Pediatric Annals

Interpreting Urinalysis in the Newborn

Abdul Jamil Khan, MD

Abstract

1. Pryles, C. V., and Lustik, B. Laboratory diagnosis of urinary tract infection. Pediatr. Clin. North Amer. 18 (1971), 233.

2. Thompson, J. Observation on the urine of the newborn infant. Arch. Dis. Child. 19 (1944), 169.

3. Royer, P., and Habib, R. The kidney in the newborn. In Royer, P., and Walsh, A. (eds.). Pediatric Nephrology. Philadelphia: W. B. Saunders Company, 1974, p. 116.

4. Rhodes, P. G., Hammel, C. L., and Berman, L. B. Urinary constituents of the newborn infant, J. Pediatr. 60 (1961), 18.

5. Rubin, M. I., and Baliah, T., Urinalysis and its clinical interpretation, Pediatr. Clin. North Am. 18 (1971), 245.

6. Rubin, M. I., and Baliah, T., Urine and urinalysis. In Rubin, M. I., and Barrat, T. M. (eds.). Pediatric Nephrology. Baltimore: The Williams and Wilkins Company, (1974), p. 85.

TABLE 1

CAUSES OF ANURIA AND OLIGURIA

TABLE 2

DISORDERS OF pH AND SPECIFIC GRAVITY

TABLE 3

ABNORMALITIES OF FORMED ELEMENTS

TABLE 4

CAUSES OF PROTEINURIA IN THE NEONATE

TABLE 5

TESTS FOR MELLITURIA AND THEIR INTERPRETATION…

Careful examination of the urine is a vital tool in the management of the sick newborn. Variations from established norms may be indicative of either primary genitourinary or systemic illness, even in the absence of obvious clinical signs. In order to properly interpret urinary findings in the neonate, the clinician must have an understanding of both normal and abnormal renal physiology. The need for prompt recognition of abnormal urinary findings cannot be overemphasized, since misinterpretations and failure to institute necessary therapeutic measures may lead to permanent damage or death.

Renal disorders during the neonatal period are of special significance because of structural and functional immaturity of tubules and glomeruli, progressive, rapid maturation of renal function, need for an urgent diagnosis to prevent permanent damage or, in some cases, death, and the special requirements of dosage and route of administration of pharmacologic agents at this age. The diagnosis of renal disorders may be delayed because signs and symptoms can be easily confused with those of neurologic, respiratory, or infectious diseases.

The most common group of clinically significant renal disorders in the newborn infant are congenital and include hydronephrosis, posterior urethral valves, polycystic and multicystic kidney, Wilms' tumor (rare), and absence of one or both kidneys. Other frequent disorders are urinary tract infection (UTI) and renal-artery or -vein thrombosis. Such diseases as nephritis or nephrosis are extremely rare in this age. In addition, the kidney may be secondarily affected by such nonrenal conditions as sepsis, asphyxia, congestive heart failure, dehydration, and metabolic disorders, such as cystinosis or glycogen-storage disease.

URINE COLLECTION AND EXAMINATION

The reliability of urinalysis in the evaluation of a neonate with suspected or proved renal disease is dependent on the method of collection, the character of recent fluid and food intake, and the freshness of the specimen.

A clean catch specimen, obtained by bagging after cleaning the perineum, is usually sufficient in most instances. However, if infection is suspected, a suprapubic bladder tap may also be indicated.1 A dipstick (Dipstix, Ames Company, Elkhart, Ind.) test is performed to determine the pH and to detect the presence of protein, glucose, blood, and acetone in the urine. The test paper is dipped in the urine once, and the change in the color is matched with the standard control provided on the bottle to quantitate (approximately) various abnormalities. Specific gravity may be determined by a T.S. meter (American Optical Company, Buffalo, N. Y.). For microscopic examination, a few drops of shaken and uncentrifuged urine is mounted on a slide. In addition, 5 ml. of the same urine is centrifuged for three minutes at 3,000 rpm/min. Supernatant (except for the bottom 0.5 ml.) is discarded. The sediment (1-3 drops) is mounted on another slide and examined under high power for the presence of white and red blood cells, bacteria, casts and crystals.

