The definition of daytime and nighttime influences the interpretation of ABPM in children.

To test if an arbitrary definition of day and night periods that differs to patient-reported awake and sleep periods leads to inaccuracies in interpretation of ambulatory blood pressure monitoring (ABPM). A single-center, retrospective review was performed comparing three different methods to classify day and night periods following a single 24-h ABPM: method A: Patient's record during monitoring; method B: 7:00 am to 11:00 pm day and 11:00 pm to 7:00 am night; method C: 8:00 am to 8:00 pm day and midnight to 6:00 am night. We included 149 studies in 149 children with a mean ± SD age of 13.0 ± 3.4 years. Reported sleep duration was 9.2  ± 1.3 h. Significant differences resulted between three methods for the means of several ambulatory BP parameters including indexed BP values during day, BP load, and nocturnal dipping status. During monitoring [median (range)], 7.5% (0.0-27.5%) readings were misclassified using method B and 0.0% (0.0-20.0%) using method C (p < 0.0001). This misclassification resulted in change of hypertension status for 11.4% (17/149) patients using method B and 9.4% (14/149) patients using method C (p = 0.70). Misclassification of measurements during ABPM can introduce significant errors in its interpretation. The clinical impact of these findings needs further evaluation in larger prospective studies.