Q: Is the specific gravity of urine a good estimate of urine osmolality?
Answer: Only if it is low
The urine osmolality is determined by the number of particles in the urine like urea, sodium, and potassium. On the other hand, the specific gravity is determined by both the number and size of the particles in the urine. e.g., if there is glycosuria or if there is a radiocontrast media in the urine, the specific gravity can be high but a urine osmolality will still be telling a diluted story.
Said that, if the urine specific gravity is reported low, it is 100% an indicative of diluted urine.
Imran S, Eva G, Christopher S, Flynn E, Henner D. Is specific gravity a good estimate of urine osmolality? J Clin Lab Anal. 2010;24(6):426-30. doi: 10.1002/jcla.20424. PMID: 21089176; PMCID: PMC6647580.