Measuring Peripheral Arthritis in PsA: OMERACT Endorses the 66/68 Joint Count

The 66/68 swollen and tender joint counts (SJC66/TJC68) is the first fully endorsed outcome measurement instrument that uses the Outcome Measures in Rheumatology (OMERACT) Filter 2.1, as well as the first instrument to be fully endorsed within the Psoriatic Arthritis (PsA) Core Outcome Measurement Set, according to results from the OMERACT 2018 Psoriatic Arthritis workshop report recently published in The Journal of Rheumatology.

Recognizing that the PsA core domain set for randomized controlled trials and longitudinal observational studies has been recently updated, and that joint counts are key to the measurement of the peripheral arthritis component of the musculoskeletal (MSK) disease activity domain, the investigators sought to evaluate the SJC66/TJC68, using the OMERACT Filter 2.1, for the following parameters: domain match, feasibility, numerical sense (ie, construct validity), and discrimination (ie, test/retest reliability, longitudinal construct validity, sensitivity in clinical trials, and thresholds of meaning). A protocol was developed to evaluate the measurement properties of the SJC66/TJC68 joint count. The 2018 OMERACT meeting was convened to seek endorsement of the SJC66/TJC68 for its inclusion in the PSA Core Outcome Measurement Set.

The OMERACT delegates endorsed use of the SJC66/TJC68 to calculate the peripheral arthritis component of the MSK disease activity domain. Of the patient research partners, 100% voted in favor of a “green” endorsement. Among those deemed “other stakeholders,” 88% voted in favor of a “green” endorsement. In fact, MSK disease activity is actually considered one of the most important domains for both patients and clinicians alike.

Study results suggest that SJC66 and TJC68 both have construct validity. Moreover, TJC68 has adequate interrater reliability, whereas SJC66 does not possess this attribute. With respect to discrimination and responsiveness, SJC66 and TJC68 change over time in response to treatment, with the change in SJC66 and TJC68 having the ability to differentiate between patients who receive an effective treatment compared with those who receive placebo.

The working group concluded that the SJC66/TJC68 fulfill the OMERACT criteria for domain match, feasibility, truth, and discrimination. The limitations of the analysis involve the relatively low interrater reliability for the SJC only and a lack of studies that address the intrarater reliability of the TJC/SJC in PsA.

The lack of standardization of the instruments used to measure peripheral arthritis in patients with PsA over the years has led to the use of different methods to evaluate peripheral arthritis in randomized controlled trials and longitudinal observational studies. The MSK disease activity domain incorporates the heterogeneous disease manifestations of PsA, including enthesitis, dactylitis, peripheral arthritis, and spondylitis/axial arthritis. Adoption of the SJC66/TJC68 will be monitored in all future published randomized controlled trials and longitudinal observational studies.

Reference

Duarte-García A, Leung YY, Coates LC, et al. Endorsement of the 66/68 joint count for the measurement of musculoskeletal disease activity: OMERACT 2018 Psoriatic Arthritis workshop report [published online February 15, 2019]. J Rheumatol. doi:10.3899/jrheum.181089