Investigations
1st investigations to order
serum sodium
Test
SIADH presents with hypotonic hyponatraemia: low serum sodium and osmolality.
Pseudohyponatraemia can occur due to hyperglycaemia-induced water shift from the intracellular to extracellular space. This should be suspected and excluded in patients with a history of diabetes mellitus, non-compliance with diabetic therapy, polyuria, and polydipsia.
Result
<135 mmol/L (<135 mEq/L)
serum osmolality
Test
SIADH presents with hypotonic hyponatraemia: low serum sodium and osmolality.
Result
<275 mmol/kg (<275 mOsm/kg) H₂O
serum urea
Test
Usually low due to mild volume expansion.
Result
<3.6 mmol/L (<10 mg/dL)
urine osmolality
Test
Values of >100 mmol/kg H₂O (>100 mOsm/kg H₂O) indicating elevated arginine vasopressin level support the diagnosis of SIADH if present in conjunction with low serum sodium and low serum osmolality.
Low levels occur in the presence of hyponatraemia due to excessive water intake.
Result
>100 mmol/kg H₂O (>100 mOsm/kg H₂O)
urine sodium
Test
Urine sodium with normal dietary salt and water intake.
If patient is euvolaemic, results of >30 mmol/L (>30 mEq/L) are consistent with SIADH.
Result
>30 mmol/L (>30 mEq/L)
Investigations to consider
diagnostic trial with normal saline infusion
Test
1 to 2 L of normal saline can be administered as a therapeutic trial if volume depletion is suspected. Serum sodium will improve in this situation, while in SIADH it will not.
A trial of normal saline should not be performed if patient is symptomatic from hyponatraemia (e.g., altered mental status, seizure, coma).
Result
serum sodium level does not improve after normal saline infusion; occasionally, serum sodium may even decrease
serum uric acid
Test
Usually not necessary.
Low serum uric acid level indicates mild volume expansion, consistent with SIADH.
Result
<238 micromol/L (<4 mg/dL)
fractional excretion of sodium
Test
Confirms euvolaemic state in people with SIADH.
Result
>1%
fractional excretion of urea
Test
Confirms euvolaemic state in people with SIADH.
Result
>55%
serum TSH
Test
Hypothyroidism should be ruled out when diagnosing SIADH.
Result
0.5 to 4.7 milli-international units/L
serum cortisol level
Test
Addisonism should be ruled out when diagnosing SIADH.
Result
morning level >138 nanomol/L (>5 micrograms/dL)
serum arginine vasopressin (AVP)
Test
AVP values vary in normal subjects with levels rising above 2.5 picograms/mL as serum sodium reaches 140 mmol/L (140 mEq/L) and beyond.
AVP not routinely recommended as urinary osmolality >100 mmol/kg H₂O (>100 mOsm/kg H₂O) is sufficient to diagnose excess AVP.[16]
Result
>2.5 picograms/mL during period of hyponatraemia
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