What is a normal QT (QT interval)/QTc (QT corrected interval) interval?

Normal QT/QTc Interval Values

Normal QTc intervals are <450 ms in men and <460 ms in women according to the American College of Cardiology, though some guidelines use slightly different thresholds. 1

Gender-Specific Normal Values

The definition of normal QTc varies slightly depending on the guideline source, but the most widely accepted values are:

• Men: Normal QTc <450 ms 1, 2

  • Some sources suggest <440 ms as the upper limit 3
  • Values ≥450 ms are considered abnormally prolonged 2

• Women: Normal QTc <460 ms 1, 2

  • Values ≥460 ms are considered abnormally prolonged 2

• Gender differences emerge after puberty (ages 12-16 years), with approximately 8-10 ms difference between males and females 1

• Children <12 years: The 98th percentile limit is approximately 450 ms without gender differentiation 1

Short QT Intervals

• QTc ≤390 ms is considered a short QT interval in both men and women 1, 2
• For short QT syndrome specifically, values of 330 ms (310 ms in children) for uncorrected QT and 360-380 ms for QTc have been proposed 3, 1

Critical Thresholds for Risk Stratification

QTc prolongation severity can be graded as follows 1:

• Grade 1: 450-480 ms
• Grade 2: 481-500 ms
• Grade 3: >500 ms

A QTc >500 ms or an increase >60 ms from baseline is associated with increased risk of Torsades de Pointes, a life-threatening ventricular arrhythmia 1, 2

The FDA recommends reporting three severity levels: >350 ms, >480 ms, and >500 ms 1, 2

QT Correction Formula Considerations

The method used to correct QT for heart rate significantly affects the calculated QTc value:

• Bazett's formula (QTc = QT/√RR) is most commonly used but has significant limitations, including overcorrection at heart rates >80 bpm and undercorrection at heart rates <60 bpm 1, 2

• Fridericia's formula (QTc = QT/∛RR) is more accurate, especially at higher heart rates, and is recommended by the FDA 1, 2

• Upper normal limits vary by formula: Using a normal ECG database, Bazett yielded 483 ms, while Fridericia, Framingham, and Hodges all yielded approximately 457-460 ms 4

Measurement Considerations and Common Pitfalls

QT corrections may be inaccurate at heart rates <40 bpm or >120 bpm 3, 1

Key pitfalls to avoid:

• Not accounting for gender differences when interpreting QTc values, particularly in post-pubertal individuals 1

• Failing to recognize that athletes often have longer uncorrected QT intervals due to lower resting heart rates, though their QTc is usually within normal limits 3, 1

• Using Bazett's formula at extreme heart rates, which can lead to misclassification—30% of normal ECGs would be reported as abnormal using a 440 ms threshold with Bazett's formula 4

• Not considering electrolyte abnormalities (particularly hypokalemia, hypomagnesemia, and hypocalcemia) that can affect QTc values 1

• Ignoring that QT interval prolongs in ventricular conduction defects, requiring adjustment for QRS duration 1, 2

Gray Zone Values Requiring Further Evaluation

QTc intervals between 440-500 ms in males and 460-500 ms in females represent a "gray zone" that requires detailed assessment to achieve a definitive diagnosis 3

This assessment should include:

• Careful family and personal history for syncope or sudden death 3
• Cascade ECG screening of family members 3
• 24-hour ECG monitoring for serial QTc evaluation 3
• Exercise testing to assess QT interval shortening during effort 3
• Evaluation for acquired causes including QT-prolonging drugs, electrolyte disorders, and metabolic changes 3

A QTc value of 500 ms, otherwise unexplained, is indicative of unequivocal long QT syndrome, regardless of family history and symptoms 3