Diabetes Insipidus: Test and Diagnosis

1st order laboratory parameters – obligatory laboratory tests.

• Plasma osmolarity
• Urinosmolarity in thirst test (8 hours of fluid abstinence) [water deprivation test]:
  • Normal: increase in urine osmolarity due to stimulation of ADH (antidiuretic hormone).
  • Diabetes insipidus: no increase in urine osmolarity (urine is unconcentrated).
  • Caveat: No thirst test if diabetes insipidus is clearly suspected (e.g., increased plasma osmolarity and hypernatremia (excess sodium) without prior fluid restriction).
• Determination of urine osmolarity after ADH injection:
  • Urine osmolarity ↑ → central diabetes insipidus.
  • Urine osmolarity does not increase → lack of or inadequate renal response to ADH = renal diabetes insipidus.
• Fasting glucose (fasting blood glucose), if necessary oral glucose tolerance test (oGTT).
• Copeptin (released together with the antidiuretic hormone arginine vasopressin (AVP) by the neurohypophysis) – for the diagnosis of central diabetes insipidus or to distinguish primary polydipsia from partial diabetes insipidus test procedure: The patient is previously infused with hypertonic saline (= hypertonic saline infusion test) until the sodium concentration has risen to at least 150 mmol/l.Interpretation:
  • Healthy patient (or those with primary polydipsia): increase in copetin and AVP as the body attempts to normalize plasma osmolality by increasing renal water reabsorption.
  • Patients with central diabetes insipidus: copetin levels remain low due to reabsorptive dysfunction.

  Test validity: the test made the correct diagnosis in 136 of 141 patients (diagnostic accuracy 96.5%; 95% confidence interval 92.1% to 98.6%), superior to the indirect dehydration test. Likewise, by means of the saline infusion test, the distinction of primary polydipsia (pathologically (pathologically) increased feeling of thirst associated with excessive fluid intake by drinking) from partial diabetes insipidus was possible in most cases (99 of 104 patients could be distinguished (95.2%; 89.4-98.1%))

Laboratory parameters 2nd order – depending on the results of the history, physical examination and obligatory laboratory parameters – for differential diagnostic clarification.

• ADH determination
• Small blood count
• Inflammatory parameter – CRP (C-reactive protein)
• Renal parameters – urea, creatinine, possibly cystatin C or creatinine clearance.

Laboratory parameters in diabetes insipidus