Background & Aims
The Visual analogue scale (VAS) and Numeric Rating Scale (NRS) are two frequently used rating scales to assess pain intensity in clinical trials and in clinical practice and are considered reliable and valid measures of pain1. Test-retest reliability and measurement error are both measures of the domain reliability2. Knowledge about measurement error is important to differentiate between true change in a score and the systematic and random error of the score. Test-retest reliability and measurement error have been described in patients with other types of neck pain but to our knowledge no previous study has described them in patients with persistent non-specific neck pain. Reliability is a characteristic of both the instrument and the population3 and there is a need to investigate these parameters in specific populations. The aim of the present study was to investigate test-retest reliability and measurement error for VAS and NRS in patients with persistent non-specific neck pain.
Methods
This is a cohort study including patients with non-specific neck pain for more than 3 months who sought rehabilitation in a primary healthcare setting. Data was collected at an ordinary visit to the physiotherapist. Neck pain “right now” was rated on a VAS (0 mm = no pain – 100 mm = worst imaginable pain)4 and a NRS (0 = no pain, 10 = worst imaginable pain)4 at two occasions with a 15-minute wait in between.
Test-retest reliability was calculated as ICC two-mixed model with absolute agreement and expressed as interclass correlation coefficient (ICC). Parameters of measurement error were standard error of measurement (SEM) and Bland Altman limits of agreement (LoA). SEM was calculated using the formula: SEM= SDpooled?1-ICC. LoA was calculated using the Bland Altman Plot where the mean of everyone’s repeated ratings on VAS and NRS is plotted on the x axis against the difference between the scores on the y-axis. LoA indicates the interval within which 95% of the plotted values fall.
Results
Forty-four patients were included in the study (13 men and 31 women). Mean age was 53 (SD15,1) years and mean duration of pain was 98 months. Mean pain intensity ratings with VAS were 38.6 mm at first rating and 46,5 mm after 15 minutes. Mean pain intensity ratings with NRS were 4,3 at first rating and 4,6 after 15 minutes.
Test-retest reliability was very good/excellent for both VAS (ICC=0,938) and NRS (ICC=0,956).
Measurement error expressed as SEM was 6,63 mm for VAS, and 0,53 points for NRS.
The Bland Altman Plot for VAS show an average difference of – 7,98. Limits of Agreement for VAS were -28,592 – 12,638. The average difference for NRS was – 0,30. Limits of Agreement for NRS were -2,259 – 1,1668. When observing the Bland Altman plots it is noticeable that the magnitude of the differences increases after 10 mm VAS and 1 NRS point respectively.
Conclusions
Test-retest reliability for VAS an NRS in patients with persistent non-specific neck pain are very good for both measures. Values of standard error of measurement indicate that patients pain intensity ratings need to change more than 6,63 mm on VAS or 0,53 points on NRS to be a true change and not caused by the difference within the measure itself. The results from the Bland Altman plot indicates that the magnitude of the differences increases after 10 mm VAS and 1 point on NRS respectively. This could be a topic to investigate in future studies.
References
1Williamsson A. Hoggart B. Pain: a review of three commonly used pain rating scales. J Clin Nurs. 2005;14(7):798-804.
2Mokkink LB, Terwee CB, Patrick DL, Alonso J, Stratford PW, Knol DL, et al. The COSMIN study reached international consensus on taxonomy, terminology, and definitions of measurement properties for health-related patient-reported outcomes. J Clin Epidemiol. 2010;63:737-745.
3de Vet HCW, Terwee CB, Mokkink LB, Knol DL. Measurement in Medicine. New York: Cambridge University Press; 2011.
4Hawker GA, Mian S, Kendzerska T, French M. Measures of adult pain. Arthritis Care & Research. 2011;63(S11): S240–S252. DOI 10.1002/acr.20543.