Urine electrolytes cut off
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urine electrolyte diagnostic cutoff urine sodium osmolality hyponatremia FENa guidelines
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Common urine electrolyte cutoffs
1. Hyponatremia workup
| Test | Cutoff | Interpretation |
|---|---|---|
| Urine osmolality | <100 mOsm/kg | Appropriately dilute urine. Think primary polydipsia or low-solute intake. |
| >100 mOsm/kg | ADH effect present, then interpret urine sodium. | |
| Urine Na | <20-30 mmol/L | Kidney is sodium avid: effective arterial volume depletion, such as vomiting, diarrhea, heart failure, cirrhosis. |
| >30 mmol/L | Renal sodium loss or euvolemic hyponatremia, including SIADH, adrenal insufficiency, diuretics, kidney disease. | |
| SIADH supportive value | Urine Na usually >30 mmol/L with urine osm >100 mOsm/kg | Only after excluding thyroid, adrenal, renal failure, and diuretic effect. |
| Urine K | Not a standalone diagnostic cutoff | High urine K suggests renal potassium loss or aldosterone effect. |
For context, urine osmolality <100 mOsm/kg suggests polydipsia, whereas >400 mOsm/kg makes AVP excess dominant. Harrison's Principles of Internal Medicine, 22E, p. 2787-2788. In hypovolemic hyponatremia from extrarenal sodium loss, urine sodium is usually <10-20 mmol/L. Tietz Textbook of Laboratory Medicine, 7th Edition, p. 627-633.
2. AKI: prerenal vs intrinsic AKI
| Index | Prerenal AKI | Acute tubular injury/necrosis |
|---|---|---|
| Urine Na | <20 mmol/L | >40 mmol/L |
| FENa | <1% | >2%, often >3% |
| FE urea | <35% | >50% |
| Urine osm | >500 mOsm/kg | <350 mOsm/kg |
Formulas
- FENa (%) =
(Urine Na × Plasma creatinine) / (Plasma Na × Urine creatinine) × 100 - FE urea (%) =
(Urine urea × Plasma creatinine) / (Plasma urea × Urine creatinine) × 100
If the patient received a loop or thiazide diuretic, FENa can be falsely high. Prefer FE urea <35% to support prerenal physiology. Goldman-Cecil Medicine, p. 2307; Frameworks for Internal Medicine, p. 3550-3578.
3. Metabolic alkalosis: urine chloride
| Urine Cl | Meaning |
|---|---|
| <10-20 mmol/L | Chloride-responsive alkalosis: vomiting, nasogastric suction, remote diuretic use, volume depletion. Usually responds to saline and potassium chloride when appropriate. |
| >20 mmol/L | Chloride-resistant alkalosis: ongoing diuretics, mineralocorticoid excess, Bartter/Gitelman syndromes. |
Some references use <25 mmol/L for chloride-responsive alkalosis. Rosen's Emergency Medicine, p. 1702-1734.
4. Hypokalemia: renal or extrarenal K loss
| Test | Cutoff | Interpretation |
|---|---|---|
| Spot urine K | <15-20 mmol/L | Appropriate renal conservation, suggesting extrarenal loss or low intake. |
| >20 mmol/L | Inappropriate renal potassium wasting. | |
| Urine K/creatinine ratio | <13 mEq/g creatinine | Extrarenal loss or appropriate conservation. |
| >13 mEq/g creatinine | Renal potassium wasting. | |
| 24-hour urine K | <15-20 mEq/day | Appropriate conservation. |
| >20-30 mEq/day | Renal K loss. |
Important cautions
- Interpret urine studies with clinical volume status, serum electrolytes, acid-base status, kidney function, recent IV fluids, and diuretic use.
- In hyponatremia, 20-30 mmol/L urine Na is a gray zone, not a definitive diagnosis.
- FENa is less reliable in CKD, sepsis, contrast-associated AKI, pigment nephropathy, glomerulonephritis, and after diuretics.
- Do not use urine electrolytes alone to decide urgent treatment. Severe symptoms of hyponatremia, hyperkalemia, or AKI need immediate clinical assessment.
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