Calculator Inputs
Formula Used
FEK (%) = ((Urine potassium × Serum creatinine) ÷ (Serum potassium × Urine creatinine)) × 100
Potassium units must match each other. Creatinine units must also match each other. This makes the ratio dimensionless.
Estimated potassium excretion per day = Urine potassium × Urine volume in liters × (24 ÷ Collection hours)
TTKG = (Urine potassium ÷ Serum potassium) ÷ (Urine osmolality ÷ Serum osmolality)
How to Use This Calculator
Enter serum potassium, urine potassium, serum creatinine, and urine creatinine from paired samples. Choose the potassium state when known. Add urine volume and collection time if you want daily potassium excretion. Add osmolality values if you want TTKG. Press calculate. Review the result under the page header. Use the CSV or PDF button to save the report.
Example Data Table
| Serum K | Urine K | Serum Cr | Urine Cr | FEK Result | Study Note |
|---|---|---|---|---|---|
| 3.0 | 45 | 1.0 | 90 | 16.67% | Higher value during low serum potassium may suggest renal loss. |
| 3.2 | 8 | 0.9 | 110 | 2.05% | Lower value may fit extra renal loss or shifting. |
| 5.8 | 70 | 1.4 | 80 | 21.12% | Urinary response appears present, but context matters. |
Fractional Excretion of Potassium Guide
Purpose
The fractional excretion of potassium calculator estimates how much filtered potassium appears in urine. It uses paired blood and urine chemistry values. The result helps describe renal potassium handling during hypokalemia, hyperkalemia, or medication review.
Clinical Meaning
Potassium balance depends on intake, cellular shifts, aldosterone activity, distal sodium delivery, and kidney function. A high value may suggest renal potassium wasting when serum potassium is low. A low value may suggest extra renal loss, poor intake, or reduced distal secretion. Interpretation must match the clinical picture.
Method
The calculation compares urinary potassium concentration with serum potassium concentration. It also adjusts for urine concentration by using creatinine values. This adjustment is useful because a random urine sample may be dilute or concentrated. The formula is not a diagnosis by itself.
Advanced Options
Advanced fields can add context. Estimated potassium excretion rate uses urine potassium, urine volume, and collection time. The optional transtubular potassium gradient needs urine and serum osmolality. It is only valid under suitable conditions, including adequate urine sodium and osmolality.
Unit Rules
Use consistent units. Serum and urine potassium should share the same concentration unit. Serum and urine creatinine should also share one unit system. Mixed units can produce a misleading percentage. Review lab labels before entering values.
Study Use
This tool is useful for teaching, study checks, and structured notes. It gives a calculated percentage, a concise interpretation, optional extra indices, and an exportable summary. It can also compare example rows. The output should support, not replace, clinical judgment.
Limitations
The formula works best with values collected near the same time. Results may be affected by diuretics, kidney disease, vomiting, diarrhea, insulin shifts, acid base disorders, mineralocorticoid states, and sampling errors. Repeat testing may be needed when results conflict with symptoms or treatment history.
Safety Review
Always review dangerous potassium values promptly. Very low or very high serum potassium can affect heart rhythm. A calculator cannot judge urgency. Use the result as one numeric clue alongside history, examination, electrocardiogram findings, medication lists, and clinician assessment. For stable comparisons, document the patient state, sampling time, and recent therapy. Note whether the sample followed supplementation, fluids, or a drug dose. These details make later review easier. They also prevent overreading one isolated laboratory snapshot. Record assumptions whenever the report is saved or shared.
FAQs
1. What does fractional excretion of potassium mean?
It estimates the percentage of filtered potassium that is excreted in urine. It helps describe kidney potassium handling when interpreted with serum potassium, kidney function, medications, and symptoms.
2. What formula does this tool use?
It uses FEK percent equals urine potassium times serum creatinine, divided by serum potassium times urine creatinine, then multiplied by 100.
3. Which units should I enter?
Use matching units. Serum and urine potassium must share one unit. Serum and urine creatinine must also share one unit. Do not mix creatinine unit systems.
4. Can this calculator diagnose renal potassium wasting?
No. It supports interpretation. Diagnosis needs clinical history, repeat labs, acid base status, kidney function, drug review, and clinician judgment.
5. Why is urine creatinine included?
Urine creatinine adjusts for urine concentration. This helps compare a random urine sample that may be dilute or concentrated at collection time.
6. What does a high value suggest?
During low serum potassium, a higher value can support renal potassium loss. The cause may include diuretics, aldosterone effect, tubular disorders, or other renal factors.
7. What does a low value suggest?
During low serum potassium, a lower value may fit extra renal loss, low intake, or potassium shifting into cells. Context is essential.
8. When should optional fields be used?
Use urine volume and collection time for daily excretion. Use osmolality fields for TTKG only when sample conditions make that index reasonable.