Excessive Thirst (Polydipsia): Test and Diagnosis

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

  • Small blood count
  • Inflammatory parameters – CRP (C-reactive protein) or ESR (erythrocyte sedimentation rate).
  • Urine status (rapid test for: pH, leukocytes, nitrite, protein, glucose, ketone, urobilinogen, bilirubin, blood), sediment, if necessary urine culture (pathogen detection and resistogram, that is, testing suitable antibiotics for sensitivity / resistance).
  • Fasting glucose (fasting blood glucose).
  • Electrolytes – calcium, potassium, sodium
  • Thyroid parameters – TSH
  • Liver parameters – alanine aminotransferase (ALT, GPT), aspartate aminotransferase (AST, GOT), gamma-glutamyl transferase (γ-GT, gamma-GT; GGT), alkaline phosphatase, bilirubin.
  • Renal parameters – creatinine, urea, uric acid, creatinine clearance if necessary.
  • Aldosterone determination – if Conn syndrome is suspected.
  • Dexamethasone test – if Cushing’s syndrome is suspected.
  • Serum and urine osmolality (20 ml each of the 2-hour urine over the experimental period) – if complete or partial central diabetes insipidus is suspected* [material: sodium, ADH (antidiuretic hormone) if necessary].
  • Copeptin (released together with the antidiuretic hormone arginine vasopressin (AVP) from 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%))

* For differential diagnosis of polydipsia and polyuria in suspected diabetes insipidus, a thirst test (two-step test) is indicated. For more information, see laboratory diagnostics under the subtopic renal diagnostics.