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Many hospital EDs currently assess adult patients’ pain using an 11-Point Numeric Rating Scale (NRS-11). With this tool, patients are asked to choose one number, on a scale from 0 (no pain) to 10 (severe pain), corresponding with the intensity of their pain. Advantages of the NRS-11 include ease of administration and scoring, multiple response options, and no age-related difficulties in using the scale.
No prior investigations have addressed similar clinical significance of the NRS-11. We sought to determine the minimum clinically significant difference (MCSD) in physician-assigned NRS-11 score for the assessment of pain in a prospective, descriptive trial.
EM resident and attending physician (EPs) volunteers were enrolled in the study over a 3-month period. All physicians were employed at the investigating institution at the time of enrollment or had recently graduated from the hospital’s EM residency program. Residents in this program do not receive formal training on pain assessment.
The hospital is a tertiary-care center with an annual ED census of 52,000 patient visits. Hospital Institutional Review Board approval was obtained for the study.
Ten written vignettes were ordered using a table of random numbers. Each described a patient presenting to an ED with acute pain, traumatic or nontraumatic in nature. Vignettes were one paragraph in length, and described the patient’s illness or injury, appearance, and brief examination. These vignettes have been used previously by Todd et al. in a study of physician-assigned changes on the VAS.
Written informed consent was obtained and subjects were instructed to read the first vignette. EPs were asked to rate their perception of the patient’s pain intensity using the NRS-11. For each additional vignette, subjects repeated this process and compared the pain intensity of each patient with the pain of the patient in the previous vignette using one of the following descriptors: “a lot more pain,” “a little more pain,” “about the same pain,” “a little less pain,” or “a lot less pain.” Participants were prevented from referring back to their previous scores as well as the prior written vignettes. This process continued for each of the 10 vignettes, resulting in nine comparisons made by each subject. Subjects were informed of the study objective on completion of the study protocol and were encouraged not to discuss the objective with colleagues until completion of the study.
The difference between pain scores and comparison scale descriptor were recorded for each pair of vignettes. We defined the MCSD as the difference in mean pain scores for the pairs rated either “a little more pain” or “a little less pain.” Demographic data on characteristics of the study participants was collected (Table 1). Data was entered into a Microsoft 97 Excel spreadsheet program (Microsoft Corp., Redmond, WA) and was analyzed using NCSS 97 statistical software (NCSS, Kaysville, UT). Analysis used descriptive statistics, including standard deviations (SD) and 95% confidence intervals (CIs).
TABLE 1Demographic Characteristics of Study Participants
Thirty-six EPs performed 324 comparisons. Of these, 176 were rated as “a little less” or “a little more” painful. These 176 comparisons were used to determine the MCSD in pain scores. Table 2 displays the mean ± SD and 95% CIs for each category on the comparison scale.
TABLE 2Mean Differences in Pain Scores by Category With 95% Confidence Intervals (95% CI)
For the 176 comparisons of interest, the mean difference in NRS-11 scores was 1.45, SD ± 0.84 (95% CI, 1.30–1.60). For each vignette, median pain score, interquartile range, and high and low scores are depicted as box plots (Fig 1).
There are several limitations to our study. The investigation was conducted at one institution where the majority of the participants were male, white, and well educated. Subjects rated the pain of vignette patients as moderate to severe. All patients described had acute illnesses or injury as their pain source. As a result, findings described might not be generalized to other physician populations or to patients with mild or chronic pain states.
Additionally, when compared with patients’ self-reporting their pain experience, it has been shown that healthcare providers often describe a patient’s pain intensity as less severe.
It can be argued that comparisons between scenarios lend themselves to value judgments and suppositions about the conditions illustrated, potentially introducing bias.
Our study provides a first estimation of the MCSD in acute pain scores as measured by the NRS-11. With this knowledge, investigators planning studies can more accurately perform power calculations to detect clinically important differences in pain states. Additional research is needed to determine the MCSD in NRS-11 scores self-assigned by ED patients experiencing painful conditions.
Acute pain management, analgesia, and anxiolysis in the adult patient.
in: Tintinalli J.E Kelen G.D Stapczynski J.S Emergency Medicine A Comprehensive Study Guide. 5th ed. McGraw-Hill,
New York2000: 251-280