First predict serum creatinine using smoothing spline, then predict GFR using CKD-EPI equation.
estimate_gfr(gfrtmp.dt)A data.table with columns gender, reth,
age_years, age_months, BSA_adj, serum_creat.
The same data.table with a gfr_est column added, containing
estimated GFR values.
Ring, Caroline L., et al. "Identifying populations sensitive to environmental chemicals by simulating toxicokinetic variability." Environment International 106 (2017): 105-118