Estimating and tracking renal function in children and adults with spina bifida

Estimated glomerular filtration rate (eGFR) in the general population is stable in children after 2 years of age until adulthood. In the first three decades after age 18, eGFR decreases by 0.3-0.8 ml/min/1.73 m²/year. Little data exists regarding eGFR changes in the spina bifida (SB) population given variability in muscle mass. In the absence of a validated SB-specific eGFR formula, the performance of different eGFR formulas may vary. We performed a cross-sectional study (1) to determine trends in eGFR with increasing age in children and adults with SB and (2) to compare eGFRs calculated using different formulas.

We retrospectively reviewed records of patients 2-50 years old with SB followed at our institution (2014-2019). We determined eGFR using four pediatric formulas (2-17 years: CKiD_SCr, CKiD_Cys, CKiD_SCr-Cys, Zappitelli_SCr-Cys) and four adult formulas (18 + years: MDRD_SCr, CKD-EPI_SCr, CKD-EPI_Cys, CKD-EPI_SCr-Cys). One eGFR per patient was included (most recent eGFR for those with serial measurements). Patients were categorized as chronic kidney disease (CKD) stage 2 (eGFR 60-89) and Stage 3+ (<60). Non-parametric tests, linear regression, and Spearman's correlation were used for analysis.

Among 209 children with SB (median age 10.3 years), depending on the formula used, eGFR decreased by -0.7 to -1.8 ml/min/1.73 m²/year (CKiD_Cys, CKiD_Scr, p ≤ 0.001), remained stable (CKiD_SCr-Cys, p = 0.41), or increased by +2.7/year (Zappitelli_SCr-Cys, p < 0.001) (Figure). The proportion of children with CKD 2 or higher varied between formulas (11.5-58.9%, p < 0.001). Correlations between pediatric formulas were negligible to moderate. Comparing any two formulas, 12.0-65.6% of children were assigned a different CKD stage. Among 164 adults (median age 26.3), eGFR decreased for each formula (range: -1.3 to -2.2/year, p ≤ 0.01) (Figure). The proportion of adults with CKD 2 or higher varied between formulas (9.2-30.5%, p < 0.001). Correlations between adult formulas were moderate to very high. Comparing any two formulas, 8.5-26.8% of adults were assigned a different CKD stage.

We cannot reliably determine whether eGFR changed during childhood. Among adults, eGFR decreased with age for every formula evaluated at greater than twice the general population rate. Without a validated SB-specific eGFR formula, we are left with formulas providing different results.

Estimated GFR among adults with SB appears to deteriorate at a higher rate than in the general population. Due to lack of precision of accepted eGFR formulas in the SB population, this may be a real phenomenon, an artifact of inaccurate eGFR formulas, or both. A validated SB-specific eGFR formula is needed.