Background: Clinical diagnosis of Paediatric flexible flatfoot is still a challenging issue for the healthcare professionals. Clarke's Angle (CA) is frequently used clinically for assessing foot posture; however there is still a debate about its validity and diagnostic accuracy in evaluation of static foot posture especially in Paediatric population with some previous studies supporting, while the others are refuting its validity. The current study aimed to investigate validity and diagnostic accuracy of CA using radiographic findings as a criterion standard measure to determine flexible flatfoot between ages of 6-18 years. Methods: A cross-sectional study conducted on 612 participants (1224 feet) with flexible flatfoot aged 6-18 years (12.36 {plus minus}3.39) was recruited. The clinical measures results were compared to the criterion standard radiographic measures and displayed on the receiver operating characteristic curve (ROC) and the area under ROC curve (AUC) was computed. Intra-rater reliability, sensitivity, specificity, predictive values, and likelihood ratios were calculated for CA. Fagan nomogram was used to detect the posttest probability. Results: Clarke's angle (CA) demonstrated higher intra-rater reliability (ICC=0.99). CA showed higher sensitivity (98.4%), specificity (98.8), positive predictive value (97.3), negative predictive value (99.3), positive likelihood ratio (84) and negative likelihood ratio (0.02). AUC was 0.98[95%CI= 0095-1.00]. The positive likelihood ratio (LR+) yielded a posttest probability of 97%, while the negative likelihood ratio (LR−) yielded a posttest probability of 0.02. Conclusions: CA is a valid with high diagnostic accuracy in diagnosis of flexible flatfoot between the ages of 6-18 years.

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