The urinary density: a useful test in children

The Urine density is a test of concentration and dilution of the kidney; it reflects the weight of the solutes in the urine measured through a refractometer. Any alteration that occurs in the urinary density is associated with damage in the concentration function of the renal tubule; its value varies throughout the day oscillating between 1,003-1,030 g/l, being higher than 1,020 in the morning due to the restriction of liquids during the night. Newborns and infants can have a urinary density between 1,005-1,010 g/l and older children 1,010-1,025 g/l.

In general terms, a child has a relative hydration when the density is less than 1.010 g/l and relative dehydration when it is higher than 1.020 g/l. Hyposthenuria is urine with urinary density less than 1010 g/l, isostenuria with urinary density of 1010-1.020 g / l and hyperstenuria with urinary density greater than 1.020 g/l.

What pathologies alter the density in the urine?

When isostenuria is permanent during the day, some renal injury must be ruled out, which may compromise the mechanisms of concentration and dilution, as in chronic kidney disease. Hyposthenuria may occur in children with acute pyelonephritis, acute renal failure, tubule-interstitial nephritis, hyperaldosteronism, use of diuretics, supra-renal insufficiency, neurogenic insipidus diabetes and over-hydration. On the contrary, hyperstenuria can occur in febrile states, dehydration, hypovolemia, overload of solutes, administration of mannitol, proteinuria, use of contrasts, liver diseases and diabetes mellitus. Some entities that persist with low urinary density are hypercalcemia, diabetes insipidus and renal tubular defects.

The Urinary osmolality

is a measure of concentration, a chemical parameter that maintains a good correlation with the values of urinary density in the different age groups, except in newborns and infants, where low urinary densities possibly occur due to the adaptation of their concentration mechanisms; its values vary between 50-1200 mOsm/Kg and its normal range is 500-850. The osmometer is used to determine urinary osmolality, a device that is not available in clinical laboratories; therefore, in an indirect way, urinary osmolality is obtained by multiplying by 33 the last two digits of the urinary density value or through the formula [urinary osmolality = (Du-1020) x40,000].

What is a refractometer?

Refractometers are optical instruments used to determine the percentage of soluble solids in a liquid solution. For this, the refractometer makes use of the principle of total refraction of light (caused by the type and concentration of dissolved substances in a liquid solution), which takes place in the boundary layer between the prism and the sample. The refractive index of the prism determines the upper limit of the measurement range, since this must always be greater than that of the sample. The refractometer therefore measures the density of liquids, the more dense a liquid, the greater the refraction, so it is the ideal instrument for the determination of urine density.

The refractometer was invented 143 years ago by Dr. Ernst Abbe, an early German scientist who described it for the first time in 1875. That is why Abbe’s refractometer is considered the “grandfather” of all modern refractometers, thanks to its simple operation and its reliability. There are two types of refractometers based on the detection of the refractive index; transparent systems and reflection systems. Portable refractometers and Abbe refractometers use transparent systems, while digital refractometers use reflection systems.

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