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Spot urine samples for evaluating solute excretion in the first week of life
V Matos1, A Drukker, J P Guignard
1Nephrology Unit, Department of Paediatrics, University Hospital Centre, Lausanne, Switzerland.
Insights
Urinary creatinine concentration is unreliable for standardizing solute excretion in newborns. Levels are high and variable in the first week of life, unlike in older children.
Area of Science:
- Pediatric Nephrology
- Neonatal Physiology
- Clinical Chemistry
Background:
- Standardizing urinary solute excretion is crucial for neonatal health assessment.
- Urinary creatinine is commonly used as a reference in older children and adults.
Purpose of the Study:
- To determine if urinary creatinine concentration reliably standardizes urinary solute excretion in spot urine samples during the first week of life.
Main Methods:
- Collected spontaneous urine samples from 48 healthy neonates (1-6 days old) and 168 older children (1 month-3 years).
- Analyzed urinary creatinine concentration and creatinine:osmolality ratios.
- Compared values between neonates and older children, and between first and repeat samples.
Main Results:
- Neonates exhibited significantly higher urinary creatinine concentrations and creatinine:osmolality ratios compared to older children.
- These values were more variable in neonates and higher in their first urine samples than in subsequent samples.
- Urinary parameters remained stable in older children across repeat samples.
Conclusions:
- Urinary creatinine concentration is high and variable in the first week of life, even when corrected for osmolality.
- This variability contrasts with the stability observed in older children and adults.
- Urinary creatinine and the creatinine:osmolality ratio are not suitable for standardizing urinary solute excretion in neonates during their first week of life.
Aim:
To evaluate whether the urinary creatinine concentration is a reliable reference value to standardise urinary solute excretion in a spot urine sample during the first week of life.
Methods:
Spontaneously voided urine specimens were obtained in 48 healthy, full term neonates, aged 1 to 6 days (median 2.4) and in 168 healthy older children with a median age of 1.5 years (range 1 month to 3 years). In 62% of the children two urine samples were available with an interval of 2 to 4 (neonates) and 7 days (older children).
Results:
In neonates both the urinary creatinine concentration and the urinary creatinine:osmolality ratios were significantly higher than in the older children, and were spread over a wider range. During the first postnatal week of life the mean urinary creatinine and urinary creatinine: osmolality ratio values in the first urine samples were also significantly higher than in the second samples. In children aged between 1 month and 3 years of age, these data were remarkably stable without any significant changes between repeat urine samples.
Conclusions:
The urinary creatinine concentration during the first days of life is high and variable, even when corrected for urinary osmolality. This is the opposite of what is found in older children and adults. Urinary creatinine and the urinary creatinine:osmolality ratio, therefore, cannot be used to standardise the urinary excretion of solutes in the first week of life.