Areliable and accurate assessment of glomerular filtration rate (GFR) is critical for diagnosing acute and chronic kidney impairment, intervening early to prevent end-stage renal failure, prescribing nephrotoxic drugs and drugs cleared by a failing kidney, and monitoring for side effects of medications. Estimation of GFR using exogenously administered substances is well established and precise, but these methods are cumbersome and time consuming [1].

Plasma creatinine is the most commonly used index for estimating renal function in the clinical practice. Due to its small size and lack of protein binding, it is freely filtered through the glomerulus. However, it is also actively secreted by the proximal tubules at unpredictable rates. Moreover, with decreasing GFR, the fraction of tubular secretion increases, leading to an over-estimation of 10-40% when compared to that of inulin clearance [2]. Especially in children, estimation of creatinine is difficult, as there is a muscle mass related increase in plasma creatinine in children after 2 years of life. Moreover, plasma creatinine may change in cases of excessive dietary intake of meat, malnourished children and anorectic adolescents [3]. On the other hand, cystatin C is produced endogeneously at a constant rate, is freely filtered by the glomerulus, and is completely reabsorbed and catabolised by the renal tubule cells. Blood levels of cystatin C have been found to be a reliable indicator of renal function. The levels of cystatin C are independent of age, height, obesity and malnutrition [3,4]. Recent studies also suggest that serum cystatin-C is better than serum creatinine in detecting acute kidney injury in critically ill children [2]. Due to high cost, difficult assay methodologies and standardization, and non-availability of definite cut-off values, cystatin C has still not replaced creatinine in the clinical practice.

It is important for pediatricians to understand that children and adolescents with early chronic kidney disease and a well-maintained fluid and electrolyte balance, the urinalysis may be entirely normal. Therefore, a reduced GFR may serve as the only clinical sign of kidney damage. Early intervention in the course of renal impairment offers the best chance of preventing end stage renal disease in children. There currently exists no equation for monitoring acute changes in GFR [8]. However, the equations developed till now, may be able to determine longitudinal changes in GFR over time. The parameters used in the equation may be used on the locally available marker, which has been standardized according to the local laboratory.

Funding: None; Competing interests: None stated.

1. Schwartz GJ, Brion LP, Spitzer A. The use of plasma creatinine concentration for estimating glomerular filtration rate in infants, children, and adolescents. Pediatr Clin North Am. 1987;34:571-90.

2. Andersen TB, Eskild-Jensen A, Frøkiaer J, Brøchner-Mortensen J. Measuring glomerular filtration rate in children; can cystatin C replace established methods? A review. Pediatr Nephrol. 2009;24:929-41.

3. Hari P, Bagga A, Mahajan P, Lakshmy R. Effect of malnutrition on serum creatinine and cystatin C levels. Pediatr Nephrol. 2007;22:1757-61.

4. Bökenkamp A, Domanetzki M, Zinck R, Schumann G, Byrd D, Brodehl J. Cystatin C — a new marker of glomerular filtration rate in children independent of age and height. Pediatrics. 1998;101:875-81.

5. Schwartz GJ, Muñoz A, Schneider MF, Mak RH, Kaskel F, Warady BA, et al. New equations to estimate GFR in children with CKD. J Am Soc Nephrol. 2009;20:629-37.

6. Hari P, Biswas B, Pandey R, Kalaivani M, Kumar R, Bagga A. Updated height- and creatinine-based equation and its validation for estimation of glomerular filtration rate in children from developing countries. Clin Exp Nephrol. 2012;16:697-705.

7. Hari P, Ramakrishnan L, Gupta R, Kumar R, Bagga A. Cystatin-C based glomerular filtration rate estimating equations in early chronic kidney disease. Indian Pediatr 2014;51:273-77.