Physical Examination
Specific Gravity and Osmolality
How specific gravity and osmolality assess urine concentration, why results may disagree, and how each method is interpreted.
Specific gravity and osmolality provide indirect information about renal concentrating ability, but they measure different physical properties. Understanding these differences explains why results from the methods may not agree.
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. Urine density is affected by both the number and the molecular mass of dissolved particles. Large molecules can therefore increase specific gravity disproportionately.
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.
What each method measures
| Method | Measures | Strength | Main limitation |
|---|---|---|---|
| Refractometry | Refractive index, which is affected by particle number and mass | Requires a small sample; rapid and quantitative | Protein, glucose, contrast media, and plasma expanders may produce high results |
| Reagent strip | Ionic strength through a polyelectrolyte pKa shift | Included on the routine reagent strip | Relatively insensitive to nonionic solutes; alkaline urine and protein can affect the result |
| Osmolality | Number of dissolved particles independent of molecular mass | Closely reflects total particle concentration | Requires a separate instrument and often a paired serum result |
Refractometry
A refractometer measures the refractive index of a small urine sample and converts the measurement to a specific-gravity result. Because refractive index is affected by solute mass, high concentrations of protein or glucose and the presence of radiographic contrast media or plasma expanders can increase the result beyond the physiologic range.
General arithmetic corrections for protein or glucose should not be applied unless they have been validated for the instrument. Results should be interpreted according to the manufacturer’s procedure, and osmolality may be more appropriate when large solutes interfere with refractometry. In addition to routine quality control, CAP-accredited laboratories verify refractometers with specific-gravity capability at least annually using solutions of known specific gravity.
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 SG 1.000 through green to yellow near SG 1.030. Results are reported in increments of 0.005.
The reagent strip estimates ionic strength rather than density. 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. Only corrections specified by the manufacturer should be applied.
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. They do not detect volatile solutes such as ethanol, whereas freezing-point instruments do.
Serum osmolality reference intervals vary slightly among laboratories and are commonly approximately 275–295 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. Many assessments of water balance require a paired serum measurement, although validated concentrating tests may use urine osmolality alone.
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.
Worked example. A refractometer measures a specific gravity of 1.035 shortly after radiographic contrast administration, while the reagent strip measures 1.010 in the same specimen. Contrast material contains large nonionic molecules that increase refractive index but are not detected by the reagent-strip method. The difference is therefore explained by the measurement principles rather than by a change in the specimen. Osmolality can provide an independent assessment of particle concentration.
References
- Kouri TT, Hofmann W, Falbo R, et al. The EFLM European Urinalysis Guideline 2023. Clin Chem Lab Med. 2024;62(9):1653-1786.
- College of American Pathologists. Urinalysis Checklist, 12/09/2025 edition, requirement URN.26100. A CAP accreditation requirement, not a regulation.