 tender joints, 5 swollen joints, a CRP of 24 and a global wellbeing score of 53 mm on a 100 mm line. Calculate her DAS here. What is her DAS28-CRP score? How active is her disease?
Simplified Disease Activity Index (SDAI) This uses the same inputs as DAS28-CRP but simply adds them together, giving a range of 0-76. Note, the CRP is in mg/dL, maximum 10. If your lab uses mg/L you will need to divide by ten.
Tender Joint count (0-28) + Swollen joint count (0-28) + CRP mg/dl (0-10) + VAS patient wellbeing (0-10)
It has excellent validity and responsiveness in RA but it has less reliability than DAS28 and is less widely used so other clinicians may have less of a feel for the significance of different values.
Exercise What is Julia’s SDAI?
Clinical Disease Activity Index (CDAI) This uses the same inputs as DAS28-CRP but includes Evaluator Global VAS in place of CRP. Like the SDAI the values are summed, giving a range of 0-76. The CDAI has the same performance as the SDAI, both being less reliable than DAS28.
Exercise If you rate Julia’s global wellbeing at 65 mm, what is her CDAI? _________________________________________________________________________ Table 1. States of disease activity (DA) defined by composite indices in RA
*As proposed by Aletaha and Smolen; Clin Exp Rheumatol 2005; 23 (Suppl. 39):S100-S108. _________________________________________________________________________
Which one to use? If you have the CRP or ESR and access to the internet, or an app on your phone (e.g. MedCalc), DAS28 is preferable. If you don’t have ESR or CRP you can use the CDAI.
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