Estimating glomerular filtration rate in children at serial follow-up when height is unknown

Abstract

Background and objectives: Estimation of GFR in children from serum creatinine measures in regional databases is limited by a lack of height data. Furthermore, the ability of GFR estimating equations to quantify changes in GFR over time is unknown. Two methods of estimating GFR when height is unknown and the ability of several GFR equations to quantify GFR changes over time were evaluated.

Design, setting, participants & measurements: This retrospective study included 195 children (mean age 11.9 ± 4.6 years, GFR = 78.8 ± 34.5) who underwent iothalamate GFR, serum creatinine, and height measurements; 93 children underwent a second GFR measurement and 47 a third GFR measurement. Four equations were evaluated for bias and precision and for quantifying GFR change over time: (1) Schwartz, using measured height; (2) Schwartz, using estimated height (based on previous height percentile); (3) a locally derived modification of a previously derived height-independent equation.

Results: The Schwartz (measured height) displayed the least bias (-2 to +7%), followed by the modified height-independent equation and Schwartz (estimated height). All equations were imprecise. All equations performed similarly at capturing change in measured GFR over time, with no significant difference between estimated and measured GFR percentage change over time. The height-estimated Schwartz formula performed similarly to the height-measured Schwartz in all aspects of equation performance.

Conclusions: Pediatric GFR follow-up studies may be possible using height-independent equations. Estimating height from prior height measurements enhances GFR estimation when height is unknown. These findings will hopefully help advance future pediatric renal function database studies.

Figure 1.

Study flow chart. Chart of the subject selection and exclusion strategy. T0, first IoGFR test; T1, second IoGFR test; T2, third IoGFR test.

Figure 2.

Plots of percentage difference between estimated and measured height versus measured height, at time 1 (a) and time 2 (b). Center line: mean difference (negative indicates estimated height underestimates measured height). Upper and lower dotted lines: upper and lower 95% confidence interval of the percentage of difference.

Figure 3.

Percentage of IoGFR change versus percentage of change in eGFR for each formula between each time period. Each graph contains the scatter plot and best fit line of the percentage of change in IoGFR versus percentage of change in eGFR (a) for the height-measured Schwartz, (b) for the height-estimated Schwartz, and (c) for the modified BCCH between two time periods. Time period T0 to T1 is plotted in black and time period T1 to T2 is plotted in gray. Pearson correlation coefficient “R” is displayed, with associated 95% confidence intervals.