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Electrocardiogram (ECG) QT Interval

Published: August 9, 2026Updated: August 16, 2026

The QT interval measures the time it takes for the heart’s electrical system to complete ventricular depolarization and repolarization. It is calculated from specific points on an ECG waveform.

Quick summary

["The QT interval helps clinicians assess heart rhythm and identify possible effects of medications or electrolyte abnormalities."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
8634-8
Short code
Q-T interval
Synonyms
Q-T interval
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
ekg
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

How the Test Works

An ECG records the heart’s electrical activity using electrodes placed on the skin. Software measures the QT interval, and a clinician reviews it along with the rest of the ECG.

Why It Matters

A prolonged QT interval can increase the risk of a rare type of abnormal heart rhythm. Possible contributors include certain medications, low potassium or magnesium levels, and underlying heart conditions.

Factors That Affect the Result

Heart rate, irregular rhythms, medications, and electrolyte abnormalities can affect the QT interval. Automated measurements may also vary slightly, so a clinician interprets the value in context.

After the ECG

Your clinician may review your symptoms, medication list, blood tests, and other ECG findings. Changes to medications should be made only with healthcare professional guidance.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

QT Interval

Frequently asked questions

Does a long QT interval mean I have a heart rhythm disorder?
Not necessarily. A prolonged QT interval can be a risk marker and may reflect medications, low electrolytes, or a heart condition, but it does not confirm that an abnormal rhythm is occurring.
Can medications affect my QT interval?
Yes. Some prescription and nonprescription medicines can prolong it. Do not stop or change a medication on your own; discuss your ECG results with the prescribing clinician.
What if my QT interval is different from a previous ECG?
Small variations can occur with heart rate, measurement method, medications, or electrolyte changes. A clinician should review the trend and determine whether follow-up is needed.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy