Normal QTc Interval for Women

The normal QTc interval for women is less than 460 ms, with values ≥460 ms considered prolonged. 1, 2

Gender-Specific Normal Values

Women have a longer normal QTc threshold than men (460 ms vs 450 ms), reflecting a physiological gender difference that emerges during adolescence when testosterone shortens the QT interval in males 1
The gender difference ranges from 6-10 ms in older adults and 12-15 ms in younger adults, but this difference practically disappears after age 40 and becomes minimal in elderly populations 1, 3
In children younger than 12 years, the 98th percentile limit is approximately 450 ms without gender differentiation 2

More refined age-specific criteria have been established based on large population studies 4:

These age-adjusted values account for the gradual narrowing of the gender gap with advancing age 1, 4

Prolonged QTc:

≥460 ms is considered prolonged in adult women using standard criteria 1, 2
460-500 ms represents a "grey zone" requiring risk factor assessment 2
>480 ms represents the 99th percentile and should be considered abnormally prolonged 3
>500 ms is highly abnormal regardless of gender and requires immediate action due to significant arrhythmia risk 3

Short QTc:

Fridericia's formula (QTc = QT/∛RR) is recommended over Bazett's formula, particularly at heart rates >80 bpm where Bazett's overcorrects 2, 5, 6
Bazett's formula produces falsely elevated QTc values at higher heart rates, leading to overdiagnosis of QT prolongation—in one study, 30% of normal ECGs would be misclassified as abnormal using a 440 ms threshold with Bazett's formula 6
Hodges formula shows the least correlation with heart rate (correlation coefficient 0.11 vs 0.33 for Bazett) and may be superior for rate correction 6

Severity grading for QTc prolongation: 2, 5

High-risk thresholds:

QTc >500 ms or increase >60 ms from baseline is associated with increased risk of Torsades de Pointes 2, 5
FDA recommends reporting three severity levels: >350 ms, >480 ms, and >500 ms 1

Measure QT in leads II, V3, or V5, using the longest value obtained 2
The end of the T-wave is identified by the intercept between the steepest tangent at the descending part of the T-wave and the isoelectric line 2
Manual measurement is required for abnormal ECGs; automated measurements are acceptable only if the ECG is otherwise normal 2
Rate correction should not be attempted when RR interval variability is large (as in atrial fibrillation) or when the end of the T-wave is unreliable 1

Not using gender-specific thresholds leads to underdiagnosis of QT prolongation in women, who are more prone to malignant arrhythmias in long QT syndrome 1, 2
Using Bazett's formula at heart rates >80 bpm results in overcorrection and false-positive diagnoses of QT prolongation 2, 6
Ignoring age-related changes in QTc thresholds, particularly in women over 40 years 4
Not accounting for pregnancy, which can increase QTc and QT dispersion, though typically remaining within normal range 7
Failing to adjust for QRS duration in the presence of bundle branch blocks or conduction delays 1, 3