We evaluated a new eGFR equation for children under 2 years of age. Using several statistical techniques, we confirmed that the eGFRunder 2 equation could be useful.
For Japanese children, there are several eGFR equations with surrogate markers, serum Cr [
3], cystatin C [
7], and β2 microglobulin [
8]. The Cr-based eGFR
Uemura equation is the most commonly used method for children aged 2–18 years. Cystatin C and β2 microgrobulin based eGFR are not always available for retrospective epidemiologic studies or common clinical practice. Therefore, we developed Cr-based eGFR for children younger than 2 years using eGFR
Uemura using a coefficient. The method reported in a previous article [
5] is based on the following idea. Physiologically, kidney function gradually increases from birth and reaches adult stage at the age of 2 years. Using the median normal reference values of GFR for each age (months) up to 2 years examined previously, we estimated the percentage of the normal adult GFR that corresponds to and calculated a regression curve using a logarithmic function. As a result, the coefficient was calculated as 0.107 × ln [age (months)] + 0.656. The aim of this study was to evaluate the eGFR
under 2 equations. We compared the values of eGFR
under 2, eGFR
Uemura, and eGFR
Schwartz equation using ME, RMSE,
P30, and Bland–Altman analysis. We divided the patients according to age: 13 patients under 2 years of age and 10 patients aged between 1 and 2 years; patient data underwent two analyses. Because eGFR
under 2 equation is for children under 2 years old, the eGFR
Uemura equation is for children aged 2–18 years and eGFR
Schwartz is for children aged 1–16 years. Therefore, in the comparison of 10 patients at the age of 1 to 2 years, we only compared eGFR
under 2 equation and eGFR
Schwartz. In the analysis of 13 patients, the ME value of eGFR
under 2 was the smallest among the three equations. The RMSE value and percent of
P30 of eGFR
under 2 and eGFR
Uemura were similar. On the other hand, the ME and RMSE values of eGFR
Schwartz were the highest, and the percentage of
P30 was the lowest among the three eGFR equations. In the analysis of 10 patients, the ME and RMSE values of eGFR
under 2 were lower than that of eGFR
Schwartz. The percentage of P
30 of eGFR
under 2 was higher than that of eGFR
Schwartz.
This study has several limitations. First, the sampling data was small particular under 1 year of age. Although we collected patients’ data from our three previous studies, there were only 13 among 373 patients (3.5%) under 2 years of age. Further, it is very rare to perform inulin clearance in patients under 2 years of age, thus limiting data collection. In addition to that, there were only two patients of a female under the age of two. However, since there was no gender difference in normal Cr values at younger ages [
9], we believed that this result was not affected. Second, to add to the data as much as possible, we also used the patient data when creating the equation of eGFR
Uemura. Third, we did not exclude preterm infants and low birth weight infant because we had no information for them. We don’t know whether the eGFR
under 2 equation can be applied to such cases or not.