From the Guidelines
To diagnose hyponatremia due to SIADH, essential laboratory tests include serum sodium, serum osmolality, urine osmolality, urine sodium concentration, and assessment of volume status, as well as thyroid and adrenal function tests, according to the NCCN guidelines insights for small cell lung cancer, version 2.2018 1. When evaluating a patient for SIADH, it is crucial to consider the clinical presentation, which may include malaise, weakness, confusion, obtundation, volume depletion, and nausea, as outlined in the guidelines 1. Key laboratory findings that support the diagnosis of SIADH include:
- Hyponatremia (serum sodium <135 mEq/L)
- Low serum osmolality (<275 mOsm/kg)
- Inappropriately concentrated urine osmolality (>100 mOsm/kg relative to serum osmolality)
- Elevated urine sodium concentration (>20-30 mEq/L)
- Euvolemia, as indicated by the absence of significant volume depletion or overload Additional tests that can help confirm the diagnosis and rule out other causes of hyponatremia include:
- Thyroid function tests to exclude hypothyroidism
- Adrenal function tests (e.g., cortisol levels) to exclude adrenal insufficiency
- Kidney function tests (e.g., BUN, creatinine) to assess renal function
- Measurement of serum uric acid, which is often low in SIADH A complete metabolic panel can also help exclude other causes of hyponatremia, such as medication effects or other electrolyte imbalances. While measurement of ADH levels can be confirmatory, it is not routinely available and is not necessary for diagnosis in most cases, as the clinical presentation and laboratory findings can establish the diagnosis, as noted in the guidelines 1.
From the Research
Labs that Diagnose Hyponatremia due to SIADH
The following labs can be used to diagnose hyponatremia due to Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH):
- Serum sodium levels: to check for hyponatremia 2, 3, 4, 5
- Serum osmolality: to check for hypo-osmolality 2, 3, 4
- Urine sodium concentration: to check for elevated urine sodium 2, 3, 4
- Urine osmolality: to check for elevated urine osmolality 2, 3, 4
- Adrenal function tests (e.g. cortisol, adrenocorticotropic hormone, renin, and aldosterone): to rule out adrenal disorders 3
- Thyroid function tests: to rule out thyroid disorders 3
- Renal function tests: to rule out renal disorders 4
- Liver function tests: to rule out liver disorders 3
Key Diagnostic Criteria
The diagnosis of SIADH is based on the following criteria:
- Hypotonic hyponatremia
- Natriuresis
- Urine osmolality in excess of plasma osmolality
- Absence of oedema and volume depletion
- Normal renal and adrenal function 4
Laboratory Tests for SIADH
Laboratory tests are necessary for the diagnosis of SIADH, but in severe, symptomatic cases of hyponatremia, patients may need treatment before the results of laboratory tests can be obtained 5.