QTc Interval Calculator
Assess QT Prolongation Risk
Calculate corrected QT interval to evaluate risk of life-threatening arrhythmia when using fluoroquinolones or macrolides. The QTc interval is critical for monitoring patients on QT-prolonging medications.
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When you take an antibiotic like azithromycin or ciprofloxacin, you expect it to kill the infection-not mess with your heart. But for some people, these common drugs can cause something dangerous: QT prolongation. It’s not a household term, but it’s one of the most underrecognized risks in outpatient and hospital care. Left unchecked, it can trigger a life-threatening heart rhythm called Torsades de Pointes. And the worst part? Many patients and doctors don’t realize the danger until it’s too late.
What QT Prolongation Actually Means
Your heart beats because of electrical signals. The QT interval on an ECG shows how long it takes for your ventricles to recharge after each beat. If that interval stretches too long, your heart’s electrical system gets unstable. That’s QT prolongation. It doesn’t cause symptoms on its own. But when it gets bad enough, your heart can start quivering instead of pumping-leading to fainting, seizures, or sudden death. Fluoroquinolones and macrolides are two of the most common antibiotic classes linked to this. Fluoroquinolones include ciprofloxacin, levofloxacin, and moxifloxacin. Macrolides include azithromycin, clarithromycin, and erythromycin. All of them block a tiny potassium channel in heart cells called hERG. That’s the same channel targeted by heart rhythm drugs like sotalol. When antibiotics do it, it’s accidental-but just as dangerous.Not All Antibiotics Are Created Equal
Some fluoroquinolones are riskier than others. Sparfloxacin was pulled from the market in the 1990s because it caused so much QT prolongation. Today, moxifloxacin carries the highest risk among those still in use. Ciprofloxacin is low risk. Levofloxacin? Mostly minimal. The same goes for macrolides: erythromycin is the worst offender, clarithromycin is moderate, and azithromycin is the safest of the group. A 2025 study of older women in long-term care found that many were prescribed levofloxacin for simple urinary tract infections-despite having multiple risk factors like low potassium, kidney problems, and other QT-prolonging drugs. That’s not just poor prescribing. It’s dangerous.Who’s at Highest Risk?
It’s not just about the drug. It’s about the person. Here’s who’s most vulnerable:- Women over 65
- People with low potassium or magnesium
- Those with heart disease, especially low ejection fraction or prior arrhythmias
- Patients on dialysis or with kidney failure
- Anyone taking other QT-prolonging drugs-antidepressants, antifungals, anti-nausea meds
- People with a family history of long QT syndrome
How to Measure QT Correctly
You can’t just look at an ECG and guess. You need to calculate the corrected QT interval, or QTc. Two formulas exist: Bazett’s and Fridericia’s. Bazett’s formula (QTc = QT / √RR) is old and flawed. It overcorrects when the heart is fast and undercorrects when it’s slow. That means you might miss danger-or panic over a false alarm. Fridericia’s formula (QTc = QT / √RR³) is more accurate. It’s now the preferred method in guidelines from the British Thoracic Society and major hospitals. If your hospital still uses Bazett’s, ask why. Also, watch out for false readings. Bundle branch blocks, pacemakers, or wide QRS complexes (>140 ms) can make QT look longer than it is. Always check the QRS width before deciding the QT is truly prolonged.When and How to Monitor
Monitoring isn’t optional. It’s standard of care for high-risk patients. For macrolides, the British Thoracic Society says: get an ECG before starting, and another one at one month. If the QTc goes above 470 ms in women or 450 ms in men, stop the drug. That’s not a suggestion-it’s a requirement. For fluoroquinolones, the timeline is different. Start with an ECG 7 to 15 days after beginning treatment. Then check again at one month, and again at three months. After that, periodic checks are enough-unless new risk factors appear. In high-risk patients-say, an 80-year-old woman on diuretics with low potassium and taking azithromycin-consider continuous telemetry during hospitalization. Don’t wait for symptoms. By the time someone faints, it’s often too late.
