Normal QTc Interval Values
The normal corrected QT interval (QTc) is <450 ms in adult men and <460 ms in adult women, with values ≤390 ms considered short QT in both sexes. 1, 2, 3
Gender-Specific Normal Limits
Adult men:
Adult women:
Both sexes:
These thresholds represent the upper limits of normal established by the American Heart Association and American College of Cardiology Foundation, based on large community-based population studies using appropriate linear regression correction formulas. 1
Age-Related Gender Differences
The gender difference in QTc values is not constant across the lifespan and follows a predictable pattern:
- Younger adults (<40 years): Women's QTc is 12-15 ms longer than men's 1, 2, 4
- Middle-aged adults (40-69 years): Gender gap narrows to 6-10 ms 1, 2
- Older adults (≥70 years): Gender difference becomes negligible and practically disappears 1, 2
The gender difference emerges during adolescence when testosterone shortens the rate-adjusted QT in boys while having minimal effect in girls. 1, 4 This physiologic difference is clinically important because women are generally more prone to malignant arrhythmias in long QT syndrome than men. 1
Clinical Risk Thresholds
Beyond the basic normal/abnormal cutoffs, specific QTc values carry different levels of arrhythmic risk:
Moderate prolongation:
Severe prolongation (highest torsades de pointes risk):
Torsades de pointes rarely occurs when QTc is <500 ms, making this threshold particularly important for clinical decision-making. 1 The FDA recommends reporting three severity levels when evaluating QT-prolonging effects: >450 ms (or >460 ms for women), >480 ms, and >500 ms. 1, 2
Critical Measurement Considerations
Correction formula selection matters significantly:
Bazett's formula (QTc = QT/√RR) is most commonly used but has substantial limitations—it overcorrects at heart rates >80 bpm and undercorrects at rates <60 bpm, potentially generating false-positive prolongations. 1, 2, 3
Linear regression functions (e.g., Framingham, Hodges) or Fridericia's formula (QTc = QT/∛RR) are superior because they effectively remove heart-rate dependence and are more accurate, especially at higher heart rates. 1, 2, 3
The American Heart Association and American College of Cardiology Foundation explicitly recommend that linear regression functions rather than Bazett's formula be used for QT-rate correction, and that the method used should be identified in ECG analysis reports. 1
Common Pitfalls to Avoid
Do not attempt QT correction when:
- RR interval variability is large (e.g., atrial fibrillation) 1, 2, 3
- The end of the T wave cannot be reliably identified 1
Bundle-branch block requires special handling:
- QT intervals must be adjusted for prolonged QRS duration, or the JT interval (QT minus QRS) should be used instead, otherwise the QT appears falsely prolonged. 2, 3
Automated measurements require verification:
- Automated QTc values should not be used alone because many devices default to Bazett's formula, which can overestimate QTc at higher heart rates. 2, 3
- Visual verification and manual measurement are essential, particularly when the ECG is abnormal. 2
Serial monitoring requires standardization:
- Time of day influences QT measurements, so serial ECGs should be performed under identical recording conditions with the same equipment to ensure reliable comparisons. 2, 3
Pediatric Considerations
For children, the normal limits differ:
- Children <12 years: 98th percentile limit is approximately 450 ms with no gender differentiation 1, 3
- Adolescents 12-16 years: Gender difference of approximately 8 ms appears in this age group 1, 3
Note that these pediatric limits are based on Bazett's formula, which may produce false QT prolongations, so interpretation requires caution. 1