Background/Purpose: Despite recommendations to use full joint counts to assess patients with PsA, reduced joint counts designed for RA are often used for reporting outcomes in PsA registry studies and are used in clinical practice to assess patients with PsA. Analysis of RCTs suggested that 28 joint counts were responsive in PsA but this analysis was based on polyarticular patients enrolled into drug trials. The aim of this analysis was to investigate full and reduced joint counts in patients with oligoarticular PsA.

Methods: The study was an analysis of baseline visits for patients in the GRACE study, an international observational cohort recruited to develop new outcome measure in PsA. Oligoarthritis was defined as less than 5 tender and swollen joints. At baseline, full 66/68 tender and swollen joint counts were assessed. Active joint counts were calculated with an active joint defined as either tender and/or swollen.  Reduced joint counts used for RA including the 28 and 44 joint count were analysed for comparison. In addition, new proposed shortened joint counts for PsA were tested to investigate their possible future use: PsA-44 (elbows, wrists, MCPs, PIPs, DIPs, knees and MTPs) and PsA-56 (as before plus ankles and PIP toes). ROC analysis was used to see how well different joint counts predicted treatment change for high disease activity. The proportion of patients with active disease missed by reduced joint counts was also analysed.

Results: In the cohort of 503 patients, 270 were classified as oligoarthritis. ROC analysis revealed that tender or active joint counts did not predict treatment change for high disease activity even when using a full 66/68 joint count (tender: AUC 0.56,p=0.18, active: AUC 0.55,p=0.23). A 66 SJC did predict treatment change (AUC 0.60, p=0.018) as did the SJC PsA44 and PsA56 (p<0.04). Neither of the RA reduced joint counts showed a significant relationship with treatment change. Of the 270 patients, 164 patients did not have any tender joints identified on a 28 joint count. However, of these 164, 38 did have 1 or more tender joints that had been missed (23%), leaving 126 patients with no tender joints. The PsA-44 and PsA-56 missed tender joints in 18 and 7 patients respectively. When considering swollen joints, 256 patients did not have any swollen joints identified on a 28 joint count but 48 of these (19%) did have swollen joints elsewhere. The PsA-44 and PsA-56 missed swollen joints in 26 and 7 patients respectively.

Conclusion: Patients with oligoarticular PsA cannot be assessed using joint counts borrowed from RA. Ideally full joint counts should be performed to assess PsA patients with the 66/68 joint count being the preferred measure due to difficulties distinguishing between the PIPs and DIPs of the toes. Reduced joint counts designed specifically for PsA do show correlation with treatment change but can still underestimate disease activity.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/reduced-joint-counts-misclassify-psoriatic-arthritis-patients-with-oligoarthritis-and-miss-significant-active-disease/