What is the normal corrected QT (QTc) interval in adults?

Normal QTc Interval Values

The normal corrected QT (QTc) interval is less than 450 ms in adult men and less than 460 ms in adult women, according to the American College of Cardiology and American Heart Association. 1

Gender-Specific Normal Thresholds

• Men: Normal QTc is < 450 ms; values ≥ 450 ms are considered prolonged 2, 1
• Women: Normal QTc is < 460 ms; values ≥ 460 ms are considered prolonged 2, 1
• The gender difference of approximately 8-15 ms emerges after puberty, driven by testosterone-mediated shortening of the QT interval in males during adolescence 1, 3

Age-Related Variations in Gender Differences

The QTc gender gap changes substantially across the lifespan:

• Younger adults (< 40 years): Women's QTc intervals are 12-15 ms longer than men's 3
• Middle-aged adults (40-69 years): The gender difference narrows to 6-10 ms 3
• Older adults (≥ 70 years): The gender difference essentially disappears 3
• Children < 12 years: The 98th percentile limit is approximately 450 ms without gender differentiation 2, 1

Short QT Interval Definition

• A QTc interval ≤ 390 ms is considered a short QT interval in both men and women 1, 3
• Short QT syndrome is defined by more extreme values of ≤ 330 ms (or 360-380 ms for QTc) 1

Alternative Percentile-Based Thresholds

The American Heart Association recommends that QTc values exceeding the 99th percentile should be considered abnormally prolonged:

• Men: 99th percentile threshold is 470 ms 2
• Women: 99th percentile threshold is 480 ms 2

These higher thresholds represent a more conservative approach to defining abnormality compared to the standard 95th percentile cutoffs (450 ms for men, 460 ms for women). 2

Critical Measurement Considerations

The correction formula used matters significantly for accurate interpretation:

• Bazett's formula (QTc = QT/√RR) is most commonly used but systematically overcorrects at heart rates > 80-90 bpm, producing falsely prolonged QTc values 2, 1, 4
• Fridericia's formula (QTc = QT/∛RR) is more accurate, especially at higher heart rates, and is recommended by the FDA and European Society of Cardiology 2, 1, 3
• Linear regression formulas (Framingham, Hodges) effectively remove heart-rate dependence and are superior to both Bazett's and Fridericia's formulas 1, 4
• Research comparing 10,303 normal ECGs found that Bazett's formula produced an upper normal limit of 483 ms, while Fridericia, Framingham, and Hodges formulas all produced limits of 457-460 ms 4

Common Pitfalls to Avoid

• Do not rely on automated QTc values alone: Many ECG machines default to Bazett's formula, which can overestimate QTc at higher heart rates; visual verification and manual measurement are essential 1, 5
• QT correction is unreliable with large RR-interval variability: Do not apply correction formulas in atrial fibrillation or other conditions with significant beat-to-beat variation 1, 3
• Bundle branch blocks require adjustment: In ventricular conduction defects, the QT interval appears falsely prolonged due to delayed depolarization; adjust for QRS duration or use the JT interval (QT - QRS) 2, 1, 3
• Measure in the correct leads: Use leads II, V3, or V5 where the T-wave is most clearly defined, and identify the T-wave end by drawing a tangent to the steepest downslope 1, 3
• Exclude discrete U waves: U waves that appear after the T wave returns to baseline should not be included in the QT measurement 1