Across- and within-session variability of ratings of painful contact heat stimuli

This study examined within- and across-session consistency of visual analog scale (VAS) pain intensity and unpleasantness ratings of contact heat stimuli in 64 subjects (32 male). Subjects participated in four sessions over 14 days, with three stimulus series per session. Two levels of painful heat (pain-lo: rated 40, and pain-hi: rated 70 on a 0–100 VAS) were delivered in randomized order during each series, with temperatures selected on an individual subject basis to equalize pain perception across subjects. Across-session ratings declined by the fourth session for both pain levels (p = 0.01). Within-session ratings declined by the third series for both pain levels (p < 0.001). While significant, changes in across- and within-session ratings were of small magnitude. Comparison of coefficients of variation (CVs) for across- and within-session ratings revealed that pain-lo ratings were more variable than pain-hi ratings (p < 0.001). Across- and within-session CVs were highly correlated for each pain level (pain-lo p < 0.001; pain-hi p = 0.001), suggesting that variability of VAS ratings is a characteristic of individual subjects over both short and long time scales. Across- and within-session CVs were significantly negatively correlated with individual ratings of the stimuli, but were not correlated with demographic or psychosocial factors. Furthermore, sex did not impact consistency of ratings, demonstrating that neither sex is more variable in ratings than the other over time. Taken together, these findings suggest that VAS ratings of painful contact heat are relatively stable over time but the variability of these ratings is significantly impacted by the perceived intensity of the stimulus.