Isosthenuria refers to the excretion of urine whose specific gravity (concentration) is neither greater (more concentrated) nor less (more dilute) than that of protein-free plasma, typically 1.008-1.012. Isosthenuria reflects renal tubular damage/failure of renal medullary function.
A closely related term is hyposthenuria, where the urine has a relatively low specific gravity, though not necessarily equal to that of plasma. Therefore, unlike isosthenuria, this condition is not associated with renal failure as the kidney tubules have altered the glomerular filtrate
Clinical significance
This is such disease states as chronic and acute kidney failure in which the kidneys lack the ability to concentrate or dilute the urine and so the initial filtrate of the blood remains unchanged despite the need to conserve or excrete water based on the body's hydration status.
Sickle-cell trait, the heterozygous form of sickle-cell disease, presents with a normal hematological picture but is associated with isosthenuria.