Bar Chart of CO measurements for non-smokers and smokers. The four lines indicate a) and b) 2 and 3 p.p.m.: the cut-off points with highest sensitivity and specificity c) 5 ppm: the new cut off for the Breathe programme and d) 8 ppm: the current cut-off point. It is possible to visualise the increase in number of subjects picked up by the test as the cut-off point is lowered. Note: The scale for the frequency of women has been truncated so as to remove the large peak of non-smokers at 1 p.p.m. (n = 1790). This allows the results for all women to be visualized more appropriately.