Hello Nygirl,

I am no expert in this area.
However, I am able to assess some online medical textbooks
that our library subscribes to…

Here is what I found…

Urine specific gravity reflects the hydration status of the patient and the concentrating ability of the kidney. With diminished renal function, the ability of the kidneys to concentrate urine lessens progressively until the specific gravity of urine reaches 1.006–1.010. However, the ability to dilute urine tends to be maintained until renal damage is extreme. Even in uremia, although the concentrating power of the kidneys is limited to a specific gravity of 1.010, dilution power in the specific gravity range of 1.002–1.004 may still be found. Determination of urine osmolality is undoubtedly a more meaningful measurement of renal function, but determination of specific gravity lends itself to office diagnosis. (From: Schwart’s surgery and Lange Urology)

Urine osmolality is a Test which measures renal tubular concentrating ability.
In the hypoosmolar state (serum osmolality Part 11. Disorders of the Kidney and Urinary Tract > Chapter 260. Acute Renal Failure[ARF] > Clinical Features and Differential Diagnosis > Clinical Assessment >Table 260–2. Useful Clinical Features, Urinary Findings, and Confirmatory Tests in the Differential Diagnosis of Major Causes of ARF)

Chronic Renal Failure
Harrison’s Internal Medicine > Part 11. Disorders of the Kidney and Urinary Tract >
Chapter 261. Chronic Renal Failure> Clinical and Laboratory Manifestations of Chronic Renal Failure and Uremia

No tables listing lab test indicators in this chapter.
However, these excerpt may be useful:
“In most patients with stable CRD, the total body contents of Na+ and H2O are increased modestly, although this may not be clinically apparent. The underlying etiologic disease process may itself disrupt glomerulotubular balance and promote Na+ retention (e.g., glomerulonephitis), or excessive Na+ ingestion may lead to cumulative positive Na+ balance and attendant extracellular fluid volume (ECFV) expansion. Such ECFV expansion contributes to hypertension, which in turn accelerates further the progression of nephron injury.”
(In my humble opinion, this seems that Chronic Renal Failure often is accompanied
by an increased urine osmolarity)

This also may be helpful:
“Patients with CRD also have impaired renal mechanisms for conserving Na+ and H2O (Chap. When an extrarenal cause for fluid loss is present (e.g., vomiting, diarrhea, sweating, fever), these patients are prone to volume depletion. Depletion of ECFV may compromise residual renal function with resulting signs and symptoms of overt uremia. Because of impaired renal Na+ and H2O conservation, the usual indices of prerenal azotemia (oliguria, high urine osmolality, low urinary Na+ concentration, and low fractional excretion of Na+) are not useful. Cautious volume repletion, usually with normal saline, returns ECFV to normal and usually restores renal function to prior levels.”

Hoping this helps some, and that others will jump in to better
clarify!

In the meantime, I am hoping this helps some.
Email me (through icon to the left) if you think I could help out more.

Wishing you the best,
Janice

they are the same thing