Background/Purpose

The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) is a validated and widely used instrument for assessing osteoarthritis (OA) knee and hip pain, stiffness and physical function.  The recall period for specific outcome measures is 48 hours.  This study aimed to examine the correlation between WOMAC pain score and reported actual pain levels one, two and 5 days prior to completing the WOMAC instrument.

Methods

Data were obtained from three OA studies:  an observational study (OBS) convenience sampling of OA and the placebo arm of two interventional randomized control trials (IRCTs).  All participants were ≥ 40 years of age, met the ACR definition for OA, and self-reported a minimum pain intensity of ≥ 4 on Numeric Rating Scale (NRS) for pain (0 – no pain; 10 – worst pain possible).  Participants were asked to report pain twice daily (IRCTs, electronic tablet) or three times daily (OBS, smartphone) over a 12-16 week period, and WOMAC was completed at regular clinic visits.  For each participant, mean pain ratings were calculated for the day of clinical exam, as well as 1 day, 2 days, and 5 days prior and compared to the WOMAC pain score for question 1.  Pearson’s correlation was computed to assess the relationship between WOMAC pain score and self-reported pain using Stata 13.

Results

In total, data were collected from 162 participants (123 studied in clinical trials and 39 studied in the observational study).  The demographics for all three groups were similar and typical of adults with OA.  Among clinical trial participants, there was a strong, positive correlation between WOMAC and self-reported pain for all time points [r=0.8505, n=392 (exam day); r=0.8482, n=474 (1 day prior); r=0.8575, n=415 (2 day prior); r=0.8173, n=267 (5 day prior); all values p < 0.05] (Figure 1).  Among observation study participants, there was a moderate, positive correlation between WOMAC and self-reported pain significant only at 1 day prior to exam [r=0.3741, n=56] (Figure 2). 

Conclusion

The findings of this study demonstrate a high correlation within individuals between WOMAC pain scores and self-reported daily preceding pain intensity in ICRT.  This correlation was independent of the look-back period (one, two or five days).  Less strong correlations were observed in OBS study settings.  These findings are consistent with lower daily pain variability in interventional trials than in the observational setting and have implications on assessing and managing pain in non-IRCT settings.