QTc Interval Calculator (Corrected QT)
Calculate corrected QT interval using Bazett, Fridericia, Framingham, and Hodges formulas. Essential drug safety check before prescribing QT-prolonging medications.
Don't rely on memory for 200+ QT-prolonging drugs
EasyClinic's drug database flags QT risk the moment you select azithromycin, citalopram, ondansetron, haloperidol, fluconazole, methadone, or any other CredibleMeds-listed drug. Built-in patient safety.
Also Known As
Why QTc Matters — The Hidden Killer in Outpatient Prescribing
The QT interval represents ventricular depolarization and repolarization on the ECG. A prolonged QT predisposes patients to Torsades de Pointes, a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation and sudden cardiac death. Because heart rate affects the QT interval, the corrected QT (QTc) is calculated to compare across different heart rates.
Hundreds of common medications prolong QT — including azithromycin, ondansetron, fluconazole, citalopram, escitalopram, haloperidol, quetiapine, hydroxychloroquine, methadone, domperidone, and many more. Indian doctors routinely prescribe combinations of these drugs, often without checking QT status. Drug-induced Torsades is one of the leading causes of preventable cardiac death in elderly Indian patients.
QTc Thresholds and Clinical Action
- Normal: Men <450 ms, Women <460 ms
- Borderline: 450-470 ms (men), 460-480 ms (women) — monitor
- Prolonged: 470-499 ms — review medications, correct K/Mg, avoid additional QT-prolonging drugs
- Severely prolonged (≥500 ms): Hold QT-prolonging drugs, ECG monitoring, electrolyte correction, telemetry, consider cardiology consult
- QT change >60 ms from baseline after starting a new drug = stop the drug
Choosing the Right Formula
Bazett's formula (QT/√RR) is the most widely used but overestimates QTc at heart rates >100 bpm and underestimates at <60 bpm. Fridericia (QT/∛RR) is more accurate at heart rate extremes and is increasingly preferred in modern cardiology. Framingham uses a linear correction and is simpler. Hodges is the best choice for atrial fibrillation and irregular rhythms.
Frequently Asked Questions
▸When should I check the QTc before prescribing?
Before starting any QT-prolonging drug in patients with: known LQTS, prior syncope, family history of sudden cardiac death, electrolyte abnormalities, hepatic/renal dysfunction, structural heart disease, or already on another QT-prolonging drug.
▸What is the "rule of thumb" for QTc on ECG?
A quick visual estimate: QT should be less than half the RR interval. If QT looks longer than half RR, calculate QTc formally.
▸Can I prescribe azithromycin to a patient with QTc 460?
Caution. Borderline QTc + addition of azithromycin is risky. Consider alternative (doxycycline, amoxicillin) if appropriate. If azithromycin necessary, check electrolytes, avoid co-prescription of other QT drugs, and consider follow-up ECG.
▸What electrolytes should I check?
Always K, Mg, and Ca. Hypokalemia and hypomagnesemia are the most common reversible causes of QT prolongation. Correct K to >4.0, Mg to >2.0 before starting QT-prolonging drugs.
▸Why does my colleague get a different QTc reading?
Different formulas. Bazett (most common) gives different values than Fridericia or Framingham, especially at HR extremes. Always specify which formula you used.
▸Where can I check if a drug prolongs QT?
CredibleMeds.org maintains the gold-standard list of QT-prolonging drugs categorized by risk (Known, Possible, Conditional). EasyClinic's drug database is integrated with CredibleMeds for real-time alerts.
References
- Bazett HC. An analysis of the time relations of electrocardiograms. Heart 1920;7:353-70
- AHA/ACCF/HRS Recommendations for Standardization of QT Interval Measurement
- CredibleMeds.org — Curated list of QT-prolonging drugs
- Indian Heart Rhythm Society Guidelines on Long QT Syndrome
Get QT-prolongation alerts before you prescribe
EasyClinic flags QT-prolonging drugs (azithromycin, ondansetron, citalopram, ondansetron, haloperidol, hydroxychloroquine + 100 more) the moment you write the prescription. Built-in safety net for every doctor.