Definition of Prolonged QTc Interval
A prolonged QTc interval is defined as ≥450 ms in men and ≥460 ms in women, with QTc >500 ms considered highly abnormal and associated with significantly increased risk of life-threatening arrhythmias. 1
Gender-Specific Thresholds
The American Heart Association and American College of Cardiology Foundation establish clear cutoffs based on sex:
- Men: QTc ≥450 ms is prolonged 1, 2
- Women: QTc ≥460 ms is prolonged 1, 2
- Both sexes: QTc >500 ms is highly abnormal and warrants immediate attention 1
These thresholds represent practical clinical limits derived from large community-based population studies using appropriate linear regression functions for rate adjustment. 1
Alternative Definitions Based on Percentiles
The American Heart Association recommends that QTc values exceeding the 99th percentile should be considered abnormally prolonged:
This percentile-based approach is preferred over mean ± 2SD because QT distributions are strongly skewed. 1 However, the more conservative thresholds of 450/460 ms remain the standard clinical definition for identifying prolonged QTc. 1
FDA Severity Classification
For drug safety monitoring, the FDA recommends reporting three severity levels:
Pediatric Considerations
In children younger than 12 years, the 98th percentile limit is approximately 450 ms without gender differentiation. 1 The gender difference of approximately 8 ms emerges in the 12-16 year age group. 1
Important Caveats
Correction formula matters significantly: Bazett's formula (most commonly used) tends to overcorrect at heart rates >85 bpm, producing falsely prolonged QTc values in hospitalized patients who often have tachycardia. 1 Linear regression functions or Fridericia's formula provide more accurate rate correction, particularly at faster heart rates. 1
Bundle branch blocks require adjustment: When QRS duration is prolonged (e.g., new bundle branch block), the QT interval increases due to delayed depolarization, not repolarization abnormality. 1 Subtract the difference in QRS widths before and after the block, or measure the JT interval (QT minus QRS duration) instead. 1
Beware of false positives: Many automated ECG algorithms label QTc >440 ms as "borderline prolonged," despite this value being exceeded by 10-20% of the normal population. 1 This is not the accepted clinical definition of prolonged QTc.