- Use this only when the patient is hypokalemic and you are asking whether renal potassium loss is appropriate or inappropriate.
- Enter a spot urine potassium (mmol/L) and spot urine creatinine from the same specimen; pick the creatinine unit (mg/dL or mmol/L).
- The tool reports the ratio in mmol/mmol and mmol/g creatinine and flags whether it supports renal potassium wasting.
- Interpret with the acid–base state, blood pressure, and magnesium — the ratio localizes, it does not diagnose.
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When to Use
In a hypokalemic patient, the spot urine potassium-to-creatinine ratio partially corrects a bare urine potassium for water concentration and estimates whether the kidney is conserving potassium (appropriate) or leaking it (inappropriate). It is a faster, more robust bedside screen than the transtubular potassium gradient (TTKG).
Appropriate setting
Documented hypokalemia (serum K below the reference range) where you need to separate renal loss (diuretics, Bartter/Gitelman, mineralocorticoid excess, RTA) from extrarenal loss or shift (GI losses, poor intake, transcellular shift).
When not to rely on it
Do not interpret when serum potassium is normal or high; in rapidly changing (non-steady-state) states; with very low or very high muscle mass (creatinine denominator distorted); or immediately after a potassium load. Thresholds vary by laboratory units — confirm locally.
Pearls & Pitfalls
High ratio during hypokalemia = renal wasting
A ratio above roughly 1.5 mmol/mmol (≈13 mmol/g creatinine) during hypokalemia supports inappropriate renal potassium loss — the kidney should be conserving, and it is not.
Low ratio = appropriate conservation
A ratio below roughly 1.0 mmol/mmol (≈9 mmol/g) during hypokalemia fits appropriate renal conservation — look extrarenal (GI loss, poor intake) or for a transcellular shift.
Critical pitfalls
(1) Thresholds are approximate and unit-dependent. (2) Correct magnesium — refractory hypokalemia often persists until hypomagnesemia is treated. (3) Add the acid–base state and blood pressure: high ratio + alkalosis + hypertension points to mineralocorticoid excess; high ratio + alkalosis + normal/low BP points to diuretics or Bartter/Gitelman.
Why Use It
A single spot urine potassium is highly flow-dependent: the same renal potassium output looks high in a concentrated urine and low in a dilute one. Dividing by urine creatinine — excreted at a relatively steady rate — cancels much of that water effect, so the ratio tracks the kidney's potassium-handling policy rather than the moment's urine flow. It answers the one question that changes the differential in hypokalemia: is the kidney part of the problem?
Spot Urine K / Creatinine Ratio
Enter spot urine potassium and creatinine from the same specimen. Interpret only when the patient is hypokalemic.
⚕ Ratio interpretable only during hypokalemia and in steady state. Approximate thresholds: >1.5 mmol/mmol (≈13 mmol/g) supports renal K loss; <1.0 (≈9 mmol/g) supports conservation. Confirm units and clinical context.
Next Steps
- Renal wasting suggested (high ratio): Check acid–base and blood pressure. Alkalosis + hypertension → renin/aldosterone axis. Alkalosis + normal/low BP → urine chloride to separate active diuretic / Bartter / Gitelman. Acidosis → consider RTA.
- Conservation suggested (low ratio): Pursue GI losses, poor intake, or transcellular shift; treat the source and replace potassium.
- Always measure and replace magnesium in refractory hypokalemia.
- Refer to nephrology for suspected inherited tubulopathy, RTA, or unexplained persistent renal potassium loss.
Evidence & References
Formula
| Quantity | Equation |
|---|---|
| Ratio (mmol/mmol) | Urine K (mmol/L) ÷ Urine creatinine (mmol/L) |
| Ratio (mmol/g) | Urine K (mmol/L) ÷ Urine creatinine (g/L) |
| Creatinine mg/dL → mmol/L | × 0.0884 (MW 113.12) |
Thresholds after Palmer & Clegg (2019). Interpretation requires documented hypokalemia and steady state.
