Calculate the urine potassium-to-creatinine ratio (urine K/Cr) from a spot urine sample in mmol/mmol and mEq/g, with creatinine in mg/dL or µmol/L.
Note: For informational purposes only. This tool performs unit conversion and algebra; it does not diagnose disease or replace clinical interpretation. Potassium and kidney-related results depend on the full clinical context (e.g., spot vs timed urine collection, hydration status, medications, and your lab’s reference ranges). If you have symptoms of high/low potassium or kidney disease, seek medical care.
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What Is the Urine K/Cr Ratio?
The urine potassium-to-creatinine ratio (K/Cr) quantifies how much potassium the kidneys excrete relative to creatinine in a single urine sample. Because creatinine excretion stays relatively constant in individuals with a stable glomerular filtration rate, dividing urine potassium by urine creatinine corrects for variations in urine concentration and dilution. A spot ratio can aid assessment when a timed collection is impractical, but variability in potassium and creatinine excretion limits its specificity. It is not universally interchangeable with a 24-hour collection.
Urine K To Creatinine Ratio Formula
Ratio (mEq/g) = urine potassium (mEq/L) ÷ urine creatinine (g/L).
Ratio (mmol/mmol) = urine potassium (mmol/L) ÷ urine creatinine (mmol/L).
When reporting in mEq/g, creatinine is converted to g/L first (mg/dL x 0.01). When reporting in mmol/mmol, creatinine in umol/L is divided by 1000 to yield mmol/L. Because potassium is monovalent, 1 mEq equals 1 mmol, so the potassium value in mEq/L can be used directly in both formulas.
Clinical Interpretation Thresholds
During hypokalemia, a high urine K/Cr ratio may support inappropriate urinary potassium loss. Reference cutoffs are approximate and require serum potassium, acid–base status, medicines, muscle mass and sampling context; the ratio alone does not diagnose a cause.
| Unit System | Threshold | Below Threshold | Above Threshold |
|---|---|---|---|
| mmol/mmol (or mEq/mmol) | 1.5 | Does not establish a cause or exclude renal loss | May support renal potassium loss during hypokalemia |
| mEq/g creatinine | 13 | Does not establish a cause or exclude renal loss | May support renal potassium loss during hypokalemia |
The calculator also retains 2.5 mmol/mmol (about 22.1 mEq/g) as a historical reference from a small study of acute hypokalemic paralysis, not a general diagnostic threshold. A previously listed 3.4 mmol/g cutoff has been removed because the cited study did not specify that unit. See Lin et al. (2004) and the KDIGO potassium conference report for context and limitations.
Differential Diagnosis by Acid-Base Status and Blood Pressure
Combining the K/Cr ratio with the patient’s acid-base status and blood pressure narrows the differential substantially. The following reference table summarizes possible clinical patterns in hypokalemia workups; these patterns are not diagnoses. Here the ratios use mmol/mmol, and 1.5 is an approximate reference rather than a definitive boundary.
| Acid-Base | K/Cr Ratio | Blood Pressure | Likely Causes |
|---|---|---|---|
| Metabolic acidosis | Low (<1.5) | Normal | Diarrhea, laxative abuse |
| Metabolic acidosis | High (>=1.5) | Normal | Diabetic ketoacidosis, distal (type 1) or proximal (type 2) renal tubular acidosis (RTA) |
| Metabolic alkalosis | Low (<1.5) | Normal | Surreptitious vomiting, remote diuretic use |
| Metabolic alkalosis | High (>=1.5) | Normal | Active diuretic use, Bartter syndrome, Gitelman syndrome |
| Metabolic alkalosis | High (>=1.5) | Hypertension | Primary hyperaldosteronism, Cushing syndrome, renal artery stenosis, Liddle syndrome, apparent mineralocorticoid excess, congenital adrenal hyperplasia |
Why K/Cr Replaced the TTKG
The transtubular potassium gradient (TTKG) was long used to estimate aldosterone activity in the cortical collecting duct. It required urine osmolality to exceed plasma osmolality and urine sodium to be above 25 mmol/L. In practice, these prerequisites frequently went unmet, and the original authors of the TTKG later acknowledged that certain assumptions underlying the calculation were invalid. Because of these limitations, most nephrology references now favor the urine K/Cr ratio as a simpler, more reliable bedside index that does not depend on osmolality measurements.
