What does low urine specific gravity in the presence of high serum osmolality indicate?

Low urine specific gravity in conjunction with high serum osmolality is indicative of diabetes insipidus. In diabetes insipidus, there is a deficiency in the production of antidiuretic hormone (ADH) or a lack of response to ADH by the kidneys. This results in the kidneys being unable to concentrate urine effectively, leading to the excretion of large volumes of dilute urine, which is characterized by low specific gravity.

High serum osmolality occurs because the body is losing excessive water through urine without a concomitant loss of solutes, causing the remaining serum to become more concentrated. This is a key difference from other conditions like syndrome of inappropriate antidiuretic hormone secretion (SIADH), where serum osmolality would typically be low due to water retention and concentrated urine.

In diabetes mellitus, even though it can lead to polyuria, it usually presents with high urine osmolality due to the presence of glucose in the urine, which causes an osmotic diuresis. Acute kidney injury might result in altered urine output, but it typically does not lead to low specific gravity and high serum osmolality in this manner. Thus, the presence of low urine specific gravity alongside high serum osmolality strongly supports the