Many people with long QT syndrome never have symptoms. Others first come to attention after a fainting episode, or when a relative is diagnosed. When symptoms do happen, they are caused by a sudden, abnormal heart rhythm, and they often have a recognisable trigger.

Fainting without warning Sudden collapse, often during physical effort, strong emotion, or a sudden fright. This is different from ordinary fainting, which usually has a warning feeling first.
Seizure-like episodes A brief period of jerking movements during a faint can be mistaken for epilepsy. Some people with long QT syndrome are first misdiagnosed with a seizure disorder.
Sudden cardiac arrest Rarely, the first sign is a cardiac arrest, or an unexplained sudden death in a young family member, including drowning in a strong swimmer.

The triggers often point to the type:
Type 1 (KCNQ1) Events usually happen during exercise, especially swimming.
Type 2 (KCNH2) Events are linked to sudden loud noises (an alarm clock or ringing phone), emotional stress, and the months after childbirth.
Type 3 (SCN5A) Events tend to happen during rest or sleep, when the heart rate is slow.

Diagnosis starts with a resting electrocardiogram. Doctors measure the “corrected QT interval,” which adjusts the reading for heart rate. A corrected QT interval of 480 milliseconds or more on repeated tracings is considered diagnostic. Borderline readings are combined with symptoms and family history in a scoring system (the Schwartz score). An exercise test or a 24-hour heart monitor may help, because some changes only show when the heart rate speeds up or slows down.

Genetic testing with a gene panel finds the cause in about 75–80% of people with a clear clinical diagnosis. International guidelines recommend genetic testing and genetic counseling for everyone diagnosed, because the gene result helps guide treatment and allows accurate testing of relatives.