General characteristics of laboratory urine diagnostics
Assessment of physical properties, chemical composition, and sediment microscopy are part of the complete urinalysis (UA). This study is a baseline method for identifying various urinary system pathologies. It is important to consider that the parameters have pronounced age-specific features, especially during the neonatal period.
Normally, in a healthy child, urine is completely transparent. Its color is influenced by pigments (urochromes, urobilins, uroerythrin, urorosein), as well as diuresis and age. Immediately after birth, urine is colorless, but by the 2nd–3rd day, it acquires an amber-brown tint due to the excretion of uric acid. In infants, urine becomes light yellow, while in older children, it acquires a straw-yellow or amber-yellow color.
Dynamics and normal urine pH values
Urine acidity undergoes significant changes in the first days of life and depending on the diet:
- In the first days of life, neonates have a predominantly acidic environment (pH 5.4–5.9), with this acidity being more pronounced in premature infants.
- On days 2–4, the pH value increases.
- Infants who are breastfed exhibit a neutral or slightly alkaline reaction (pH 6.9–7.8).
- Infants who are formula-fed have an acidic environment (pH 5.4–6.9), which is medically regarded as physiological acidosis.
- In older children, the urine reaction is usually slightly acidic, less frequently neutral.
Interrelation of pH, salts, and uric acid infarct
The physicochemical properties of urine are closely linked to salt crystallization processes in the inorganic sediment layer. Sediment formation depends on colloidal equilibrium, the current pH value, and the state of the urothelium.
During the neonatal period, massive excretion of uric acid leads to the formation of characteristic brick-red stains on diapers — the so-called "uric acid infarct of the kidneys." This condition is due to the fact that uric acid is the end product of purine and pyrimidine metabolism, which undergo intensive breakdown due to the destruction of nuclear cellular structures in the first days of a child's life. In older children, the appearance of uric acid sediment is not normal, but it may transiently occur secondary to excessive meat consumption, fever, or intense physical exertion.
Breast milk is alkaline (up to 7.8), formula from a bottle is acidic (up to 6.9).
Class and exam questions
What are the normal urine pH values in neonates?
In neonates, the urine environment is acidic, with pH values ranging from 5.4 to 5.9. In premature infants, this acidity is more pronounced, and on the 2nd–4th days of life, the pH value increases.
How does the feeding method affect the urine reaction?
With breastfeeding, the urine reaction is neutral or slightly alkaline (pH 6.9–7.8). With formula feeding, an acidic reaction predominates (pH 5.4–6.9), which is considered physiological acidosis.
Why might newborn urine leave brick-red stains?
This is associated with the massive excretion of uric acid ("uric acid infarct of the kidneys"). Uric acid is formed due to the breakdown of cellular nuclear structures in the first days of the child's life.
What else to study on this topic
- Relative density and specific gravity of urine in the neonatal period
- Physiological proteinuria of newborns: causes and resolution timelines
- Urinary sediment microscopy: organic and inorganic elements
- Two-glass test for topographic diagnostics
- Clinical urinalysis in leptospirosis and other renal pathologies
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