I assess the severity of SIADH by looking at the electrolyte content of the urine. Here is a doozy:
- Urine sodium: 134
- Urine Potassium: 62
- Urine osmolality: 777
But this strategy suffers from a similar limitation, the urine electrolyte are affected by factors beyond the severity of the SIADH. Patients with SIADH are in sodium balance, that means that all the sodium they ingest is excreted. Increased sodium intake will be reflected by increased urine sodium, the same goes for dietary potassium and urinary potassium. The above labs came after the patient was given isotonic saline for 24-hours. The serum sodium fell from 128 to 117 with saline.
The true measure of SIADH severity probably is simply the urine osmolality.