A growing body of evidence suggests that real-time electronic assessments of pain are preferable to traditional paper-and-pencil measures. We used electronic assessment data derived from a study of patients with fibromyalgia (FM) to examine variability of pain over time and to investigate the implications of pain fluctuation in the context of a clinical trial.


The study group comprised 125 patients with FM who were enrolled in a randomized, placebo-controlled trial of milnacipran. Pain intensity levels were captured in real time by participants using electronic diaries. Variability in pain was assessed as the standard deviation of pain entries over time (pain variability index [PVI]).


Substantial between-subject differences in pain variability were observed (mean ± SD PVI 1.61 ± 0.656 [range 0.27–4.05]). The fluctuation in pain report was constant over time within individuals (r = 0.664, P < 0.001). Individuals with greater variability were more likely to be classified as responders in a drug trial (odds ratio 6.14, P = 0.006); however, this association was primarily attributable to a greater change in pain scores in individuals receiving placebo (r = 0.460, P = 0.02) rather than active drug (r = 0.09, P > 0.10).


Among individuals with FM, there were large between-subject differences in real-time pain reports. Pain variability was relatively constant over time within individuals. Perhaps the most important finding is that individuals with larger pain fluctuations were more likely to respond to placebo. It is not clear whether these findings are applicable only to patients with FM or whether they may also be seen in patients with other chronic pain conditions.