How can I differentiate between SIADH and other causes of hyponatremia in my patients?

Differentiating between Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) and other causes of hyponatremia primarily involves a careful assessment of the patient's fluid status and specific laboratory tests NICE CKS, Peri 2019.

Key Differentiating Factors for SIADH:

• Fluid Status: Patients with SIADH are typically euvolemic, meaning they do not show clinical signs of fluid overload (e.g., oedema) or dehydration (e.g., reduced skin turgor, orthostatic hypotension) NICE CKS, Peri 2019, Warren et al. 2023. This is a crucial distinction from hypovolemic or hypervolemic hyponatremia NICE CKS.
• Plasma Osmolality: SIADH is characterised by a low plasma osmolality, typically less than 275 mOsm/kg NICE CKS, Peri 2019.
• Urine Osmolality: Despite low plasma osmolality, the urine osmolality in SIADH is inappropriately high, usually greater than 100 mOsm/kg NICE CKS, Peri 2019, Warren et al. 2023. This indicates the kidneys are concentrating urine excessively due to ADH NICE CKS.
• Urine Sodium Concentration: Urine sodium concentration is typically high, often greater than 30 mmol/L, reflecting the kidney's inability to conserve sodium despite the hyponatremia NICE CKS, Peri 2019, Warren et al. 2023.
• Exclusion of Other Causes: A diagnosis of SIADH requires the exclusion of other conditions that can cause hyponatremia NICE CKS, Peri 2019. This includes ruling out adrenal insufficiency, hypothyroidism, significant renal impairment, and recent diuretic use NICE CKS, Peri 2019. Normal renal, adrenal, and thyroid function are expected in SIADH NICE CKS.

Differentiation from Other Common Causes of Hyponatremia:

• Hypovolemic Hyponatremia: Patients present with signs of dehydration, such as dry mucous membranes, reduced skin turgor, or orthostatic hypotension NICE CKS. The urine sodium concentration is typically low (less than 30 mmol/L) as the kidneys attempt to conserve sodium in response to volume depletion NICE CKS. Causes include gastrointestinal losses (e.g., vomiting, diarrhoea) or excessive sweating NICE CKS.
• Hypervolemic Hyponatremia: Patients exhibit signs of fluid overload, such as peripheral oedema, ascites, or pulmonary oedema NICE CKS. This is commonly seen in conditions like heart failure, cirrhosis, or nephrotic syndrome NICE CKS.
• Pseudohyponatremia: This occurs when severe hyperlipidaemia or hyperproteinaemia interfere with laboratory measurements, leading to a falsely low sodium reading NICE CKS. In these cases, the plasma osmolality will be normal, which helps differentiate it from true hyponatremia NICE CKS.
• Hyponatremia due to Hyperglycemia: High blood glucose levels can cause water to shift from cells into the extracellular space, diluting serum sodium NICE CKS. In this scenario, the plasma osmolality will be elevated NICE CKS.

Therefore, a comprehensive assessment including clinical fluid status and specific laboratory parameters (serum sodium, plasma osmolality, urine osmolality, and urine sodium) is essential for accurate differentiation NICE CKS, Peri 2019.