Background <p>The AAP recommends that infants born &lt;37 weeks gestation or with risk factors for positional cardiorespiratory instability undergo a car seat tolerance screen (CSTS) before discharge. Understanding characteristics of CSTS failure may inform screening practices.</p> Objective <p>Compare clinical, oxygen saturation, and heart rate features in infants who failed vs. passed CSTS.</p> Methods <p>We retrospectively reviewed CSTS data from a single level IV NICU (2018–2024). Clinical characteristics of infants who failed vs. passed their first CSTS were compared. For a subset with SpO₂ and HR data, we analyzed hypoxemia and bradycardia events the day before and during CSTS.</p> Results <p>We analyzed 2861 CSTSs from 2523 infants. First CSTS failure occurred in 111 infants (4.4%), most commonly due to oxygen desaturation in 85%. Of 111 infants who failed, 77% passed a second screen a mean of 1.9 days later, and overall length of stay was not significantly different based on CSTS result. Among 1136 infants with physiologic data, those who failed had more desaturation events the day before CSTS, and deeper and longer desaturations during the CSTS.</p> Conclusions <p>CSTS failure was associated with more oxygen desaturations the day prior. Most infants passed a subsequent screen within a short time.</p> Impact <p><UnorderedList Mark="Bullet"> <ItemContent> <p>This is the largest analysis of NICU car seat tolerance screens (CSTS) using continuous physiologic data.</p> </ItemContent> <ItemContent> <p>Hypoxemia and bradycardia events occurred in some CSTS designated as “passed” but events were deeper and longer in screens designated as “failed”.</p> </ItemContent> <ItemContent> <p>Infants that failed their first CSTS had lower mean SpO₂ and more desaturation events in the 24 hours before the screen.</p> </ItemContent> </UnorderedList></p>