This figure comprises a line graph showing how procalcitonin levels change in a patient with an infection. The horizontal axis is labelled time (hours) and is marked from 0 to 72 hours in 12 hour intervals. The vertical axis is labelled [PCT] micrograms per litre and is marked from 0 to 5 in intervals of 1. At 16 hours there is a vertical orange dashed line which is labelled antibiotic treatment started. An arrow at 0 hours indicated the onset of infection. The levels of PCT are indicated with a blue line which slopes steeply upwards from the onset of infection to a peak of 5 micrograms per litre PCT at 16 hours. The line then slopes gradually downwards towards the horizontal axis ending below 1 microgram per litre PCT at 72 hours. At the top of the graph the region before antibiotic treatment started is labelled Assessing infection likelihood and severity. The region after antibiotic treatment is labelled Assessing effectiveness of treatment.

 4.3.1 Detecting the patient infection response