Painful differences between different pain scale assessments: The outcome of assessed pain is a matter of the choices of scale and statistics.

OBJECTIVES: Perceived pain is a multi-factorial subjective variable, commonly measured by numeric rating scales, verbal descriptive scales (VDS), or by a position on an analogue line (VAS). A major question is whether an individual's VAS and VDS pain assessments, on the same occasion, could be comparable. The aim was to compare continuous and discretized VAS pain data with verbal descriptive pain datasets from the Oswestry Disability Index (ODI) and the European Quality of Life Scale (EQ-5D) in paired pain datasets.

METHODS: The measurement level of data from any type of scale assessments is ordinal, having rank-invariant properties only. Non-parametric statistical methods were used. Two ways of discretizing the VAS-line to VAS-intervals to fit the number of the comparing VDS-categories were used: the commonly used (equidistant VAS,VDS)-pairs and the (unbiased VAS,VDS)-pairs of pain data. The comparability of the (VAS,VDS)-pairs of data of perceived pain was studied by the bivariate ranking approach. Hence, each pair will be regarded as ordered, disordered, or tied with respect to the other pairs of data. The percentage agreement, PA, the measures of disorder, D , and of order consistency, MA, were calculated. Total interchangeability requires PA = 1 and MA = 1.

RESULTS: The wide range of overlapping of (VAS,VDS)-pairs indicated that the continuous VAS data were not comparable to any of the VDS pain datasets. The percentage of agreement, PA; in the (equidistant VAS,ODI) and (equidistant VAS, EQ-5D) pairs were 38 and 49%, and the order consistency, MA, was 0.70 and 0.80, respectively. Corresponding results for the (unbiased VAS,VDS)-pairs of pain data were PA: 54 and 100%, and MA: 0.77 and 1.0.

CONCLUSION: Our results confirmed that perceived pain is the individual's subjective experience, and possible scale-interchangeability is only study-specific. The pain experience is not possible to be measured univocally, but is possible for the individual to rate on a scale.