QTc

QTc is the QT interval after it has been corrected for heart rate. Because the raw QT naturally shrinks when the heart speeds up and stretches when it slows down, comparing QT values across different heart rates would be misleading. The correction estimates what the QT would be at a standard rate, so that one number can be judged fairly.

The most familiar correction divides the measured QT by the square root of the time between beats (Bazett's formula), but several formulas exist and they can disagree, especially at very fast or very slow rates. Whichever is used, the goal is the same: to express repolarization time on a level playing field so that abnormal prolongation is not missed or imagined.

A prolonged QTc is a recognised marker of risk for a potentially dangerous ventricular rhythm. Clinicians track it especially when prescribing medicines known to lengthen it, when potassium or magnesium are low, or when an inherited long-QT condition is suspected. Importantly, a borderline QTc is interpreted in context, not as an automatic alarm, since measurement and correction both carry some imprecision.