Patient-Derived Disease Activity Scores Related to DAS28, CDAI

A recent report in Rheumatology described the establishment of thresholds for rheumatoid arthritis (RA) disease activities and response criteria for the patient reported outcome measure (PROM), Patient-based Disease Activity Score (PDAS) 1 and 2. PDAS is a recently developed, validated patient self-assessed score of RA disease activity initially developed as an alternative to assessor-based disease activity measures, such as the Disease Activity Score in 28 joints (DAS28).¹ PDAS1 and PDAS2 differ in their of inclusion erythrocyte sedimentation rate (ESR) as a component.

Guidelines from the European League Against Rheumatism (EULAR) and American College of Rheumatology (ACR) currently recommend the use of a treat-to-target strategy in the treatment of patients with RA.  In this strategy, clinicians use validated composite measures of disease activity to guide the adjustment of therapies until a pre-established treatment target of remission or low disease activity is met. 

Commonly used instruments for measuring disease activity in RA involve clinical assessment of the patient’s tender joint count (TJC) and swollen joint count (SJC), and may also include laboratory findings. This necessitates that patients make frequent clinic visits, as often as once per month for patients with high/moderate disease activity.²

The PDAS, a self-assessment whose use by patients requires no prior training, was first proposed in the journal Arthritis & Rheumatism in 2008 as a “a valid, reliable, sensitive, and feasible alternative to conventional assessment by clinicians for determining individual clinical disease activity and responses to therapy with anti-rheumatic drugs.”³

In that paper, Ernest H. Choy, MD, FRCP, and colleagues described the problems inherent in the commonly-used DAS28 assessment, which depends on joint counts performed by physicians, nurses, or therapists. In addition to requiring clinic visits by patients, clinician-reported joint counts are subject to substantial interobserver variability and are less sensitive than patient-reported measures in discriminating between active treatment and placebo.³

The investigators for the recently published study in Rheumatology, led by Alexander M. H. Leung, MD, Queen Elizabeth Hospital, Kowloon, Hong Kong, established thresholds for PDAS1 and PDAS2 that correspond to DAS28 and Clinical Disease Activity Index (CDAI) disease criteria for remission, mild, moderate and high disease activity states.

To accomplish this task, they utilized receiver operating characteristic (ROC) curve analysis to evaluate the accuracy of a diagnostic test in discriminating between diseased and non-diseased states.⁴ They also determined optimal thresholds for PDAS1 and PDAS2 corresponding to EULAR good and moderate responses to treatment, utilizing data from patients with RA who had started on conventional disease-modifying drugs or biologic agents. The investigators utilized Cohen’s kappa, a statistic used for interrater or intrarater reliability testing,⁵ to assess agreement with DAS28, CDAI and EULAR response criteria.¹

Summary and Clinical Applicability

“Thresholds for disease activity statuses and response to treatment for PDAS1 and PDAS2 have been established. They have comparable agreement to assessor-based criteria,” Dr. Leung and colleagues concluded.  “Defining self-assessed disease activity states may be useful to prompt patients to seek medical advice when RA is active. However, further studies will be needed to test the clinical effectiveness of PDAS in implementing treat-to-target strategy.”

Limitations and Disclosures 

The investigators declared no conflicts of interest. 

References

1. Leung AMH, Farewell D, Lau CS, Choy EHS. Defining criteria for rheumatoid arthritis patient-derived disease activity score that correspond to Disease Activity Score 28 and Clinical Disease Activity Index based disease states and response criteria. Rheumatology (Oxford). 2016 Jul 31. doi:10.1093/rheumatology/kew279 [Epub ahead of print]doi:10.1093/rheumatology/kew279.
2. Smolen JS, Aletaha D, Bijlsma JWJ, et al. Treating rheumatoid arthritis to target: recommendations of an international task force. Ann Rheum Dis. 2010;69(4):631-637. doi:10.1136/ard.2009.123919.
3. Choy EH, Khoshaba B, Cooper D, MacGregor A, Scott DL. Development and validation of a patient-based disease activity score in rheumatoid arthritis that can be used in clinical trials and routine practice. Arthritis Rheum. 2008;59(2):192-199. doi:10.1002/art.2334
4. Schoonjans F. MedCalc: ROC curves. MedCalc. https://www.medcalc.org/manual/roc-curves.php. Accessed November 1, 201
5. McHugh ML. Interrater reliability: the kappa statistic. Biochem Medica. 2012;22(3):276-282.

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