Height is measured in centimeters. Serum creatinine is measured in milligram per deciliter. This formula is most useful when body habitus and muscle mass are reasonably normal andwhen renal function is relatively stable. This formula may bemost accurate in the range of 15 to 75 mL/min per 1.73 m2. Values greater than 75 should be reported as such rather than as aspecific number. Creatinine clearance ([UCr]V/[PCr]) estimatesGFR but overestimates GFR when renal function is decreased.
Urinalysis is a useful screen for renal abnormalities. In addition to urine color and turbidity, macroscopic urinalysis uses a urine dipstick for pH and the presence of protein, glucose, ketones, blood, and leukocytes. Dilute urine may result in a false-negative result for protein; false-positive results may occur with extremely alkaline or concentrated urine or if there is a delay in reading the test. Dipsticks are exquisitely sensitive to the presence of hemoglobin (or myoglobin); there are few false-negative test results but many false-positive results. Glucose is detected via a glucose oxidase-peroxidase reaction, and leukocytes are detected via a leukocyte esterase reaction. The nitrite test may detect bacteriuria if the bacteria reduce nitrate to nitrite and have long contact time with the urine. False-negative results occur with frequent voiding, low urine bacterial count, urinary tract obstruction, and infection with bacteria unable to generate nitrite. Gross hematuria or prolonged contact (uncircumcised boys) may result in a false-positive nitrite test. Microscopic urinalysis is used to confirm pyuria and hematuria and detect casts and crystals.
Proteinuria can be further defined by determination of a spot urine protein/creatinine (UPr/Cr) in a single urine specimen.This unit-less value correlates very well with 24-hour urine protein excretion: spot UPr/Cr approximates 24-hour urine protein/ m2/day (normal <0.20; nephrotic range >2.0 in children).
Ultrasound reliably assesses kidney size, determines degree of dilation, and differentiates cortex and medulla. The bladder also can be visualized. Pulsed Doppler studies assess arterial and venous blood flow and can be used to calculate the kidneys’ resistive index.
A voiding cystourethrogram involves repeated filling of the bladder to detect vesicoureteral reflux and to evaluate the urethra. Computed tomography and magnetic resonance imaging have mostly replaced the intravenous pyelogram to evaluate kidney structure and function. Radionuclide studies can define renal size, scars, and renal function/excretion.
Small amounts of protein are found in the urine of healthy children (<4 mg/m2/hour or UPr/Cr <0.2). Nephrotic proteinuria in children is defined as protein greater than 40 mg/m2/hour or U Pr/Cr >2.0. Proteinuria between these levels is mildly to moderately elevated but not nephrotic.
Proteinuria may be transient or persistent, asymptomatic or symptomatic, and orthostatic (present in the upright position but not in the recumbent position) or fixed (present in all positions). Proteinuria may be glomerular (disruptions of the normal glomerular barrier to protein filtration) or tubular (increased filtration, impaired reabsorption, or secretion of proteins).