Laboratory parameters of the 1st order – obligatory laboratory tests.
- Small blood count
- Electrolytes – potassium, sodium [hyponatremia: < 135 mmol/l]
- Urine sodium in spontaneous urine.
- Total protein in serum (serum protein; serum protein).
- Urinary and serum osmolality (U-osm, H-osm).
- Glucose
- Urea
Laboratory parameters 2nd order – depending on the results of the history, physical examination, etc. – for differential diagnostic clarification.
- Inflammatory parameters – CRP (C-reactive protein).
- Urine status (rapid test for: pH, leukocytes, nitrite, protein, glucose, ketone, urobilinogen, bilirubin, blood), sediment.
- Renal parameters – urea, creatinine, cystatin C or creatinine clearance if necessary.
- Pancreatic parameters – amylase, elastase (in serum and stool), lipase.
- SIADH diagnostics: sodium, chloride, urea, urinary and serum osmolality [hyponatremia, hypouricemia, decreased serum osmolality, hypochloridemia, urinary osmolality > 100 mosmol/L].
S-osm = 2 x S-Na+ + urea + glucose (concentrations in mmol/L) S-osm = 2 x S-Na+ + urea/2.8 + glucose/18 (urea and glucose in mg/dL).
Difference between calculated and measured serum osmolarity = osmotic gap [≤ 10 mosmol/L].
Procedure:
- If hyponatremia → exclude hypertonic hyponatremia: osmotic gap must be ≤ 10 mosmol/L
- Determination of urine sodium:
- Hypovolemia: clin. Volume depletion (water loss):
- Urine Na < 30 mmol/L = extrarenal cause.
- Urine Na > 30 mmol/L = renal cause
- Euvolemia: clin. Signs are usually not groundbreaking
- Urine Na > 30 mmol/L
- Hypervolemia: clin. Edema, heart failure, liver cirrhosis, nephrotic syndrome.
- Urine Na < 20 mmol/L
- Hypovolemia: clin. Volume depletion (water loss):