Isosthenuria refers to the excretion of urine, the specific gravity (concentration) of which is neither greater (more concentrate) nor less (more dilute) than that of protein-free plasma. This is the hallmark of such disease states as chronic and acute renal 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 and increased frequency of urinary tract infections.