Specific Gravity and Osmolality

A refractometer can read 1.035 on a urine while the reagent strip reads 1.010 on the same specimen. Both results can be correct, because the methods measure different properties. Specific gravity reflects urine density, and osmolality reflects the number of dissolved particles. Large solutes can therefore raise specific gravity without producing a proportional increase in osmolality.^{11 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.}

Specific gravity is the ratio of the density of urine to the density of an equal volume of water, which has a specific gravity of 1.000. Both the number and the molecular mass of dissolved particles affect urine density.

Glomerular filtrate entering the renal tubules has a specific gravity near 1.010. A persistent value near this level is described as isosthenuric; values below it are hyposthenuric, and values above it are hypersthenuric. Random urine specimens commonly range from approximately 1.003 to 1.035, although very dilute physiologic urine may approach 1.001. Specific gravity alone cannot establish that a specimen has been substituted. ^{11 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.}

For the anatomic basis of these findings, see Nephron Structure and Urine Formation.

Refractometry

A refractometer measures the refractive index of a small urine sample and converts the measurement to a specific-gravity result. Because solute mass affects refractive index, high concentrations of protein or glucose and the presence of radiographic contrast media or plasma expanders can increase the result beyond the physiologic range.

Do not apply general arithmetic corrections for protein or glucose. Apply only corrections validated for the instrument and specified in the manufacturer's procedure. When large solutes interfere with refractometry, osmolality measures particle concentration without the effect of molecular mass. In addition to routine quality control, College of American Pathologists (CAP)-accredited laboratories verify refractometers with specific-gravity capability at least annually using solutions of known specific gravity.^{22 College of American Pathologists. Urinalysis Checklist. 2025.}

Reagent strip

The reagent-strip pad contains a polyelectrolyte in an alkaline medium. As ionic strength increases, the polyelectrolyte releases hydrogen ions. Bromothymol blue detects the resulting pH change, with the pad changing from blue near a specific gravity of 1.000 through green to yellow near 1.030. Results are reported in increments of 0.005.

The reagent strip estimates ionic strength. It is therefore relatively insensitive to nonionic solutes such as glucose, urea, and contrast media. Alkaline urine and high protein concentrations can also affect the result. Apply only the corrections specified by the manufacturer.

Osmolality

Osmolality measures the number of dissolved particles per kilogram of solvent, independent of their molecular mass. It therefore reflects total particle concentration more directly than a density measurement. Urea illustrates the distinction: it is a small nonelectrolyte and a major urinary osmole. Each urea molecule contributes one particle to osmolality, while its mass also contributes to specific gravity.

Freezing-point osmometers cool the sample several degrees below its freezing point, often to about −7 °C, and then initiate crystallization. The instrument measures the brief thermal plateau at the true freezing point. One mole of a nonionizing solute per kilogram of water lowers the freezing point by 1.86 °C. Vapor-pressure osmometers instead measure dew point using microliter samples. Freezing-point instruments detect volatile solutes such as ethanol, and vapor-pressure instruments miss them.

Serum osmolality reference intervals vary slightly among laboratories and are commonly approximately 275 to 295 mOsm/kg. Urine osmolality changes substantially with hydration and therefore does not have a single reference interval; reported values range from approximately 50 to 1400 mOsm/kg. Evaluation of renal water handling commonly compares urine and serum osmolality. A validated concentrating test may use urine osmolality without a paired serum measurement. ^{11 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.}

Urinometry, which uses a weighted float, is now primarily of historical interest. Oscillation densitometry remains a valid method for measuring density, but refractometry is more common in routine urinalysis.