ACE2 and acute kidney injury: On the restricted cubic spline plots demonstrating the associations of the ACE2 concentration with ICU mortality and AKI within 48 hours after ICU admission, knots were placed at the serum ACE2 concentrations of 0.2, 1.1, 2.5, 6.6, and 20.0 ng/mL, corresponding to the 2nd, 25th, 50th, 75th, and 98th percentiles.