Inter-Rater Variability in Nocifensive Behaviour Testing of Rodents

Chronic pain is a critical health problem with a need for new effective treatments. Nocifensive behaviour testing is used to evaluate animal responses to noxious or painful stimuli and is important in basic pain research for understanding pain mechanisms and developing new treatments. A major problem with this technique, however, is that there can be large differences in baseline measurements even when the same sex and strain of rodent is used. We posit that these differences may be due to variation between testers. This project explored factors that contribute to potential inter-rater variability observed in nocifensive behaviour. Adult Sprague Dawley rats underwent evoked somatosensory testing using von Frey filaments and the simplified up-down (SUDO) approach with two different testers, while other environmental conditions such as location, testing time, and acclimation periods were carefully controlled for. The paw withdrawal threshold between testers was different with baseline scores ranging from 3 g to 18.5 g for individual rats. These results suggest that most observed variation in behavioural outcomes is due to tester differences relative to any other factors. Tester differences that could contribute to this variation include sex, genetics, or odour. These results highlight the importance of maintaining one tester for a series of experiments. We conclude that inter-rater variability is an important factor that must be considered when conducting von Frey nocifensive behaviour testing.