Elimination

Renal drug clearance is reduced in infants and slowest in premature neonates, due to immature renal function, resulting in the need for longer dosing intervals for renally cleared medications, such as vancomycin, to prevent accumulation. Glomerular filtration rate (GFR) is lowest in premature neonates, increasing with age and peaking at 3 to 12 years of age, after which there is a gradual decline to approximate adult value. For example, vancomycin is often given every 18 to 24 hours in a low birth weight (LBW) premature neonate, every 6 hours in children with normal renal function, and every 8 to 12 hours in adult patients with normal renal function. Children with cystic fibrosis also present with greater renal clearance of drugs such as aminoglycosides, compared to children without the disease, requiring higher doses by weight and more

frequent dosing intervals.

Pediatric creatinine clearance (CrCl), an indicator of GFR, is normalized due to variable body size (mL/min/1.73 m ). The use of the Cockroft-Gault or Jelliffe equations for estimating CrCl in adults is not recommended for patients less than 18 years 22,23

of age. ' Schwartz's equation is a common method of estimating pediatric CrCl for LBW infants up to 21 years of age (Fig. 3-2). This equation utilizes patient length (cm), serum creatinine (mg/dL), and a constant, k, which is dependent on age for all patients and also gender for those greater than 2 years of age. 4 Urine output is also a parameter used to assess renal function in pediatric patients, with a urine output of greater than 1 to 2 mL/kg/h considered normal.

Age

r

Low birth weight <1 year of age

0.33

Full lerm ¿1 year of age

0.45

2-12 years (male or female) or 13-21 years (female)

0.55

13-21 years (male)

0.70

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