IL6 and susceptibility to pneumonia measurement: Using a cut-off value of 77·5 and the incidence of severe pneumonia (34·8% in this study) as the pre-test probability, the resulting Fagan plot (Figure 5) shows that interleukin-6 levels can be clinically informative, as it increases the probability of the patient being classified into the hypoxemia group by up to 89% when positive, and lowers the probability by up to 22% when negative.