Coronary pressure never lies.

Fractional Flow Reserve (FFR), calculated by coronary pressure measurement, is the invasive gold standard to assess the hemodynamic significance of a coronary stenosis. FFR reliably indicates whether a stenosis is responsible for inducible ischemia and if percutaneous coronary intervention is appropriate. False positive or negative FFR is rare. Occasionally, however, a clinical or angiographic condition is encountered in which in first instance the FFR measurement contradicts the intuitive feeling of the interventionalist. Further examination in such cases, however, often reveals a clear physiologic explanation and in this manner FFR "lifts the veil". Two such patients are presented in this case report and the reasons for apparent or actual false positive or negative FFR are discussed. (c) 2008 Wiley-Liss, Inc.