Endocrine Abstracts

The biochemical investigation of suspected arginine vasopressin deficiency (AVPD, formerly cranial diabetes insipidus) has historically been with water deprivation and its effect on serum and urine osmolality. The test is lengthy and unpleasant for patients and is demanding of staff time. It often yields indeterminate results due to impairment of the concentrating ability of the renal medulla where osmotic pressure across the distal convoluted tubule and collecting duct has been diminished due to ‘washout’ caused by high volume urine output over time. Thus diagnosis is often not aided by this investigation and is made clinically on the basis of symptoms, history and the ruling out of other potential aetiologies. The measurement of copeptin, a cleaved peptide fragment from the arginine vasopressin (AVP) prohormone, offers a novel alternative biomarker for AVP (in)sufficiency. Provided here is the data from Plymouth Hospitals NHS Trust (PHNT), accumulated over 3 years, obtained from the osmotic stimulation (by L-arginine hydrochloride) of copeptin in patients in whom a diagnosis of AVPD was suspected. The protocol is a modified version of that proposed by Winzeler et al. Copeptin is measured at baseline and at 30-, 45-, 60-, 90- and 120-minutes post infusion. Biochemical outcomes are considered along with clinical details to support or refute diagnoses. A total of 20 patients have undergone this test. 6 patients had copeptin levels below the 60-minute threshold at which sufficiency is considered and are receiving treatment with DDAVP. The remaining 14 patients had a diagnosis of primary polydipsia or were normal. The experience of PHNT is that this is a reliable test offering diagnostic certainty.