Spot Urine vs. 24-Hour Collection
Studies of hospitalized hypertensive patients show that the potassium-to-creatinine ratio in a random spot sample correlates closely with the same ratio measured in a timed 24-hour collection. A spot ratio is an adjunct; it does not establish that an individual result is equivalent to timed potassium excretion. However, its discriminative value diminishes when 24-hour potassium excretion is only mildly elevated; the correlation is strongest at higher excretion levels. For borderline results, a full 24-hour urine may still be warranted.
Factors That Affect the Result
Several variables can shift the K/Cr ratio independent of true renal potassium handling. High dietary potassium intake or supplementation raises urinary potassium and inflates the ratio. Dehydration concentrates the urine, potentially altering the relationship between potassium and creatinine. Reduced muscle mass (common in elderly or cachectic patients) lowers creatinine production and can artificially elevate the ratio. Certain medications, notably loop and thiazide diuretics, directly increase renal potassium excretion. An improperly collected or contaminated specimen will also produce unreliable values.
Emerging Research: (UK/UCr) / SK Squared
Recent research has proposed refining the urine K/Cr ratio by dividing it by the square of the synchronous serum potassium concentration: (UK/UCr) / SK^2. This adjusted index showed significantly higher values in patients with confirmed renal potassium loss compared to those with extrarenal causes, with a particularly strong separation observed in female patients. The concept aims to account for the expected renal response to a given serum potassium level, improving diagnostic accuracy beyond what the raw urinary ratio provides alone. Validation studies are ongoing.
Example Calculation
A patient presents with hypokalemia. The spot urine returns potassium of 40 mEq/L and creatinine of 100 mg/dL.
First, convert creatinine to g/L: 100 mg/dL x 0.01 = 1.0 g/L.
Then divide: 40 mEq/L / 1.0 g/L = 40 mEq/g.
The result is above the commonly used approximate 13 mEq/g reference and may support inappropriate urinary potassium loss during hypokalemia. A clinician must interpret it with acid-base status, blood pressure, medicines and the wider clinical picture; it does not establish a diagnosis.
Frequently Asked Questions
When is this test ordered?
The urine K/Cr ratio is most commonly ordered during the evaluation of unexplained hypokalemia (serum K below 3.5 mEq/L). It helps clinicians determine whether the kidneys are appropriately conserving potassium or inappropriately wasting it. It may also be ordered to monitor patients on diuretics, to investigate refractory hypokalemia, or during the workup of suspected mineralocorticoid excess.
What sample is needed?
A spot urine specimen can support assessment. A timed collection may be needed depending on the clinical question and uncertainty. The sample should be sent to the laboratory for both urine potassium and urine creatinine measurements. Ideally, a simultaneous serum potassium and basic metabolic panel should be drawn to provide context for interpretation.
Can this test distinguish between specific causes of renal potassium wasting?
The K/Cr ratio alone cannot reliably establish that the kidney is the source of potassium loss or identify its cause. To differentiate between conditions like primary aldosteronism, Bartter syndrome, or diuretic use, additional testing is needed: serum bicarbonate (acid-base status), blood pressure measurement, plasma renin and aldosterone levels, and urine chloride concentration. The differential diagnosis table above outlines how these additional data points help narrow the cause.
Is there a difference between mEq/g and mmol/mmol results?
Yes, the numeric value differs substantially because the denominator is expressed in different units (grams of creatinine vs. millimoles of creatinine). Using a creatinine molar mass of 113.12 g/mol, 1.5 mmol/mmol converts to about 13.26 mEq/g; 13 mEq/g converts to about 1.47 mmol/mmol. Published 1.5 and 13 cutoffs are approximate clinical conventions, not exactly equivalent diagnostic boundaries. Always check which unit system your laboratory uses before comparing your result to published cutoffs.
