What are the expected lab results for a patient with Syndrome of Inappropriate Antidiuretic Hormone (SIADH) secretion?

Expected Laboratory Findings in Syndrome of Inappropriate Antidiuretic Hormone (SIADH) Secretion

The classic laboratory findings in SIADH include hyponatremia (serum sodium <134 mEq/L), plasma hypoosmolality (<275 mOsm/kg), inappropriately concentrated urine (urine osmolality >100 mOsm/kg and typically >500 mOsm/kg), and elevated urinary sodium concentration (>20 mEq/L), all in a clinically euvolemic patient. 1

Key Laboratory Abnormalities

Serum Findings:

• Sodium: Decreased (<134 mEq/L)
• Plasma osmolality: Decreased (<275 mOsm/kg)
• BUN: Typically low
• Uric acid: Reduced in approximately 70% of cases
• Anion gap: Lower than expected
• Total CO2: Nearly normal despite dilution
• Potassium: Usually normal
• Creatinine: Normal (absence of renal insufficiency)

Urine Findings:

• Urine osmolality: Inappropriately high (>100 mOsm/kg, often >500 mOsm/kg)
• Urine sodium: Elevated (>20 mEq/L, typically >40 mEq/L)
• Fractional excretion of sodium: Often elevated (>0.5% in 70% of cases) 2

Diagnostic Criteria

The complete diagnostic criteria for SIADH include:

1. Hyponatremia with corresponding hypoosmolality
2. Inappropriately concentrated urine relative to plasma osmolality
3. Urinary sodium excretion that matches sodium intake (typically >20-40 mEq/L)
4. Clinical euvolemia (absence of edema, dehydration, or hypovolemia)
5. Normal renal, adrenal, and thyroid function
6. Absence of recent diuretic use 1, 3

Severity Classification

Hyponatremia in SIADH can be classified as:

• Mild: 126-135 mEq/L
• Moderate: 120-125 mEq/L
• Severe: <120 mEq/L 1

Special Considerations

• Urine osmolality patterns: Patients with very high urine osmolality (>600 mOsm/kg) may respond better to vasopressin receptor antagonists, while those with lower urine osmolality may benefit more from fluid restriction or urea therapy 2

• Urea levels: Typically low in SIADH, though this finding is less specific in elderly patients who may have higher values due to reduced clearance 2

• Acid-base status: Unlike hyponatremia from other causes (such as hypocorticism), SIADH patients typically maintain near-normal total CO2 levels despite dilution 2

• Response to saline: Patients with urine osmolality >530 mOsm/kg may paradoxically worsen with isotonic saline administration, while those with lower urine osmolality often improve 4

Pitfalls to Avoid

1. Misdiagnosis: Failure to exclude other causes of hyponatremia (adrenal insufficiency, hypothyroidism, renal failure)

2. Incomplete evaluation: Not checking both serum and urine parameters simultaneously

3. Overlooking medication effects: Many drugs can cause SIADH, including antidepressants, antipsychotics, anticonvulsants, and chemotherapeutic agents 1, 5

4. Misinterpreting urine sodium: Low urine sodium can occasionally be seen in SIADH patients with poor salt intake, which may lead to diagnostic confusion 2

5. Failure to assess volume status: Clinical euvolemia is a key diagnostic feature that distinguishes SIADH from other hyponatremic states

By recognizing these characteristic laboratory findings, clinicians can accurately diagnose SIADH and distinguish it from other causes of hyponatremia, leading to appropriate management strategies.