LAUSR

BACKGROUND Long QT syndrome (LQTS) is an autosomal dominant disorder characterized by a prolonged QT interval. ECG screening in the first 48 hours of life may be misleading, even in newborns with a genotype-positive LQTS parent. OBJECTIVE To determine the ECG's diagnostic accuracy in the first 48 hours of life for neonates born to a parent with LQTS. METHODS Retrospective review of all neonates born at Mayo Clinic to a parent with ≥1 pathogenic variant in a LQTS-causative gene who had least 1 ECG in the first 48 hours and genetic test results were available. Sensitivity and specificity of the diagnostic ECG were calculated using QTc thresholds of 440, 450, 460, and 470 ms. RESULTS Overall, 74 infants (36 [49%] females) were included (mean QTc on first ECG 489 ± 54 ms; 68% LQTS genotype-positive). Mean QTc in the first 48 hours for neonates that ultimately were genotype-positive was greater (506 ± 52 ms) compared to genotype-negative neonates (455 ± 41 ms; p=0.0004). When using a recommended threshold QTc of ≥ 440 ms, 6/50 (12%) genotype-positive neonates were missed (underdiagnosed) and 17/24 (71%) genotype-negative neonates were overdiagnosed (sensitivity: 88%, specificity: 29%). CONCLUSIONS The newborn ECG should not be used in isolation to make the diagnosis of LQTS since it will result in many misclassifications. Genetic testing must be initiated prior to discharge, and proper anticipatory guidance is vital while awaiting test results.