Arginine Vasopressin (AVP) Laboratory Test
The arginine vasopressin lab test measures the level of antidiuretic hormone (ADH) in blood or urine to diagnose disorders of water balance, particularly diabetes insipidus and syndrome of inappropriate antidiuretic hormone secretion (SIADH).
What the Test Measures
- Arginine vasopressin (AVP), also called antidiuretic hormone (ADH), is a peptide hormone that regulates water balance by controlling renal water excretion through V2 receptors and aquaporin-2 water channels 1
- The hormone is primarily regulated by plasma osmolarity, with additional non-osmotic stimuli including blood volume and stress 1
Clinical Indications for Testing
Suspected Diabetes Insipidus
- Order when patients present with polyuria, polydipsia, and inappropriately dilute urine (urine osmolality <200 mOsm/kg H₂O) combined with high-normal or elevated serum sodium 2, 3
- Extremely low AVP levels are found in cranial (central) diabetes insipidus 4
Suspected SIADH
- Order when evaluating euvolemic hypoosmolar hyponatremia with inappropriately concentrated urine 1
- Elevated AVP values are found in patients with conditions like bronchial carcinoma and hyponatremia 4
- In SIADH, 90% of cases show hypersecretion of vasopressin, while 10% have undetectable vasopressin despite antidiuresis 1
Practical Measurement Considerations
Plasma AVP Measurement Challenges
- Direct AVP measurement is technically challenging because the hormone is unstable and the assay is cumbersome 2
- Normal plasma AVP range in healthy volunteers is 0.8-14.3 pmol/L (approximately 3.3 pg/ml under mild water diuresis) 4, 5
- Blood should be centrifuged promptly, as AVP levels are highest in the lowest plasma layer because platelets serve as a reservoir for the hormone 4
Plasma Copeptin as Alternative
- For adults, measure plasma copeptin instead of direct AVP, as copeptin (CT-proAVP) is released in equimolar 1:1 ratio with AVP and is more stable 2
- Baseline plasma copeptin >21.4 pmol/L is diagnostic for nephrogenic diabetes insipidus in adults 2, 3
- Plasma copeptin <21.4 pmol/L suggests central diabetes insipidus or primary polydipsia and requires further testing with hypertonic saline or arginine infusion 2, 3
- Limited data exists for copeptin reference values in pediatric populations 2
Urinary AVP Measurement
- Urinary AVP can be measured in a 4 ml urine sample after extraction with cation exchanger, with overall recovery of 72% 6
- Normal daily urinary excretion is approximately 34 ng in women and 70 ng in men 6
- Urinary AVP clearance in healthy subjects is 7.5 ± 1.2 ml/min with fractional clearance of 6.4 ± 1.0% 5
- Urinary AVP rises during water deprivation and osmolar stimulation, with women showing more pronounced elevation than men 6
- Extremely high urinary AVP values are found in Schwartz-Bartter syndrome (SIADH) associated with bronchogenic tumors 6
Important Caveats
- Water loading generally lowers AVP levels in normal individuals 4
- In chronic renal failure, urinary clearance of ADH is significantly reduced (3.4 ± 0.6 ml/min vs 7.5 ± 1.2 ml/min in healthy subjects) 5
- Inappropriate ADH secretion can occur in conditions like asthmatic attacks, creating risk of water intoxication during fluid therapy 7
- Genetic testing is now strongly recommended as first-line for suspected nephrogenic diabetes insipidus, particularly in children, as it can avoid the need for challenging and potentially harmful diagnostic procedures like water deprivation testing 2, 3