Preventing immortal time bias in observational studies: a matter of design.

We thank Diab and colleagues for raising awareness for immortal time bias (ITB) in studies on cardiosurgical treatments in the European Heart Journal. 1 The authors elaborate on ITB in research on infective endocarditis (IE) and tricuspid regurgitation (TR), stating that a prospective, intention-to-treat design is the only practical way to detect and avoid ITB. In this letter, we elaborate on the causes of ITB and outline additional methods to prevent this bias in observational studies. ITB is introduced when researchers deviate from basic principles of study design. At the start of follow-up (i.e. ‘time-zero’), treatment status should be determined and eligibility criteria should be met for all participants. For a fair comparison, time-zero