QTc Interval Normal Values and Clinical Interpretation

Normal Reference Ranges

The American Heart Association defines a normal corrected QT interval (QTc) as < 450 ms in men and < 460 ms in women; a QTc ≤ 390 ms is considered a short QT in both sexes. 1
In children younger than 12 years, the 98th‑percentile QTc is approximately 450 ms, with no gender differentiation. 1

A physiologic testosterone‑mediated shortening makes male QTc about 10 ms shorter than female QTc during adolescence; this gap narrows to 6–10 ms in middle‑aged adults and disappears after age 70. 1
In adolescents (12–16 years), a gender difference of roughly 8 ms emerges. 1
In adults aged ≥ 70 years, gender‑related QTc differences are essentially negligible. 1

A QTc > 500 ms markedly increases the risk of torsades de pointes; offending medications should be discontinued immediately. 1
An increase of > 60 ms from a patient’s baseline QTc is clinically significant regardless of the absolute value and warrants prompt evaluation. 1
The U.S. Food and Drug Administration recommends reporting QT‑prolonging drug effects at three severity levels: > 350 ms, > 480 ms, and > 500 ms. 1

Fridericia’s formula (QTc = QT / RR³ᐟ³) is the preferred correction method, especially when heart rate exceeds 80 bpm. 1
Bazett’s formula (QTc = QT / √RR) systematically over‑corrects at heart rates > 80–90 bpm and under‑corrects at rates < 60 bpm, leading to falsely elevated QTc values. 1
QT measurement should be performed from the onset of the QRS complex to the end of the T wave in leads II, V3, or V5, where the T wave is most distinct. 1
QT correction is unreliable in the presence of marked RR‑interval variability (e.g., atrial fibrillation). 1
In bundle‑branch block, adjust the QTc for the prolonged QRS duration or use the JT interval (QT minus QRS) to avoid overestimation. 1