Long QT syndrome (LQTS) is usually diagnosed with an electrocardiogram (ECG), which is a picture of the electrical activity of heartbeats. QT refers to a time interval measured on the ECG. People with this disorder have a longer QT interval than other people.
The ECG isn't a perfect test for the condition. Some people with the disorder will have normal tests or it might be normal on some days and abnormal on other days.
LQTS is usually inherited, which means it may affect multiple family members. Each child, sister, or brother of a person with the disorder has a 50 percent chance of inheriting the genetic mutation that causes the condition.
Once a family member is identified with the condition, it is very important to test all other family members. All close relatives should see a cardiologist and have an ECG.
Genetic testing can also help diagnose LQTS. Once genetic testing has identified the genetic mutation causing the condition in a family, other family members can be tested for that same genetic mutation to see if they also inherited it.
If a relative of a person with the disorder has genetic testing and is found not to carry the genetic mutation for the condition, then that relative need not be followed by a cardiologist. This can relieve a lot of stress and worry.
Reviewed by health care specialists at UCSF Medical Center.