The yield of genetic testing in long QT syndrome phenotype-positive patients is 50% to 86%, with the higher range present in patients with marked QT prolongation or positive family history of SCD.S7.9.1.1-17,S7.9.1.1-21,S7.9.1.1-53 A negative genetic test does not exclude the diagnosis of long QT syndrome, which relies on the clinical evaluation. In asymptomatic patients with oth- erwise unexplained prolonged QTc 480 ms on serial ECGs, genetic testing may help con- firm the diagnosis and supplement prognostic information in addition to clinical symptoms and QTc duration.S7.9.1.1-5,S7.9.1.1-18 S7.9.1.1-20,S7.9.1.1-30, 5. In a prospective, observational study of patients with suspected long QT syndrome, patients with a history of syncope or cardiac arrest and either an affected first-degree relative or a borderline or prolonged QTc interval underwent exercise treadmill testing and bicycle exercise, with ECGs recorded before, during, and after exercise, as well as in different positions.S7.9.1.1-27 long QT syn- drome was confirmed by genetic testing in all affected individuals. Among patients with border- line-to-normal resting QTc intervals, prolongation of the 4-minute recovery QTc 445 ms had high sensitivity for correctly identifying patients with long QT syndrome.S7.9.1.1-27 A study in younger patients demonstrated QTc prolongation >460 ms at 7 minutes of recovery predicted long QT syndrome type 1 or long QT syndrome type 2 patients versus controls.S7.9.1.1-23 In a study using burst bicycle exercise, patients with latent long QT syndrome had a significantly greater increase in QTc with exercise than either controls or those with QTc prolongation at baseline.S7.9.1.1-24 These findings can be useful in establishing whether long QT syndrome is present. 