Table 2. Main clinical and laboratory findings that allow the differentiation between classic SIAD (type A) and osmostat reset (SIAD type C).
| Clinical and laboratory findings | Classic SIAD | Osmostat reset |
|---|---|---|
| Hypotonic hyponatremia (POSM < 275 mOsm/L) | Yes | Yes |
| Euvolemia | Yes | Yes |
| UOSM > 100 mOsm/L | Yes | Yes |
| Significant variations in UOSM | No (UOSM “fixed”) | Yes |
| UNa > 30 mEq/L | Yes | Yes |
| Normal renal, thyroid, and adrenal functions | Yes | Yes |
| No recent use of diuretics | Yes | Yes |
| PAU < 4 mg/dL | Common | No |
| FEAU > 11% | Yes | No |
| Plasma urea levels | Low-normal (< 30 mg/dL) | Normal |
| FEUREA | > 55% | < 55% |
| Worsen of hyponatremia due to sodium chloride 0.9% intravenous infusion | Yes | No |
| Improvement of hyponatremia due to water restriction and high solute diet | Yes | No |
| Inappropriately high ADH and copeptin levels regarding plasma osmolarity | Yes | Yes |
| Response to water load test | < 80% | > 80%* |
*Usually reaches values < 100 mOsm/L.