Prediction Equations for Glomerular Filtration Rate in Children Based on Serum Cystatin C and Body Cell Mass

Today, children's kidney function (glomerular filtration rate, GFR) can be monitored by two methods:

1. indirectly by serum creatinine, or
2. directly by injection of a radioactive substance followed by several blood samples.

The first method is inaccurate with many drawbacks, whereas the latter is precise but time-consuming and unpleasant for the child. Therefore, there is a need for a new method for investigating GFR in children.

Serum cystatin C is a small protein that is produced with a constant rate in all nucleated cells in the body. It meets many of the characteristics of an ideal marker of GFR because of the way it is excreted in the kidneys. However, earlier studies have not proven serum cystatin C to be convincingly better than serum creatinine. Why? If there is considerable extra renal elimination, serum cystatin C alone isn't enough to estimate GFR. Therefore, the individual production rate and possible extra renal elimination of cystatin C are included in this study. To assess these factors, the children are submitted to bioelectrical impedance spectroscopy (BIS) to estimate their body composition, including body cell mass as cystatin C is produced in all nucleated cells. To validate the BIS data, dual energy x-ray absorptiometry (DEXA) will be conducted on 100 of the included children.

Based on serum cystatin C and the individual, age-corrected extra renal elimination rate of cystatin C, new algorithms to calculate GFR can be developed.