FIGURE 3.

Decision curves for the four intervention strategies for THA and TKA, by outcome. The four intervention strategies are performing a G&S and post‐operative blood test on all patients, on no patients, only on patients with pre‐operative anaemia, or on patients at risk of post‐operative anaemia. Data are presented for 2011–2018 for transfusion threshold analyses, and 2013–2018 for the blood transfusion analysis. Analysis was not performed for a 70 g/L transfusion threshold for TKR due to the paucity of event rates. The y‐axis is the benefit (net benefit), and the x‐axis is the preference (threshold probability). The benefit of an intervention strategy is that is correctly identifies which patients had a post‐operative Hb < 70 g/L, <80 g/L or had a blood transfusion (and hence actually needed a G&S and postoperative blood test). The preference refers to how clinicians value different outcomes and what risk they are willing to take to not have blood when it is needed for a given patient, which varies. For example, if Clinician A was concerned about undiagnosed post‐operative anaemia in their THA (Hb < 70 g/L) patient, they may only be willing to accept a 0.1% risk to not have blood available when it is needed, compared to Clinician B who is more concerned about unnecessary blood test and would be willing to accept a slightly higher 1% risk of not having blood when it is needed. In this case for Clinician A, there is little difference between the intervention strategies of performing a G&S and post‐operative blood test on all patients, only on patients with pre‐operative anaemia, or on patients at risk of post‐operative anaemia. However, Clinician B would be better to use the intervention strategies of performing a G&S and post‐operative blood test only on patients at risk of post‐operative anaemia based on pre‐operative Hb measurement.