Tests and diagnosis

By Mayo Clinic Staff

To diagnose long QT syndrome, your doctor will review your symptoms, review your medical and family history, and conduct a physical examination. If your doctor suspects that you have long QT syndrome, you might need several tests to confirm the diagnosis. These include:

  • Electrocardiogram (ECG). During an ECG, doctors attach sensors (electrodes) that can detect the electrical activity of your heart to your chest. An ECG measures the timing and duration of each electrical phase in your heartbeat.

    You might have this test while at rest or during an exercise stress test, in which doctors monitor your heart activity as you exercise on a treadmill or a stationary bicycle. Your doctor may also suggest your family members have ECGs.

  • Holter monitor. This portable ECG device can be worn for a day or more to record your heart's activity as you go about your routine.
  • Event monitor. This portable ECG device is attached to your body and is intended to monitor your heart activity over a few weeks to a few months. When you have symptoms, you press a button. This allows your doctor to check your heart rhythm at the time of your symptoms.

In some people with suspected long QT syndrome, the ECG doesn't show a visibly lengthened QT interval. Other testing may be necessary to diagnose long QT syndrome, such as:

  • A nonexercise (medication) stress test. This ECG test is done while you're given a medication such as epinephrine (Adrenalin) that stimulates your heart in a way similar to exercise. In this test, doctors monitor the effects of the medication on the way your heart recharges.

    This test can help doctors to diagnose people with suspected long QT syndrome and may help determine which genes are associated with the condition. It may also be used to diagnose people with long QT syndrome who have a gene associated with long QT syndrome but who have a normal QT interval (recharging time) at rest.

  • Genetic testing. A genetic test for long QT syndrome is available and may be covered by some private and governmental insurance plans. Genetic tests for long QT syndrome can generally find the genetic cause for about 3 out of every 4 cases of long QT syndrome. However, genetic tests can't detect all cases of long QT syndrome.

    If your genetic cause of long QT syndrome is discovered through a positive genetic test, then your doctor may recommend that your family members be tested to determine whether they inherited the same genetic mutation.

  • A second opinion. You might want to seek a second opinion before proceeding with treatment if your doctor diagnoses you with long QT syndrome. Studies have found that people can sometimes be misdiagnosed as having long QT syndrome when it's not actually present.
Oct. 27, 2015