Normal QTc Interval Values
The normal QTc interval is less than 450 ms in adult men and less than 460 ms in adult women, with values of 390 ms or shorter considered short QT in both sexes. 1, 2
Gender-Specific Normal Limits
- Adult men: Normal QTc < 450 ms 1, 2, 3
- Adult women: Normal QTc < 460 ms 1, 2, 3
- Short QT interval: ≤ 390 ms for both men and women 1, 2, 3
These thresholds represent the practical clinical limits recommended by the American Heart Association and American College of Cardiology Foundation for distinguishing normal from abnormal QT intervals. 1
Age-Related Considerations
The gender difference in QTc values varies significantly with age:
- Younger adults (under 40 years): The gender difference is most pronounced at 12-15 ms, with women having longer QTc intervals 1, 4
- Middle-aged adults (40-69 years): The gender difference decreases to 6-10 ms 1
- Older adults (over 70 years): The gender difference practically disappears 1
The QTc shortens in adolescent males due to testosterone effects, while females show little change during this period, which explains why the gender difference emerges during adolescence. 1, 4
Clinical Thresholds for Prolonged QT
Beyond the basic normal limits, several severity thresholds exist for clinical decision-making:
- Prolonged QT: ≥ 450 ms in men, ≥ 460 ms in adult women 1, 2, 3
- Moderate prolongation: > 480 ms (FDA Level 2) 1, 3
- Severe prolongation: > 500 ms (FDA Level 3, highest risk for torsades de pointes) 1, 3
- Change from baseline: An increase of > 60 ms from baseline is clinically significant 3
The FDA recommends reporting these three severity levels when evaluating QT-prolonging effects of drugs. 1
Important Measurement Caveats
Several factors can affect accurate QTc interpretation:
- Correction formula matters: Linear regression functions or Fridericia's formula are more accurate than Bazett's formula, which tends to overcorrect at heart rates above 80-90 bpm and may produce false QT prolongations 1, 2, 3
- Heart rate variability: QT correction should not be attempted when RR interval variability is large, such as in atrial fibrillation 1, 2
- Bundle branch blocks: QT interval prolongs in ventricular conduction defects and requires adjustment for QRS duration, or use of the JT interval (QT minus QRS duration) 1, 2
- Lead selection: Choose a lead with T-wave amplitude of at least 2 mm and well-defined T-wave end for accurate measurement 2
Pediatric Normal Values
In children younger than 12 years, the 98th percentile limit is approximately 450 ms without gender differentiation. 1, 3 The gender difference of approximately 8 ms appears in the 12-16 year age group. 1