How reliable is pain as the fifth vital sign?

Although many health care organizations require routine pain screening (eg, "5th vital sign") with the 0 to 10 numeric rating scale (NRS), its accuracy has been questioned; here we evaluated its accuracy and potential causes for error.

We randomly surveyed veterans and reviewed their charts after outpatient encounters at 2 hospitals and 6 affiliated community sites. Using correlation and receiver operating characteristic analysis, we compared the routinely measured "5th vital sign" (nurse-recorded NRS) with a research-administered NRS (research-recorded NRS) and the Brief Pain Inventory (BPI).

During 528 encounters, nurse-recorded NRS and research-recorded NRS correlated moderately (r = 0.627), as did nurse-recorded NRS and BPI severity scales (r = 0.613 for pain during the last 24 hours and r = 0.588 for pain during the past week). Correlation with BPI interference was lower (r = 0.409). However, the research-recorded NRS correlated substantially with the BPI severity during the past 24 hours (r = 0.870) and BPI severity during the last week (r = 0.840). Receiver operating characteristic analysis showed similar results. Of the 98% of cases where a numeric score was recorded, 51% of patients reported their pain was rated qualitatively, rather than with a 0 to 10 scale, a practice associated with pain underestimation (chi2 = 64.04, P < .001).

Though moderately accurate, the outpatient "5th vital sign" is less accurate than under ideal circumstances. Personalizing assessment is a common clinical practice but may affect the performance of research tools such as the NRS adopted for routine use.