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*The diagnosis is by
maesurement of the [[Conduction#The_QT_interval|heart rate corrected QT interval]] on the ECG, which can be calculated with the [[QTc calculator]].* Sometinmes the QT interval can be difficult to assess. Read the [[Difficult_QT|guidelines for measurement of difficult QT interval]].
*A QTc of > 500ms in patients with Long QT Syndrome is associated with an increased risk for sudden death.<cite>Priori</cite>
*In patients suspected of LQTS (e.g. family members of known LQTS patients) a QTc > 430ms makes it likely that a LQTS gene defect is present.<cite>Hofman</cite>
;SCD: Sudden Cardiac Death
Long before the genes involved were known, two syndromes
had been described that were associated with a prolonged QT interval on the ECG.* Anton Jervell and Fred Lange-Nielsen from Oslo described in 1957 a autosomaal recessive syndrome that was associated with QT interval prolongation, deafness and sudden death: the now called '''Jervell-Lange-Nielsen syndrome'''. <cite>Lang1957</cite>
* '''Romano-Ward syndrome''' is a long QT syndrome with normal auditory function and autosomal dominant inheritance.
* In a genotype–phenotype study by Moss et al. that studied type-1 LQTS, it was found that mutations located in the transmembrane portion of the ion channel protein and the degree of ion channel dysfunction caused by the mutations are important independent risk factors influencing the clinical course of this disorder.<cite>moss</cite>