Reliability, Concurrent Validity, and Minimal Detectable Change of a Smartphone Application for Measuring Thoracic Kyphosis

Objective: To assess the intra - and inter-rater reliability and concurrent validity, and to estimate minimal detectable change of a smartphone application for measuring thoracic kyphosis angle. Methods: A total of 80 healthy university students were evaluated. Two raters measured the thoracic kyphosis angle using a digital inclinometer and the smartphone application. Intra - and inter-rater reliability were assessed using the intraclass correlation coefficient (ICC) with 95% confidence interval. The standard error of measurement (SEM) and the minimal detectable change at the 95% confidence level (MDC95) were also calculated. The concurrent validity between the digital inclinometer and the smartphone application was assessed by the linear regression analysis and Bland and Altman's 95% limits of agreement method. Results: The intra - and inter-rater reliability were excellent for the digital inclinometer and the smartphone application (ICC > 0.75). The SEM values for the digital inclinometer and the smartphone application were close together. The MDC95 values for the smartphone application were 5.11 and 6.30 degrees, and 9.02 degrees for intra - and inter-rater, respectively. The digital inclinometer and the smartphone application showed a positive correlation (R-2 = 0.85). The Bland-Altman plot showed a good agreement between the instruments. Conclusion: The smartphone application used in this study is a cost-effective, practical, reliable, and valid instrument for measuring the thoracic kyphosis angle. More than 9 degrees in the value of the thoracic kyphosis angle measured by the smartphone application can be considered as a true change.