However, the 12-lead ECG remains the universal initial diagnostic test in the evaluation of ... and clarifies current recommendations regarding diagnosis of so-called “borderline” QT interval ...

A prolonged QT interval has a variety of causes and can be a very serious ECG finding, as it may lead to the potentially fatal arrhythmia polymorphic ventricular tachycardia (Torsades de Pointes).

Lower heart rates also result in a longer QT interval. A quick way to distinguish a prolonged QT interval is to examine if the T wave ends beyond the halfway point between the RR interval.