Blood for this test MUST NOT be collected on outpatients or GP patients without FIRST consulting a Chemical Pathologist or Registrar (3163 8607 or 3163 8872). It is usually paired with fasting serum and urine osmolality so if these are requested they should all be collected together (ie do not collect the samples for osmolality and ask the patient to return for ADH/vasopressin). Financial consent should also be obtained (vasopressin/ADH on the price list).
A separate tube must be collected for this test. A MINIMUM of 4 mL of blood must be collected for this test (6 mL for adults). Place specimen in a COLD pack immediately after collection. This test must arrive at the laboratory within 30 minutes of collection.
APPLICATION: ADH measurement is only useful in the context of a water deprivation test to diagnose diabetes insipidus or in some cases of suspected SIADH. In diabetes insipidus the test may be used, in conjunction with plasma osmolality, to distinguish between central and nephrogenic diabetes insipidus. Random ADH levels are not interpretable.
INTERPRETATION: Central diabetes insipidus is associated with low antidiuretic hormone (ADH) levels when plasma osmolality is elevated. Nephrogenic diabetes insipidus is associated with increasing levels of ADH as plasma osmolality increases.