Characteristics of electronic visual analogue and numerical scales for ratings of experimental pain in healthy subjects and fibromyalgia patients.

Comparisons of measurement characteristics were made for three types of electronic pain scales: (a) visual analogue scale (VAS), (b) VAS combined with an electronic number box (VAS-N; 0-100), and (c) electronic number box scale (NUM). The three scales were capable of discriminating pain sensations from very small (0.5 degrees C) temperature steps in 13 healthy males, 26 healthy females, and 16 female fibromyalgia (FM) patients. All scales provided monotonic functions when used by subjects to rate pain from 5s nociceptive temperatures (45-49 degrees C), thereby demonstrating the generality of these results across different demographic groups. As expected, FM patients rated heat pain sensations higher on all scales in comparison to healthy females, demonstrating the capacity of these scales to detect well-established group differences in pain sensitivity that exist across these two groups. However, in comparison to male subjects, healthy females gave higher NUM but not VAS or VAS-N ratings to the range of nociceptive presented temperatures. We interpret this difference as a selective scaling bias of female subjects for NUM. Finally, all three groups (total of 55 subjects) found the scales easy to use after brief instructions, though subjects strongly preferred the use of VAS-N or VAS in comparison to NUM scale.