I saw Dr. Spencer Rosero in
Rochester,
NY who is a specialist in long qt. When he measured my qts by hand, the highest he found is 440 for qtc. He was sure I didn't have it and he has never been wrong. Nevertheless, I ended up getting the genetic test.. low and behold it turned out negative! If you were low on potassium it definitely could have caused the high qtc reading. Since your other readings were all normal, I wouldn't be concerned. Have you seen an electrophysilogist?