To assess the reliability of the KidZ Health Castle formula (KHC-F) to determine the correct probe position of a multichannel intraluminal impedance pH.

A retrospective cohort study was performed on 222 children between 1 month and 18 years of age undergoing multichannel intraluminal impedance pH. The primary outcome was the comparison of the pH sensor location determined by the KHC-F with the radiological target position. The margin of error was defined as 1 cm from the target position. Performance of the KHC-F and existing formulas was determined via the percentage with a correct position, mean error, 95% limits of agreement (Bland-Altman plots), and Spearman correlation. A post hoc analysis was performed with an updated KHC-F v2, subtracting −0.5 cm from the KHC-F.

Positioning with KHC-F was correct in two-thirds of the participants, with a very strong correlation (ρ = 0.91) with the target position. Bland-Altman plots showed good agreement between KHC-F and target position (mean error of −0.44 cm, lower limit −3.2 cm, upper limit 2.3 cm). A post hoc analysis with the KHC-F v2 showed a correct positioning in 74% of patients. Comparison with other formulas showed a stronger performance of KHC-F and KHC-F v2 on correct positioning, mean error, and 95% limits of agreement.

The KHC-F leads to reliable results. KHC-F v2 outperforms all other existing formulas in children, thereby reducing the need for repositioning and the amount of x-ray exposure. The age distribution of the sample may be a limitation, as well as the retrospective nature of the study.