Misreading Disease Activity With DAS28

March 17, 2014
Philip Gardiner MD

The DAS28 disease-activity score has made the transition from research to clinic, but in some circumstances it may be misleading. A tool developed by a rheumatologist can help you explore how changes in components like ESR affect the score in unexpected ways.

Over the past few years, rheumatologists have embraced ‘treat to target’ protocols for early arthritis. The key to this strategy is the regular use of disease activity scores, and among these the Disease Activity Score for 28 joints (DAS28) has now made the transition from the research arena into daily use in the clinic.

The DAS28 was developed and validated by Piet van Riel and colleagues, using data from clinical trials.1 However, a tool that is valid for comparing groups of patients with very active rheumatoid arthritis (RA) may not work as accurately for patients in the clinic.

So when is a DAS28 not a reliable measure of disease activity?

•  When the arthritis is more active in the feet than the hands

This is an obvious drawback with the DAS28, but the score is a compromise, and for most patients this is not a major problem. I haven’t yet met a clinician who would prefer to carry out a 44-joint traditional DAS score in the clinic!

•  When the erythrocyte sedimentation rate (ESR) is within the normal range

Most clinicians would be surprised to learn that the ESR component of the DAS28 score changes by 1.85 within the range 1-15 mm/h, but only by 1.64 between 20 and 50 mm/h (Figure 1). The logarithmic transformation of the ESR performs well at the higher end of the scale, but most people would not expect a normal ESR to contribute so much to the overall score.

  When the patient has fibromyalgia as well as RA

One of the more controversial features of the DAS28 formula has been the double weighting given to the tender joint count over the swollen joint count. This makes the score more responsive to patient ‘distress,’ but it can skew results in patients with chronic pain syndromes such as fibromyalgia. For this reason, I have developed a DAS28 component tool to scrutinize the components of the DAS28 score, which helps me to see when the tender joint count and VAS dominate the score.

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Using this tool, you can experiment with different scores to better understand how scores are made up in different situations. Here are a few examples that might surprise you:

Case 1: Inactive RA but no remission: A patient with a normal ESR of 14 and no swollen joints will not attain DAS28 remission with one tender joint and a VAS of 20.

Case 2: Very active RA but ineligible for access to biologics (DAS28 <5.1). A patient with an ESR of 5, 10 swollen and 10 tender joints and a VAS of 90 would not qualify. (This situation has occurred more than once in my clinic).

Case 3: Fibromyalgia + Inactive RA – A patient with no swollen joints and a normal ESR of 15 would qualify for biologics (DAS28 5.1) with 12 tender joints and a VAS of 90. A ‘drop’ in the ESR to 2 would count as a significant response in the DAS28 score!

  When your patient needs a very high DAS28 score to qualify for access to biologic drugs

The DAS28 score was never intended to be an arbitrary test to allow a patient to access the most appropriate treatment. However, in the UK patients with RA can receive biologic treatment only when their DAS28 ESR score is over 5.1 on two consecutive visits (NICE guidance, UK). For patients with unusually low ESRs, this is impossible to achieve. This anomaly has been a very significant issue for several of my patients in recent years. (In many other countries a cut off DAS28 score of 3.2 allows the rheumatologist to treat patients more fairly and appropriately.)

 

The validity of using a ‘Swollen to Tender Joint Count Ratio’ to predict response to biologics has recently been confirmed in a large dataset from a Swedish database.2 This observation suggests that in the clinic we should be using a disease activity score that gives at least equal weighting to the swollen joint count.

The SDAI3 is an alternative, but has a skewed distribution and no clear correlation with EULAR criteria or NICE guidelines.4 For now the DAS28 is the best tool we have, but it should to be interpreted with care.

Disclosures: