A urine osmolality of less than 400 mOsm/(kg-H₂O) means that the patient may have impaired renal tubular function. Urine osmolality refers to the total number of particles of all osmotically active solutes in the urine, which reflects the concentrating and diluting function of the kidneys. The general range of urine osmolality in a normal person is 600-1000 mOsm/(kg-H₂O) after abstinence from drinking. A persistent decrease in urine osmolality may imply that the concentrating and diluting function of the kidneys is severely impaired. Diseases with severely impaired renal concentrating function such as chronic pyelonephritis, chronic renal failure, polycystic kidneys, urinary tract obstructive nephropathy, uric acid nephropathy, uremic avalanches, and primary glomerulopathy need to be considered. The presence of decreased urine osmolality, it is recommended that patients seek further medical examination to clarify the condition and actively treat under the guidance of physicians.