Which diagnostic test is used for ADH deficiency?

• A. Overnight Water Deprivation Test
• B. Water Loading Test
• C. Serum Osmolality Test
• D. Urine Osmolality Test

Answer: (A)

ADH deficiency causes the kidneys to lose their ability to concentrate urine, so urine stays dilute even as the body becomes dehydrated. To reveal this defect, the Overnight Water Deprivation Test is used. By withholding water overnight, plasma osmolality rises and the kidneys should respond by concentrating urine if ADH is present. In the presence of ADH deficiency (central diabetes insipidus), the urine remains dilute despite dehydration. To confirm central origin, administering desmopressin will markedly increase urine concentration, while nephrogenic DI would show little to no response. The other tests don’t specifically diagnose ADH deficiency on their own: serum osmolality shows the body’s hydration status but doesn’t prove ADH deficiency; measuring urine osmolality alone doesn’t reveal the cause; and a water loading test isn’t the standard approach to uncover ADH deficiency.