Glomerular filtration rate in infants

The use of plasma creatinine concentration for estimating glomerular filtration rate in infants, children, and adolescents

The formula GFR = kL/Pcr can be used to estimate GFR in infants, children, and adolescents who have grossly normal body habitus and are in steady-state condition. GFR is expressed in ml/min per 1.73 m2 BSA, L represents body length in cm, Pcr represents plasma creatinine concentration in mg per dl and k is a constant of proportionality that reflects the relationship between urinary creatinine excretion and units of body size. The value of k varies as a function of age and sex being 0.33 in preterm infants, 0.45 in full-term infants, 0.55 in children and adolescent girls, and 0.70 in adolescent boys. The advantages of rapid determination, reasonable accuracy, and the avoidance of urine collection justify the use of this formula in pediatric patients.

Reliability of plasma creatinine measurement in infants and children

Plasma creatinine (Pcr, mg/dl) is often used to estimate glomerular filtration rate (GFR) in children. To establish whether the clinician can rely on the commonly used methods for measuring Pcr, the authors analyzed data from their own modified Technicon Autoanalyzer reference method and compared them to those obtained simultaneously from a Beckman Astra 8 kinetic Jaffe technique or a Technicon continuous flow Jaffe endpoint SMAC method. The SMAC method consistently overestimated Pcr by 0.1 mg/dl, whereas the kinetic method resulted in a large spread around the reference values. Neither laboratory gave consistent results for Pcr below 0.55, the normal range for infants and young children. The SMAC technique tended to underestimate GFR by 20 to 30 percent, whereas the kinetic method resulted in a great deal of scatter (only 37% of the measurements fell within +/- 25% of the values for GFR obtained by the reference method). The results suggest that the subtraction of 0.1 mg/dl from the Pcr measured on the SMAC system would give a value similar to that obtained with the reference method. This correction would permit the use of an estimate of GFR from kL/Pcr, where L is body length in cm and k is a constant (equalling 0.45 in term infants, 0.55 in children, and 0.7 in adolescent boys). The kinetic method requires repetitive determinations of Pcr before any firm conclusions can be drawn about GFR because of the scatter. The reliability of the clinician's laboratory can be tested by sending half the plasma to the laboratory on one day and the other half the next.(ABSTRACT TRUNCATED AT 250 WORDS)

A simple estimate of glomerular filtration rate in low birth weight infants during the first year of life: noninvasive assessment of body composition and growth

The management of the preterm infant often requires rapid assessment of glomerular filtration rate (GFR). We sought to develop a screening test using GFR = kL/Pcr, where GFR is expressed as ml/min/1.73 m2, L is body length in centimeters, Pcr is plasma creatinine concentration, and k is a constant that depends on muscle mass. The value for k in 118 appropriate for gestational age preterm infants (0.34 +/- 0.01 SE) was significantly less than that of full-term infants (0.43 +/- 0.02, P less than 0.001). There was no difference between 12- to 24-hour single-injection inulin clearance and either 0.33 L/Pcr or creatinine clearance in preterm infants. We compared the body habitus of preterm and full-term infants using the assessment of muscle mass from urinary creatinine excretion (UcrV) and from upper arm muscle area (AMA) and volume (AMV), and that of fatness from the sum of five skinfold thickness measurements. During the first year of life, premature infants were found to have a lower percentage of muscle mass than term infants did. On the other hand, they took on a relatively greater amount of subcutaneous fat. There was a very good correlation between AMA or AMV and urinary creatinine excretion (r = 0.91 and 0.94, respectively) in 68 infants with heterogeneous body composition during the first year, indicating the validity of the urinary creatinine measurement. Absolute GFR (ml/min) was also well estimated from AMA or AMV factored by Pcr. We conclude that GFR can be well estimated from 0.33 L/Pcr in preterm infants. The lower value for k reflects the smaller percentage of muscle mass in preterm versus term infants. As a screening test, 1.5 X k or 0.05 L/Pcr predicted low values of GFR with an efficiency of 73%, specificity of 67%, and sensitivity of 88%.

Estimation of digoxin dosage in VLBW infants using serum creatinine concentrations

Digoxin steady state plasma concentrations (Css) and the corresponding serum creatinine concentrations were studied in 17 VLBW infants. Birth weight was in the range of 760-1 500 g (mean 1 068 g), gestational age ranged from 26 to 32 weeks (mean 28.7 weeks). Digoxin steady state plasma concentrations were found in the range of 0.5-6.5 ng/ml (mean 1.88 ng/ml) during maintenance therapy with 1.6-8.4 micrograms/kg BW/24 h (mean 4.4 micrograms/kg BW/24 h) given in two divided doses intravenously. No digoxin-like immunoreactive substance could be detected in the plasma of 18 infants (10 patients with a birth weight less than or equal to 1 500 g, 8 patients with a birth weight of 2 100-4 730 g) that were not treated with digoxin. The calculated digoxin clearance ranged from 0.38-4.03 ml/min/kg BW. Serum creatinine concentrations were found in the range of 35-274 mumol/l (0.4-3.1 mg/100 ml). A hyperbolic correlation may be derived from the digoxin clearance and the corresponding serum creatinine concentration. A linear relationship was observed between the dose normalized digoxin concentrations (y = Css/dose in 24 h) and the respective creatinine concentrations x (y = 0.52x-0.05; n = 17; s = 0.24; r = 0.86; p less than 0.01). According to this equation we suggest a dosing schedule for digoxin in VLBW infants with impaired renal function. Digoxin maintenance dose is derived from the digoxin target and the creatinine serum concentration. This dose recommendation proved reliable on four VLBW infants (birth weight 770-1 260 g) with decreased renal function.