We previously found a correlation coefficient (R) of 0.57 between CSF MCP-1 and tau increases from before to 24 hours after surgery in neurosurgery patients,14 although this correlation may be smaller in nonneurosurgery patients.A 150-patient study will provide more than 90% power todetect R ≥ 0.3 (a low moderate correlation) between CSF tauand MCP-1 levels after surgery, as well as changes between CSF monocyte-to-lymphocyte ratio and CSF tau levels beforeand after surgery. Postoperative tau change is nonnormallydistributed30 and thus requires nonparametric analysis.