From the Guidelines
To diagnose diabetes insipidus (DI) and syndrome of inappropriate antidiuretic hormone secretion (SIADH), the initial laboratory work-up should include serum sodium, serum osmolality, and urine osmolality, as recommended by recent expert consensus statements 1. When suspecting DI or SIADH, it's crucial to start with these basic labs to understand the patient's hydration status and renal concentrating ability.
- For DI, look for hypernatremia (sodium >145 mEq/L), high serum osmolality (>295 mOsm/kg), and inappropriately dilute urine (urine osmolality <300 mOsm/kg) 1.
- For SIADH, expect to find hyponatremia (sodium <135 mEq/L), low serum osmolality (<275 mOsm/kg), inappropriately concentrated urine (urine osmolality >100 mOsm/kg, typically >300), and elevated urine sodium (>20-30 mEq/L). Additional tests may be necessary based on the initial findings, such as a water deprivation test for DI or thyroid function tests, cortisol levels, and kidney function tests for SIADH. The detection of inappropriately diluted urine, in combination with high–normal or elevated serum sodium, is pathognomonic for the diagnosis of diabetes insipidus (nephrogenic or central) and warrants early genetic testing if nephrogenic DI is suspected 1. Measuring ADH/vasopressin levels can be helpful but isn't routinely available. These patterns reflect the underlying pathophysiology: excessive water loss in DI due to ADH deficiency or resistance versus water retention in SIADH due to excess ADH action.
From the FDA Drug Label
Prior to treatment with desmopressin acetate injection, assess serum sodium, urine volume and osmolality. Intermittently during treatment, assess serum sodium, urine volume and osmolality or plasma osmolality Ensure the serum sodium concentration is normal before starting or resuming desmopressin acetate. Measure serum sodium within 7 days and approximately 1 month after initiating therapy, and periodically during treatment.
To check for Diabetes Insipidus and SIADH, the following labs should be checked:
- Serum sodium
- Urine volume
- Urine osmolality
- Plasma osmolality 2
From the Research
Laboratory Investigations for Diabetes Insipidus and SIADH
To diagnose and differentiate between diabetes insipidus (DI) and syndrome of inappropriate antidiuretic hormone (SIADH), several laboratory tests can be employed. The following are some of the key investigations:
- Serum and urine osmolality: Measurement of serum and urine osmolality can help in diagnosing DI and SIADH 3.
- Sodium content: Assessing sodium levels in serum and urine is useful in differentiating between various forms of DI and SIADH 3.
- Water deprivation test: This test is commonly used to investigate DI, although it has some drawbacks, such as being cumbersome and sometimes producing equivocal results 4.
- Direct measurement of AVP: Measuring arginine vasopressin (AVP) levels has theoretical advantages but is often limited to specialist centers due to pre-analytical and analytical challenges 4.
- Copeptin measurement: Copeptin, a proxy marker for AVP, is more stable and easier to measure than AVP. Recent studies have shown that copeptin measurement during stimulation tests offers improved diagnostic performance compared to the water deprivation test 4.
- Urine solute output: Daily variations in solute output can affect urine volume in patients with DI and SIADH, which can, in turn, influence serum sodium levels 5.
Differentiating Between Central and Nephrogenic DI
Differentiating between central diabetes insipidus (CDI) and nephrogenic diabetes insipidus (NDI) is crucial for effective treatment. The following tests can help in differentiation: