Most Appropriate Initial Test for Polyuria with Intense Thirst
The most appropriate initial test is fasting plasma glucose (option a) to rule out diabetes mellitus, followed by a water deprivation test (option b) if diabetes mellitus is excluded. 1, 2
Diagnostic Reasoning
The clinical presentation of polyuria with intense thirst (polydipsia) and very dilute urine, without systemic symptoms or abnormal urinalysis, requires a systematic approach to differentiate between the major causes:
Step 1: Rule Out Diabetes Mellitus First
- Fasting plasma glucose must be the initial test because diabetes mellitus with hyperglycemia and glucosuria is the most common cause of polyuria through osmotic diuresis 1, 2
- The American Diabetes Association emphasizes that diabetes mellitus causes polyuria through osmotic diuresis from glucosuria, not ADH deficiency, and must be ruled out with proper fasting glucose or HbA1c testing 2
- This patient's very dilute urine suggests water diuresis rather than osmotic diuresis, but diabetes mellitus must still be excluded first as it is far more common 1
Step 2: Water Deprivation Test After Excluding Diabetes
- If fasting glucose is normal, proceed to water deprivation test to differentiate between central diabetes insipidus, nephrogenic diabetes insipidus, and primary polydipsia 2, 3
- The water deprivation test remains the gold standard for differentiating these conditions, with a urine osmolality threshold of >680 mOsm/kg after water deprivation having 100% sensitivity for diagnosing primary polydipsia 3
- The test requires simultaneous measurements of serum sodium, serum osmolality, and urine osmolality during polyuria to accurately assess the kidney's concentrating ability 2
Why Other Options Are Less Appropriate Initially
Urine Osmolality (option c)
- While urine osmolality is useful, a single random measurement without context is insufficient for diagnosis 4, 5
- Urine osmolality must be interpreted in conjunction with serum osmolality and clinical context during a structured test like water deprivation 2, 4
Urine Electrolyte Measurement (option d)
- This is indicated for suspected Bartter syndrome or electrolyte-driven solute diuresis, which typically present with additional electrolyte abnormalities not described in this case 1
- Not appropriate as an initial test for isolated polyuria-polydipsia 5
Brain MRI (option e)
- MRI is premature without first establishing whether diabetes insipidus is present 2
- Brain imaging is indicated only after confirming central diabetes insipidus through water deprivation testing to identify hypothalamic or pituitary lesions 2, 6
Critical Clinical Pitfall
Never assume a diagnosis based on symptoms alone - polyuria, polydipsia, and dilute urine can occur in diabetes mellitus, diabetes insipidus, and primary polydipsia, requiring systematic laboratory differentiation 1, 2. Starting with fasting glucose prevents the error of pursuing expensive testing for diabetes insipidus when the patient actually has undiagnosed diabetes mellitus.