Determination and Interpretation of the QT Interval

Area of Science:

• Cardiology
• Genetics
• Medical Diagnostics

Background:

• Long QT syndrome (LQTS) is a genetic disorder associated with life-threatening arrhythmias.
• Accurate diagnosis of LQTS can be challenging due to varying methods for QT interval assessment.
• QT interval measurement methods, specifically tangent and threshold, have not been directly compared for their diagnostic performance in LQTS.

Purpose of the Study:

• To compare the tangent and threshold methods for QT interval analysis in LQTS diagnosis.
• To assess the similarities and differences between QT interval measurements using these two methods.
• To provide data to aid in accurate QT assessment for LQTS.

Main Methods:

• Included 1484 individuals from 265 families with confirmed pathogenic variants in LQTS-associated genes (LQT1, LQT2, LQT3) and genotype-negative family members as controls.
• Analyzed electrocardiograms (ECGs) using both tangent and threshold methods to determine inter- and intrareader validity and diagnostic accuracy.
• Established cutoff values based on population percentiles, corrected for heart rate, age, and sex.

Main Results:

• The tangent method yielded QT intervals that were, on average, 10.4 ms shorter than the threshold method (95% limits of agreement ±20.5 ms).
• Both methods demonstrated high inter- and intrareader validity (ICC >0.96) and diagnostic accuracy (AUC >0.84).
• Current guideline cutoffs (QTc interval 480 ms) resulted in similar specificity but different sensitivity between the two methods, with tangent method cutoffs being lower.

Conclusions:

• The QT interval measurement method significantly impacts results, although both tangent and threshold methods are valid for LQTS diagnosis.
• Current guideline cutoffs may lead to misdiagnosis due to method-dependent variations in sensitivity and specificity.
• Adjusted cutoff values specific to the measurement method, correction formula, age, and sex are recommended for accurate LQTS diagnosis. An online calculator is available to assist.