Background/Purpose:

Participation, including work disability, is an important patient centred outcome and is in the middle ring of the Outcome Measures in Rheumatology (OMERACT) psoriatic arthritis core set of domains. The Work Productivity and Activity Index Specific Health Problem (WPAI-SHP) is a feasible, patient reported measure of work disability but there is little data on the validity of the WPAI-SHP in PsA. We set out to determine the validity of the WPAI-SHP and the minimally important difference (MID) for improvement in PsA.

Methods:

LOPAS II is a UK multicentre observational study of work disability in PsA. Working patients were included for analysis. Construct validity was determined by correlation with other clinical and patient reported outcomes using spearman correlation coefficients. Responsiveness was calculated using the standard error of the mean (SEM). The MID was determined by the health based anchor method (mean change in score amongst patients who improved) and the receiver operating characteristic curve (ROC) method.

Results:

Analyses were undertaken on 177 of 229(77.3%) working participants with complete data at baseline and three months follow up. The mean age was 48 years and median disease duration 6.0 years (IQR 2.0 – 12.0). The mean change of presenteeism was -7.9 (sd30.0), SEM 16.2 and SRM -0.26. Correlations with other outcomes are reported in table 1. Amongst 105 (59.3%) patients who reported no change between baseline and follow up the mean change in WPAI was; absenteeism 2.9 (sd30.7), presenteeism 4 (26.5), productivity loss 7.7 (27.2), activity impairment 1.9 (24.4). The MID for improvement amongst 72 (40.7) patients who reported improvement was; absenteeism -6.1 (26.2), presenteeism -20.8 (28.2), productivity loss -21.7 (30.8) and activity impairment -27.2 (29.5). The MID using the ROC method for presenteeism, productivity loss and activity impairment was -5.0 (AUC 0.75), -11.3 (AUC 0.76) and -25.0 (AUC 0.79) respectively (Figure 1).

Conclusion:

We report data on the validity of the WPAI-SHP in PsA and an estimate of the MID for improvement. Presenteeism and productivity loss are moderately correlated with clinical measures. Responsiveness was small to moderate amongst the group as a whole.