Long QT syndrome is diagnosed through a combination of electrocardiogram (ECG) readings, clinical scoring, family history evaluation, and sometimes genetic testing or provocation tests. No single test confirms the diagnosis on its own. Instead, doctors piece together electrical measurements of your heart, your personal symptom history, and whether close relatives have experienced unexplained fainting or sudden cardiac death.
What the ECG Measures
The cornerstone of diagnosis is a standard 12-lead ECG, which records the electrical activity of your heart. Doctors focus on the QT interval, the stretch of time it takes your heart’s lower chambers to recharge between beats. Because heart rate affects this measurement, the raw QT interval is adjusted using a mathematical formula to produce a “corrected” value called the QTc.
Normal QTc values are below 430 milliseconds for males and below 450 milliseconds for females. A QTc above 450 milliseconds in males or above 470 milliseconds in females is considered prolonged. The higher the QTc climbs above those thresholds, the stronger the evidence for long QT syndrome. A QTc at or above 480 milliseconds carries the most diagnostic weight.
The most widely used correction method is the Bazett formula, which divides the QT interval by the square root of the time between heartbeats. It performs well across a range of heart rates, especially in children and younger patients. The FDA recommends an alternative formula (Fridericia) for drug safety trials, but in everyday clinical practice Bazett remains the standard. Importantly, the two formulas use different thresholds for what counts as “prolonged,” so knowing which formula was applied matters.
A single ECG can miss the diagnosis. Some people with long QT syndrome have a normal or borderline QTc on any given day. That’s why doctors may order repeat ECGs or move on to provocation testing when suspicion is high but the resting ECG looks unremarkable.
The Schwartz Diagnostic Score
Because a QTc number alone doesn’t tell the whole story, doctors use a structured scoring system developed by Peter Schwartz. It assigns points across three categories: ECG findings, clinical history, and family history. The points add up to a total that classifies your probability of having the condition.
ECG findings carry the most weight. A QTc of 480 milliseconds or above earns 3.5 points. A QTc between 460 and 479 earns 2 points. For males specifically, a QTc between 450 and 459 earns 1 point. A prolonged QTc during the recovery phase of an exercise stress test adds 1 point. Other ECG features also contribute: a dangerous rhythm called torsades de pointes (3 points), alternating T-wave patterns (1 point), notched T waves visible in three or more leads (1 point), and a resting heart rate that’s unusually low for your age (0.5 points).
On the clinical side, fainting episodes triggered by stress or exertion score 2 points, while fainting without a clear trigger scores 1 point. Congenital deafness adds 0.5 points, since certain genetic forms of long QT syndrome also affect hearing. For family history, having a first-degree relative with a confirmed diagnosis (clinical or genetic) earns 1 point, and unexplained sudden cardiac death in an immediate family member under age 30 adds 0.5 points.
A total score of 3.5 or higher indicates high probability. Scores between 1.5 and 3 fall into intermediate probability, while scores of 1 or below suggest low probability. Intermediate scores typically prompt further testing rather than a definitive yes or no.
Ruling Out Acquired Causes
Before concluding that you have the inherited form of long QT syndrome, your doctor needs to rule out reversible causes of QT prolongation. A wide range of medications can lengthen the QT interval, including certain antibiotics, antifungals, antidepressants, antipsychotics, and anti-nausea drugs. Electrolyte imbalances, particularly low potassium, low magnesium, or low calcium, are another common culprit. Other acquired causes include severe slowing of the heart rate, heart muscle damage from reduced blood flow, stroke, subarachnoid hemorrhage, extreme dieting, and certain infections.
If your QTc normalizes after correcting an electrolyte imbalance or stopping a medication, the prolongation was acquired rather than congenital. This distinction changes your treatment and long-term outlook entirely.
Exercise Stress Testing
When a resting ECG is normal or borderline but clinical suspicion remains, an exercise stress test can unmask a hidden long QT phenotype. You walk or jog on a treadmill while your heart’s electrical activity is continuously recorded. The key diagnostic window isn’t during peak exercise but during the recovery phase afterward.
In a healthy heart, the QT interval stays the same or shortens during recovery. In long QT syndrome, the interval stretches. The updated Schwartz criteria use a QTc of 480 milliseconds or above at the four-minute mark of recovery as a diagnostic cutoff. Some studies have found that a QTc above 445 milliseconds at four minutes, or above 460 milliseconds at one minute of recovery, can reliably distinguish people with the condition from those without it.
The pattern during exercise also differs depending on the genetic subtype. In the most common form (type 1), QTc prolongation is greatest at peak exercise. In type 2, the prolongation peaks during posture changes rather than at maximum effort, and there may be no significant prolongation during exercise itself. In type 3, the QTc can actually shorten during peak exercise. These distinct patterns can guide doctors toward a suspected genetic subtype even before genetic testing is done.
The Epinephrine Provocation Test
For patients who can’t exercise or whose stress test results are inconclusive, an epinephrine (adrenaline) challenge can serve as an alternative. During this test, a small dose of epinephrine is injected intravenously while a 12-lead ECG and blood pressure are monitored continuously. The test mimics the effect of a sudden adrenaline surge, which is a common trigger for dangerous heart rhythms in people with long QT syndrome.
Doctors look at how the QTc responds in two phases: the initial peak effect within the first couple of minutes and the steady-state effect at three to five minutes. A QTc increase of 35 milliseconds or more during the peak phase that stays elevated through steady state suggests the most common genetic subtype (type 1). A larger spike of 80 milliseconds or more that doesn’t persist into steady state points toward type 2. If the QTc doesn’t rise by at least 35 milliseconds, the test is considered negative. This test is performed under close monitoring in a hospital setting because of the small risk of triggering an abnormal rhythm.
Genetic Testing
Genetic testing can confirm the diagnosis and identify the specific subtype, which influences treatment decisions and helps predict which situations are most likely to trigger symptoms. Three genes account for roughly 75% of all confirmed cases. Mutations in KCNQ1 cause about 35% of cases (type 1), KCNH2 mutations account for about 30% (type 2), and SCN5A mutations underlie roughly 10% (type 3). Each gene codes for a different ion channel protein that controls how electrical signals flow through heart cells.
The practical value of knowing your subtype goes beyond confirming the diagnosis. Type 1 is most commonly triggered by exercise, especially swimming. Type 2 is often triggered by sudden loud noises or emotional stress. Type 3 events tend to happen during rest or sleep. Knowing your subtype helps you and your doctor tailor lifestyle precautions.
Genetic testing does have limits. About 25% of people with a clinically confirmed diagnosis have no identifiable mutation in any known gene. A negative genetic test does not rule out the condition if the clinical evidence is strong. Conversely, a positive genetic test in the absence of any ECG abnormality or symptoms creates a gray area that requires careful interpretation.
Family Screening
Because long QT syndrome is inherited, a confirmed diagnosis in one person triggers screening of close relatives. Children and young adults in the family who have experienced unexplained fainting, seizures, or cardiac arrest should be tested first. Other family members with a history of fainting or seizures, even if previously attributed to other causes, also warrant evaluation.
Screening typically starts with an ECG and may progress to genetic testing if the specific family mutation is known. Identifying the mutation in one family member makes genetic screening of relatives straightforward: a targeted test for that single mutation is faster, cheaper, and more definitive than sequencing all known genes. Relatives who carry the mutation can then be monitored and treated before symptoms ever appear.