Mayo Clinic published an institution wide study here. Of note, patients with long QT intervals had a much higher all cause mortality (mean follow up 224 days). Only 10% had known congenital LQTS. Beyond the usual suspects (drugs, electrolyte disturbances) other less well appreciated conditions were cited as contributory to QT prolongation (DM, ACS, ESRD and others). The threshold for identifying patients as having long QT was generous----500 ms.
Related article in the same issue here.