Correlation of specific gravity and osmolality of urine in neonates and adults.

Abstract

SUMMARY There is good correlation between the specific gravity and osmolality of urine obtained from neonates and adults, if specimens contain neither protein nor glucose. The regression equations between specific gravity and osmolality, however, differ considerably between the two age groups, maturation to the adult regression occurring within the first 5 years of life. Specifically derived regression equations should therefore be used when predicting urine osmolality from measurement of specific gravity in neonates.The osmolality of urine is the definitive measure of its concentration. Under normal physiological conditions low urine osmolality equates with hydration, and high urine osmolality with dehydration. The ability of neonates to concentrate urine is lower than that of adults, but the capacity to dilute urine is similar. In a non-uraemic, normonatraemic, and normoglycaemic neonate, therefore, urine osmolality maintained between 75 mosmol/kg-300 mosmol/ kg indicates adequate hydration; values outside these ranges suggest overhydration and dehydration, respectively.'The necessity for monitoring urine osmolality in neonates to estimate fluid balance may make heavy demands on the laboratory. Osmometers are expensive and unsuitable for use in the wards without the help of a technician. The specific gravity of urine, traditionally measured in the ward, also reflects concentration, but the prediction of urine osmolality from specific gravity measurement may be unreliable in unskilled hands.2 In addition, abnormal concentrations of glucose and protein in the urine cause appreciable increases in specific gravity without affecting osmolality. Finally, measurement by traditional flotation hydrometry requires a large volume of urine. All these factors militate against the use of specific gravity as a measure of urine concentration in neonates.Despite recent interest in the measurement of specific gravity as an indicator of urine concentration in children, the influence of age on the relation between specific gravity and osmolality has niot been defined.3 We studied the association between urine specific gravity measured by refractometry, and osmolality, to assess the practicability of measuring the urine concentration of neoniates accurately in the ward; we also assessed the influence of age on the relation between specific gravity and osmolality. Material and methodsWe collected 100 urine samples from 21 neonates in the special care baby unit; the babies had been born at 28 to 32 weeks' gestation and mean (SD) weight was 2100 (940) g. Single specimens of urine were collected from 22 healthy babies born at full term, 88 children from a paediaitric ward (age range 1-70 months), and from 59 healthy adults. The specimens were not taken at a particular time, nor were they preserved. Urine was tested for the presence of glucose and protein using the Ames Uristix, and those specimens showing positive results were excluded.Osmolality wCas measured on fresh urine slmples by freezing point depression osmometry (Advanced Ins...

Validation of Surrogates of Urine Osmolality in Population Studies

“…Another study showed that UD was well correlated with mU osm but the wide dispersion made it “impossible to use UD as a dependable clinical estimate of U osm ” [46]. Moreover, UD or specific gravity cannot be used if urine contains proteins or glucose [47]. …”

Measurement of urinary concentration: a critical appraisal of methodologies

“…While evaluating the correlation between the refractometer measured SG and the osmolality it was observed that at the same osmolality younger children had higher SG readings as compared to older children and adults (Fig. 5) [14,25]. A similar observation of a higher SG reading at the same osmolality was also made with the reagent strip method [13].…”

“…Although the same changes are expected to cause lower refractometer SG readings, in reality refractometer SG readings in young children are higher due to high concentrations of free amino acids and low molecular weight proteins in their urine. This discrepant association between SG and osmolality shows a rapid progression towards the adult pattern during the first 2 years of life and is completed around 5 years of age [25]. It is, therefore, obvious that the equation used for estimating osmolality from the SG value in older children and adults is not valid for younger children.…”

Urine Osmolality Estimated Using Urine Urea Nitrogen, Sodium and Creatinine Can Effectively Predict Response to Tolvaptan in Decompensated Heart Failure Patients

“…We emphasize that the formula is valuable regardless of the presence of HF. Urine SG has been known to correlate with U-OSM traditionally, 16 but it appears to be too crude to estimate U-OSM precisely, as shown in previous studies. 17, 18 In the present study, urine SG was not a good predictor of TLV response.…”