High correlation but inadequate point-to-point agreement, between conventional mechanical and electronical visual analogue scale for assessment of acute postoperative pain after general surgery

SummaryBackgroundAlthough visual analogue scales in conventional mechanical form (mVAS) and electronical VAS (eVAS) on PDA devices are used to evaluate pain in surgical patients, it is not clear whether results of both instruments are comparable. Therefore, the level of agreement between both VAS instruments was evaluated in patients undergoing general surgery.MethodsAfter general surgery, 53 patients assessed their pain at rest and while coughing by mVAS and eVAS at the same time. mVAS and eVAS data were correlated and the level of agreement between both was calculated according to the method of Bland and Altman. Thirty-three female and 20 male patients in the median age of 51 (18–78) years participated in this study and a total of 185 data sets (112 at rest, 73 while coughing) were evaluated.ResultsWhen data at rest and while coughing were analysed together, correlation between mVAS and eVAS was very good (r = 0.902, p < 0.001). While the mean VAS score was 4.5 cm, mVAS and eVAS differed from each other by only 0.22 cm (4.4% of mean VAS score). However, standard deviation of the difference (“repeatability coefficient”) was 1 cm (22% of the mean VAS) and the lower and upper level of agreement ranged from −0.21 to 0.17 cm (−47 to 38%). Separate analysis for mVAS and eVAS during rest and while coughing revealed similar results.ConclusionAfter elective general surgery, correlation between mVAS and eVAS was high. While measurements with both instruments differ in average only by less than 5%, those differences may vary considerably in individual measurements. The type of VAS (mVAS or eVAS) should not be changed within one trial. Results obtained with different forms of VAS should be compared only with caution.