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Is specific gravity a good estimate of urine osmolality?
Sethi Imran1, Goldwater Eva, Shutty Christopher
1Berkshire Medical Center, Internal Medicine, Pittsfield, Massachusetts, USA. imransethi79@yahoo.co.in
Journal of Clinical Laboratory Analysis
|November 20, 2010
Summary
Urine specific gravity (USG) correlates moderately with urine osmolality, but this relationship weakens in pathological urines. Direct osmolality measurement is recommended for accurate results in such cases.
Area of Science:
- Clinical Chemistry
- Urology
- Laboratory Medicine
Background:
- Urine specific gravity (USG) is a common clinical tool for estimating urine osmolality.
- USG can be measured using refractometry or reagent strips.
Purpose of the Study:
- To evaluate the correlation between USG and urine osmolality using both refractometry and reagent strip methods.
- To identify factors influencing the USG-osmolality relationship.
Main Methods:
- Retrospective analysis of 504 urine specimens with simultaneously measured USG and osmolality.
- Linear regression analysis was used to assess correlations, adjusting for variables like pH, protein, and ketones.
- USG measurements were performed using automated refractometry and reagent strips.
Main Results:
- Both refractometry and reagent strip methods showed a correlation of approximately 0.75 with urine osmolality.
- Factors such as pH, ketones, bilirubin, glucose, protein, and hemoglobin affected the correlation depending on the USG method.
- Correlation was significantly poorer in pathological urines compared to "clean" urines.
Conclusions:
- While USG provides a reasonable estimate of urine osmolality in general, its accuracy diminishes in pathological conditions.
- Direct measurement of urine osmolality is advised for patients with pathological urines to ensure diagnostic accuracy.
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