What to Do If QT Prolongation Shows Up
If QTc exceeds 500 ms-or increases by more than 60 ms from baseline-stop the antibiotic immediately. Don’t wait. Don’t wait for a cardiologist. Don’t wait for lab results. Stop it. Then fix what you can:- Correct potassium to above 4.0 mmol/L
- Correct magnesium to above 2.0 mg/dL
- Stop all other QT-prolonging drugs if possible
- Check thyroid function-hypothyroidism worsens QT prolongation
Why This Matters in Real Life
The FDA has issued multiple warnings about fluoroquinolones since 2013. Yet, they’re still overused-especially for simple infections like UTIs. A 2025 study found that in long-term care facilities, nearly 40% of fluoroquinolone prescriptions for UTIs in older women were inappropriate. Many of these women were on multiple QT-prolonging drugs. That’s not just a prescribing error. It’s a systemic failure. Antimicrobial stewardship programs are the answer. Hospitals and clinics need protocols that force clinicians to ask: Is this antibiotic necessary? Is there a safer alternative? Has the patient’s ECG been checked? Are electrolytes normal?What’s Next?
Research is moving toward personalized risk scores. Imagine a tool that takes your age, sex, kidney function, electrolytes, and current meds-and spits out your personal QT risk level. That’s coming. In the meantime, the best tool you have is an ECG, a calculator, and the discipline to use it. Don’t assume your patient is safe because they’re young or otherwise healthy. Don’t assume azithromycin is harmless because it’s "milder." QT prolongation doesn’t care about your assumptions. It only cares about the numbers-and whether you’re looking at them.What is considered a dangerous QTc interval?
A QTc interval above 500 ms is considered clinically dangerous and requires immediate action. Some guidelines also recommend intervention if the QTc increases by more than 60 ms from baseline, even if it hasn’t reached 500 ms. For men, QTc over 450 ms is abnormal; for women, it’s over 470 ms. These thresholds come from the 2023 British Thoracic Society guidelines and are widely adopted in clinical practice.
Which fluoroquinolone has the lowest risk of QT prolongation?
Ciprofloxacin has the lowest risk among commonly used fluoroquinolones. Levofloxacin is considered minimal risk, while moxifloxacin carries the highest risk. Sparfloxacin was withdrawn from the market due to severe QT prolongation. When choosing between fluoroquinolones, ciprofloxacin is preferred for patients with cardiac risk factors.
Is azithromycin safer than erythromycin for QT prolongation?
Yes, azithromycin is significantly safer than erythromycin. Erythromycin strongly blocks the hERG potassium channel and can prolong the QT interval as much as class III antiarrhythmics. Azithromycin has much weaker hERG inhibition and is associated with far fewer cases of Torsades de Pointes. It’s still not risk-free, especially in high-risk patients, but it’s the safest macrolide option.
Should I always do an ECG before prescribing a macrolide?
Yes, according to the 2023 British Thoracic Society guidelines, an ECG should be performed before starting any macrolide therapy to rule out pre-existing QT prolongation. A follow-up ECG is recommended one month after starting treatment. This is especially critical for patients over 65, women, and those with heart disease or electrolyte imbalances.
Can QT prolongation be reversed?
Yes, in most cases, QT prolongation caused by antibiotics is reversible. Stopping the offending drug and correcting electrolyte imbalances-especially potassium and magnesium-can normalize the QT interval within days. However, if Torsades de Pointes develops, immediate medical intervention is required, including magnesium sulfate infusion and cardiac pacing.
Are there alternatives to fluoroquinolones for UTIs?
Yes. For uncomplicated UTIs, nitrofurantoin and fosfomycin are first-line alternatives with no QT prolongation risk. Trimethoprim-sulfamethoxazole is another option, unless resistance is common in your region. Fluoroquinolones should be reserved for complicated infections or when no safer alternatives exist, especially in older women and patients with cardiac risk factors.
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