About one-third of long QT syndrome (LQTS) patients do not have any symptoms. Coupled with the fact that many cases of LQTS are diagnosed in children or young adults, there is often no reason to perform an electrocardiogram (EKG), so a diagnosis is often not made until a serious fainting (syncope) incident has occurred or the LQTS-related death of a family member.

A red flag for physicians is raised when there is a family history of any of the following:

The diagnosis for LQTS is usually made through an EKG evaluation. An EKG is noninvasive, quick, safe and painless. As a stand-alone test, it takes about 5 to 10 minutes.

However, the EKG is not always able to detect the condition. In some cases, the physician will request multiple EKG tests. If LQTS is still not found, but suspected, the physician may order an exercise stress test. This is an EKG that is performed in conjunction with exercise and is often abnormal among patients with LQTS. A new testing protocol, using short bursts of exercise (maximum exertion for one minute) has further improved the accuracy of traditional exercise stress testing. If exercise is not possible, a pharmacological stress test may be performed. During this test, the heartbeat is elevated through the use of medications, including epinephrine. In some cases, patients may also be required to wear a Holter monitor, which allows the physician to monitor the heart rate over a fixed period of time (e.g., 24 hours).

However, individuals with LQTS can also have a normal EKG. This contradiction has led to a search for another way of diagnosing the condition. One method being explored involves the use of a pharmacological stress test, in which increasing doses of epinephrine are given to speed up the heart. Epinephrine is the hormone released by the body in response to emotional and physical stress. It can provoke the serious effects of LQTS.

Researchers are also working on ways to identify LQTS using a genetic test. Currently, seven specific genetic abnormalities have been identified among patients with LQTS. Tests are being designed to detect these abnormalities. They are still experimental, however, and not commercially available.