APPLE Study Protocol: Post-market Validation of the PreemieTest® Device for Neonatal Screening in Diverse Birth Scenarios — A Multicenter, Prospective Cohort Study

Background: Reliable assessment of post-natal gestational age (GA) remains a significant gap in resource-poor settings, where early obstetric ultrasound (US) is often unavailable or unreliable. Because fetal skin maturation follows a sigmoid trajectory that alters optical properties near term, noninvasive skin reflectance can serve as an indicator of GA, which can help in stratifying early respiratory risk. Objective: To report a protocol designed to validate in a real scenario a portable optoelectronic device (PreemieTest®), which estimates GA within 24 to 48 hours after birth, and examines its association with respiratory outcomes at birth, 3 months, and 6 months. Method and analysis: The APPLE Study is a multicenter prospective cohort study in Brazil and Honduras. We will compare the GA estimated by the PreemieTest® device with traditional approaches, such as gold-standard first-trimester US (when available), date of last menstrual period (LMP), and a postnatal clinical score, using correlation, Bland-Altman plots, and concordance analyses. We will be modeling with respiratory outcomes (hospitalizations, clinical diagnoses, and use of ventilatory support) with multivariable logistic regression adjusted for prespecified confounders (birth weight, sex, prematurity, mode of delivery, and US-based GA, considering clustering by location. Usability will be measured with the System Usability Scale among midwives. Data will be captured in Research Electronic Data Capture (REDCap) with standardized quality checks and monitored for completeness. Missing data will be addressed through multiple imputations and sensitivity analyses. Discussion: This study will provide real-world evidence on the accuracy and clinical utility of a portable, low-cost optical approach for GA assessment in diverse birth settings, including remote communities in the Amazon. By clarifying performance across different phototypes and levels of care, and evaluating usability, the APPLE study aims to inform implementation pathways that promote equitable neonatal care in settings were dating uncertainty compromises early clinical decisions. Trial Registration: Registered in the Brazilian Clinical Trials Registry, section of Observational Studies (Brazilian Clinical Trials Registry (ReBEC) — RBR-10ch623r, registered on April 24, 2024; UTN U1111-1302-9740). Ethical approval was obtained prior to the initiation of the study in both Brazil and Honduras.