3.1 – Study selection
The search retrieved 355 individual studies that were filtered by language, title and abstract leaving 21 original studies. Nine studies were then excluded at full text, mostly for not reporting on patients who had undergone coronary angiography although one study was excluded for unclear results(2) and another excluded for reporting on the same patients as another study(11). This left 12 original studies published between 1974 and 2017 included in this review. Figure 2 shows the selection process.