When to suspect and how to diagnose SIAD?

Ally Prebtani, MD, is an endocrinologist, internist, and professor of medicine at McMaster University. He has developed clinical practice guidelines as part of Diabetes Canada and Hypertension Canada. He is actively involved in teaching, advocacy work, clinical work, and research.

When to suspect and how to diagnose syndrome of inappropriate antidiuresis (SIAD)?

Ally Prebtani, MD: The first thing is to assess the patient’s volume status. Patients with SIAD are often euvolemic and often, to make the diagnosis, they need to have a low sodium, of course, low serum osmolality, inappropriately high urine osmolality, high urine sodium in the absence of diuretics, and you have to rule out adrenal insufficiency, and you have to rule out severe hypothyroidism, and they have to have normal renal function, and ideally do not have any confounding diuretics such as thiazide or thiazide-like diuretics.

This is a stepwise approach to making the diagnosis of SIAD. Sometimes you can’t tell whether they’re hypovolemic or euvolemic, and you’re not sure between SIAD and hypovolemic causes. Then, sometimes you may need to give them a volume challenge of some kind of a crystalloid, eg, 1 to 2 L of normal saline or Ringer lactate, and to see what the response is in 1 or 2 days. If they respond, it is probably a hypovolemic state and not SIAD, but if they don’t respond, it is probably SIAD. Sometimes you can have both, but that’s the general approach to making the diagnosis.