Understanding the Feline Grimace Scale: A study of dimensional structure, importance of each action unit and variables affecting assessment

The Feline Grimace Scale (FGS) is a facial expression-based scoring system for acute pain assessment in cats. This study aimed to investigate the dimensionality, importance of each action unit (AU), and variables affecting pain assessment using the FGS. One hundred images of cat faces were scored using the FGS by five veterinarians and five veterinary students. Cats were classified as painful or pain-free, whether the cut-off for analgesia was reached during real-time assessment. Scale dimensionality was studied using principal component analysis and Horn's parallel analysis. Item-total correlation investigated correlations between each AU and total FGS scores. Linear mixed models assessed responsiveness for each AU and variables influencing scores (age, gender, pain, and group). AUs had loading values ≥ 0.6, demonstrating an association for each AU with the first principal component of the PCA. All AUs and the FGS total ratio scores were increased in painful versus pain-free cats (p < 0.001). Female raters gave higher FGS scores than male raters (p = 0.02). Muzzle tension was the only AU with sensitivity below 70 %, whereas whiskers change was the only AU with specificity below 70 %. Similarly, whiskers change and muzzle tension had the lowest area under the curve values and Youden index. The FGS is a unidimensional scale, with total scores influenced by the rater's gender and pain. FGS demonstrated strong consistency and a high correlation between the AUs and total scores. However, muzzle tension and whiskers change are less discriminative than other AUs.