How to calculate actual risk of Torsades des Pointes with QT prolongation

• FACT: Computer generated QT/QTc calculations are unreliable and should not be used.
• Measure QT manually( yes use calipers!)
• There is no common agreement which QT value to use, I prefer to use the longest QT, but other advocate average of all leads( time consuming and QT cannot be measures usually in all leads), or use lead II ( QT can be measures) as this is lined up with mean QRS vector.
•  Calculate QTc according to Fredericia or Hodges formula ( Bazzett's formula over corrects QTC at higher heart rates often present in critically ill patients). 
• Example 
  
• For the above example the measured QT in leads I, II aVF, V2, V4, V6 are 320, 340, 320, 300, 340 and 320 respectively. This gives a median QT of 320ms. Note the computer generated QT is longer at 360ms, correcting with Bazett’s formula to 485 – which would be considered prolonged. However plotting this on the nomogram shows a level which is well below the risk line. Bazett’s QTc over corrects in tachycardia and under corrects for slow heart rates and should not used in toxicology patients to determine overall risk of TdP. 
  
• The QT nomogram has a sensitivity of 97% and specificity of 99% for predicting TdP following toxicological ingestions and has been found to be more accurate in predicting TdP than using Bazett’s QTc of 440ms or 500ms. 

General references:

1. Drug-Induced Prolongation of the QT Interval

2. What Clinicians Should Know About the QT Interval

Go this this website for any question , CredibleMeds,on QTc risk of drugs. Meds are classified in 4 risk categories