[1] The accurate measurement of the QT interval is subjective[2] because the end of the T wave is not always clearly defined and usually merges gradually with the baseline.

[3] With the increased availability of digital ECGs with simultaneous 12-channel recording, QT measurement may also be done by the 'superimposed median beat' method.

Bazett's formula is often given in a form that returns QTc in dimensionally suspect units, square root of seconds.

[8] However, reporting QTc using this formula creates a "requirement regarding the units in which the original QT and RR are measured.

For a heart rate of 70 bpm, QT would roughly be expected to be equal to or below 0.40 s. Likewise, for 80 bpm, QT would roughly be expected to be equal to or below 0.38 s.[13] Prolonged QTc causes premature action potentials during the late phases of depolarization.

[21] The autosomal recessive syndrome of Jervell and Lange-Nielsen is characterized by a prolonged QTc interval in conjunction with sensorineural hearing loss.

The mechanism of action of certain antiarrhythmic drugs, like amiodarone or sotalol, involve intentional pharmacological QT prolongation.

[30] A possible interaction between selective serotonin reuptake inhibitors and thiazide diuretics is associated with QT prolongation.

[31] Hypothyroidism, a condition of low function of the thyroid gland, can cause QT prolongation at the electrocardiogram.

Traditionally, the QT interval had been evaluated by having an individual human reader measure approximately nine cardiac beats per clinical timepoint.

However, a substantial portion of drug approvals after 2010 have incorporated a partially automated approach, blending automated software algorithms with expert human readers reviewing a portion of the cardiac beats, to enable the assessment of significantly more beats in order to improve precision and reduce cost.

[35] As the pharmaceutical industry has gained experience in performing TQT studies, it has also become evident that traditional QT correction formulas such as QTcF, QTcB, and QTcLC may not always be suitable for evaluation of drugs impacting autonomic tone.

In the general population, there has been no consistent evidence that prolonged QTc interval in isolation is associated with an increase in mortality from cardiovascular disease.

The researchers proposed that inflammation prolonged the QTc interval and created arrhythmias that were associated with higher mortality rates.