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%.

A simple estimate of glomerular filtration rate in adolescent boys.

We reexamined the relationship between creatinine clearance (Ccr) and body habitus in 212 girls and 356 boys, including 181 boys and 69 girls between 13 and 21 years of age. The use of formula Ccr = k L/Pcr, where k = 0.55 for the calculation of GFR, resulted in a significant underestimation of GFR in adolescent boys but was suitable for girls. In 51 adolescent boys the equation Ccr = 0.7 L/Pcr resulted in an accurate estimate of GFR. Regression analysis in 133 boys aged 3 to 21 years showed that the constant k increased gradually and linearly with age (r = 0.35, P less than 0.01). GFR could be better estimated for boys of any age by the linear bivariate equation Ccr = 1.5 (age) + 0.5 (L/Pcr), where age is given in years (r = 0.82, P less than 0.001). This equation yielded slightly better results than did 0.7 L/Pcr in 91 additional clearance studies performed in adolescent boys with native kidneys or functioning renal transplants. The larger value for the constant k (0.7) and the age correction for GFR reflect the greater rate of urinary creatine excretion (and thus muscle mass) per unit of body mass in adolescent boys.

A simple estimate of glomerular filtration rate in full-term infants during the first year of life.

An estimate of glomerular filtration rate has been derived for children from body length (L, in centimeters) and plasma creatinine (Pcr, in milligrams per deciliter): GFR = 0.55 L/Pcr. The near universality of this estimate in children led us to seek a similar formula for estimating GFR in full-term infants during the first year of life. We measured Pcr in 137 healthy infants and performed creatinine clearance (Ccr) studies in 63 of them aged greater than or equal to 5 days. Beyond the first week, Pcr averaged 0.39 +/- 0.01 (0.10 SD) mg/dl. The estimate of GFR from 0.55 L/Pcr overestimated Ccr by 24% (P less than 0.001). Based on the calculation of a new constant from Ccr X Pcr/L, GFR was more accurately estimated from 0.45 L/Pcr (mean difference of Ccr - 0.45 L/Pcr = -0.4 +/- 3.7 (SE) ml/min X 1.73 m2) in full-term infants between 1 and 52 weeks of age. Because the constant 0.45 and Pcr do not change significantly during this period, GFR can be approximated at the bedside from body length of the healthy full-term infant (GFR = 0.45 L/0.39 = 1.1 L).

Late metabolic acidosis: a reassessment of the definition.

The term "late metabolic acidosis" is generally used to define a population of apparently healthy LBW infants who fail to grow and have a base deficit in excess of 5 mEq/l (CO2TOT less than 21 mM). A relationship between hypobasemia and the lack of appropriate growth was postulated. This conclusion was reached, however, in the absence of adequate information regarding the distribution of acid-base variables in healthy LBW infants. The results of this study demonstrate that the CO2TOT of LBW infants (n = 114) rises between birth and three weeks of life from a mean of 18.6 to 20.3 mM. The frequency distribution of CO2TOT values did not show any significant deviations from normality, and 2 SD included values as low as 14.5 mM. No difference in the rate of growth was detected between "hypobasemic" infants given a solution of bicarbonate calculated to bring their blood CO2TOT to greater than 21 mM and those given similar amounts of isotonic saline solution. The ability of the LBW infants to excrete an ammonium chloride load was not related to their acid-base status and was comparable to that of term infants. It is apparent that the definition of late metabolic acidosis needs to be reconsidered.

Geometric method for measuring body surface area: a height-weight formula validated in infants, children, and adults.

Estimates of body surface area were made based on measurement of 81 subjects, ranging from premature infants to adults. SA was calculated geometrically for each subject from 34 body measurements, and the values obtained compared with those based on previously published formulas and graphs. The most widely used formula, that of Du Bois and Du Bois, increasingly underestimated SA as values fell below 0.7 m2; the disparity was greatest in the newborn infant (7.96%). Closer agreement was obtained with the equations and nomograms of Body, Brody, Faber and Melcher, and Sendroy and Cecchini, although minor deviations were noted in some age ranges. The formula SA (m2) = weight (kg)0.5378 X height (cm)0.3964 X 0.024265, derived from the measured data by multiple regression analysis, gave a good fit for all values of SA from less than 0.2 m2 to greater than 2.0 m2 (r = 0.998). This formula was used to construct nomograms for estimation of SA in infants, children, and adults from height (length) and weight.

Subtherapeutic dicloxacillin levels in a neonate: possible mechanisms.

A neonate treated initially with oxacillin intravenously for two weeks and who was receiving phenobarbital for a seizure disorder subsequently failed to achieve therapeutic levels of orally administered dicloxacillin, even when the dosage was as high as 175 mg/kg/day. Intestinal absorption was documented by high serum peak levels. The low trough levels correlated with a high urinary excretion rate. The possibillity that renal tubular transport of dichloxacillin was stimulated by administration of penicillin derivatives (and/or phenobarbital) is suggested, and the need for careful monitoring of serum levels of antibiotics in neonates is emphasized.