Exploring the outcome "days with bothersome pain" and its association with pain intensity, disability, and quality of life.

Objectives: The aim of this study is to investigate the criterion validity, specifically the concurrent validity of the measure of days with bothersome pain, by investigating its association with measures of pain intensity, disability, and quality of life.

Methods: We used two separate cohorts to study the concurrent validity of "days with bothersome pain," by investigating its association with pain intensity, disability, and quality of life: (1) 321 patients with low back pain (LBP), using follow-up data at 1 year, and (2) 170 pregnant women with pelvic girdle pain (PGP), using data at 12 and 30 weeks of their pregnancy. In both studies, weekly text messages asked for the number of days with bothersome pain the previous week. Pearson's correlation, univariable, and multivariable linear regression were used to assess the association between days with bothersome pain per week and pain intensity, disability, and quality of life. Non-linear associations were explored.

Results: Days with bothersome pain were moderately and inversely correlated with quality of life (r = -0.45, p < 0.001), and moderately correlated with pain intensity (r = 0.70, p < 0.001) and disability ( r = 0.51, p < 0.001), among patients with LBP, and to a lesser degree among pregnant women with PGP (corresponding values at 18 weeks of pregnancy for quality of life; r = -0.27, p = 0.005, for pain intensity r = 0.41, p < 0.001, and for disability r = 0.41, p < 0.001). Furthermore, it was best explained by pain intensity for LBP patients, and by pain intensity and disability for pregnant women with pelvic pain. For the latter cohort, non-linear analyses suggested that days with bothersome pain could not distinguish between individuals with different high pain intensities and disabilities and low quality of life.

Conclusions: We consider the concurrent validity of "days with bothersome pain" to be moderate in the correlations with pain intensity, disability, and quality of life in patients with LBP and in pregnant women with PGP. Ceiling effects may be an issue in populations with high pain severity. The differences between the cohorts suggested that also other constructs are involved in the concept of days with bothersome pain.