Volume. Urinary output is relatively low on the first day of life (15-30 ml./kg.).2-3 This is probably due to both a low glomerular filtration rate and low fluid intake. Failure to pass urine (anuria) or decreased urinary output (oliguria) may be secondary to urinary-tract obstruction, renal agenesis or hypoplasia, multicystic kidney, hypotension, sepsis, cortical or tubular necrosis, or renal- vein or -artery thrombosis. Urinary output increases progressively after the first day, so that volumes that may be normal during the early neonatal period can be considered indicative of oliguria at an older age.

Physical and chemical characteristics. The first voided urine is usually dark and slightly turbid, becoming more pale and yellow with advancing age. The average specific gravity is 1.012 during the first three to four days of life and then decreases to about 1.002 to 1.006. The maximum specific gravity observed during the neonatal period is 1.018 to 1.021. Urinary pH ranges from 6 to 7 initially but can normally decrease to 5 or less after the first few days. Transient proteinuria during the first three days is not uncommon (about 21 per cent) in both normal-term newborns and premature infants.4 Beyond this age, small amounts of protein (10-25 mg./24 hours, or 5-10 mg./dl.) are present in the urine of healthy newborn infants. This is associated with a negative or, at most, trace reaction on dipstick testing. A false-positive dipstick reaction for protein during the first week may be due to the presence of urates. Transient glycosuria also may be found in 25 per cent of newborn infants. Leukocyturia is normally absent, but centrifuged urine may contain up to 10 WBC/cu. mm. during first week of life. Uric acid crystals are commonly seen.

ABNORMALITIES OF URINE

Various abnormal findings, including presence of protein, blood, sugar, and formed elements may occur alone or in combination. They are often accompanied by oliguria or anuria.

Abnormal volumes. Absent or decreased urination results from a variety of causes (Table 1). A normal infant failing to void at the end of 24 hours, and especially after 48 hours, should be strongly suspected of a serious nephrourologic disorder often accompanied by azotemia and electrolyte disturbances. Such an infant should be transferred to a neonatal intensive-care unit if adequate facilities for diagnosis and treatment are not available at the hospital of birth.

Abnormal pH. Urinary pH varies from maximally acid (4.5) to maximally alkaline (8.5); therefore one or two pH readings by themselves may not be of any value. Various acidbase disorders of renal and nonrenal origin (Table 2) may be suspected on the basis of a persistently abnormal pH.

Specific gravity. The maximum concentrating ability of the neonatal kidney is about 600 to 700 mOsm/L. This increases over the first three months of life until a normal adult value of 1,200 mOsm/L is attained. A number of disorders (Table 2) may lead to poor concentrating ability, with a specific gravity below 1.006. Occasionally, unusually high urinary specific gravities may be encountered. This may be secondary to glycosuria, proteinuria, presence of radiopaque dyes in the urine, and inappropriate secretion of antidiuretic hormone.

Table

TABLE 1CAUSES OF ANURIA AND OLIGURIA

TABLE 1

CAUSES OF ANURIA AND OLIGURIA

Table

TABLE 2DISORDERS OF pH AND SPECIFIC GRAVITY

TABLE 2

DISORDERS OF pH AND SPECIFIC GRAVITY

Hemoglobinuria and hematuria. The dipstick reacts to free hemoglobin or myoglobin, as well as to intact red blood cells. The test is extremely sensitive, detecting a hemoglobin concentration as low as 0.003 mg./L. The presence of more than two or three red cells per high-powered field in fresh, centrifuged urine is abnormal and can be found in renal as well as nonrenal conditions (Table 3). The presence of red-cell casts indicates glomerular pathology, but its absence does not eliminate such disorders.

Presence of free hemoglobin in the urine may or may not accompany hematuria. Most renal disorders presenting with hematuria usually are associated with biochemical disturbances, such as changes in BUN, serum creatinine, and electrolyte concentrations. A neonate with hematuria certainly needs further investigation, including urologie procedures for the diagnosis and treatment.

Table

TABLE 3ABNORMALITIES OF FORMED ELEMENTS

TABLE 3

ABNORMALITIES OF FORMED ELEMENTS

Proteinuria. Transient proteinuria may occur in the absence of pathology or may accompany asphyxia, cardiac failure, dehydration, fever, massive doses of penicillin, and urinary-tract infection. Persistent proteinuria is extremely rare and is observed in neonatal nephritis and the congenital nephrotic syndrome. Proteinuria (primarily globulins) accompanying tubular disorders (Table 4) is usually of a lesser magnitude than that found with glomerular disorders. Aminoaciduria is not uncommon in the presence of tubular dysfunction.

Persistent proteinuria requires an immediate diagnosis, which includes a quantitative determination of 24-hour urinary protein excretion, tests of glomerular and tubular functions and, if necessary, a renal biopsy.

Mellituria. Both glucose and other sugars (such as fructose and galactose) may be present in the urine of newborn infants. The reagent embedded in the dipstick is glucose oxidase, which is specific for glucose (at a minimum) urinary concentration of 100 mg./dl. and does not detect other reducing substances. In order to detect the presence of other sugars, the use of Benedict's solution or Clinitest tablets (Ames Company) is required. In testing urine for the presence of sugars, care must be taken to rule out both false-positive and false- negative findings (Table 5).

Table

TABLE 4CAUSES OF PROTEINURIA IN THE NEONATE

TABLE 4

CAUSES OF PROTEINURIA IN THE NEONATE

MICROSCOPIC EXAMINATION

The microscopic examination of urine should be performed on a freshly voided specimen. If this is not possible, the specimen should be immediately refrigerated. The person performing the examination must be able to properly identify both cellular (blood cells and bacteria) and noncelluar elements.

Table

TABLE 5TESTS FOR MELLITURIA AND THEIR INTERPRETATION

TABLE 5

TESTS FOR MELLITURIA AND THEIR INTERPRETATION

Pyuria and bacteriuria. The presence of one or more WBCs/high-power fields in uncentrifuged urine or five or more in urinary sediment is abnormal1,5,6 and has been correlated with bacterial colony counts of more than 105 organisms/ml. This finding is indicative of urinary-tract infection about 50 per cent of the time;1 noninfectious factors are responsible for the remaining 50 per cent (Table 2).

Casts. The presence of occasional hyaline or granular casts may be normal. Large numbers of these casts (cylindruria) or the presence of white- or red-blood-cell, bacterial, or fatty casts are usually abnormal findings (Table 3), and may point to specific disorders.

CrystaIIuria. Usually, identification of crystals in the urine of a newborn infant is of little clinical importance. However, the presence of cystine, uric acid, oxalate, xanthine, leucine, or tyrosine crystals may point to an underlying metabolic disorder.

A very high acid urine may lead to precipitation of uric acid crystals and may be a cause of abdominal pain.

CONCLUSION

Routine urinalysis, including microscopic examination, should be performed on every newborn infant, since abnormalities may exist even in the absence of signs and symptoms of illness. In sick newborn infants, repeated careful examinations of the urine are necessary for optimal medical management. In general, it can effectively enable a clinician to identify newborn infants with significant renal disease(s) and refer the patient to the proper medical facility.

BIBLIOGRAPHY

1. Pryles, C. V., and Lustik, B. Laboratory diagnosis of urinary tract infection. Pediatr. Clin. North Amer. 18 (1971), 233.

2. Thompson, J. Observation on the urine of the newborn infant. Arch. Dis. Child. 19 (1944), 169.

3. Royer, P., and Habib, R. The kidney in the newborn. In Royer, P., and Walsh, A. (eds.). Pediatric Nephrology. Philadelphia: W. B. Saunders Company, 1974, p. 116.

4. Rhodes, P. G., Hammel, C. L., and Berman, L. B. Urinary constituents of the newborn infant, J. Pediatr. 60 (1961), 18.

5. Rubin, M. I., and Baliah, T., Urinalysis and its clinical interpretation, Pediatr. Clin. North Am. 18 (1971), 245.

6. Rubin, M. I., and Baliah, T., Urine and urinalysis. In Rubin, M. I., and Barrat, T. M. (eds.). Pediatric Nephrology. Baltimore: The Williams and Wilkins Company, (1974), p. 85.

TABLE 1

CAUSES OF ANURIA AND OLIGURIA

TABLE 2

DISORDERS OF pH AND SPECIFIC GRAVITY

TABLE 3

ABNORMALITIES OF FORMED ELEMENTS

TABLE 4

CAUSES OF PROTEINURIA IN THE NEONATE

TABLE 5

TESTS FOR MELLITURIA AND THEIR INTERPRETATION

10.3928/0090-4481-19790201-07

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