Urine Sodium (mEq/L) Enter the sodium concentration in the urine (mEq/L)

Plasma Sodium (mEq/L) Enter the sodium concentration in the blood (mEq/L)

Urine Creatinine (mg/dL) Enter the creatinine concentration in the urine (mg/dL)

Plasma Creatinine (mg/dL) Enter the creatinine concentration in the blood (mg/dL)

Quick Examples:

Typical Prerenal (0.71%) Typical Intrinsic Renal Failure (2.29%)

Calculate Clear

This calculator is a screening tool only, not a diagnostic instrument. It is not intended to replace professional medical evaluation. Consult a healthcare provider.

What Is Fractional Excretion of Sodium

Fractional Excretion of Sodium, or FENa, is a test that measures how much sodium passes through your kidneys and into your urine. It compares the amount of sodium in your urine to the amount of sodium in your blood. Doctors use this number to help figure out the cause of acute kidney injury. It helps them see if the problem is caused by low blood flow or direct damage to the kidney tissue.

How Fractional Excretion of Sodium Is Calculated

Formula

The calculation works by comparing the clearance of sodium to the clearance of creatinine. Creatinine is used as a marker because it is filtered by the kidney but not reabsorbed in large amounts. By comparing sodium to creatinine in both the blood and urine, the formula finds the exact percentage of filtered sodium that the kidneys let pass through. This helps show if the kidneys are holding on to sodium to save water, which happens in some types of kidney issues.

Why Fractional Excretion of Sodium Matters

Knowing your FENa value helps doctors tell the difference between the two main types of acute kidney problems. This distinction is important because the treatments for these types are very different. Using this number can help guide the doctor toward the best care plan for the specific kidney issue.

Why Accurate Classification Is Important for Treatment

If the FENa is low, it may mean the kidneys are trying to hold onto sodium because blood flow is low. In this case, giving fluids might help. If the FENa is high, it may mean the kidney tissue is damaged and cannot hold onto sodium. Giving fluids in this case could be harmful. Sorting this out correctly helps the doctor choose the right treatment to avoid further stress on the kidneys.

For Patients on Diuretics

If a patient is taking water pills called diuretics, the FENa result may not be correct. Diuretics force the kidneys to release sodium, which makes the FENa look high even if the kidney problem is from low blood flow. Doctors often use other tests, like the Fractional Excretion of Urea (FEUrea), for patients taking these medicines.

Example Calculation

Let us look at a patient with signs of a kidney issue who needs a test. The lab results show a Urine Sodium of 100 mEq/L, Plasma Sodium of 140 mEq/L, Urine Creatinine of 100 mg/dL, and Plasma Creatinine of 1.0 mg/dL.

The calculator takes the Urine Sodium (100) and multiplies it by the Plasma Creatinine (1.0), which equals 100. Then, it multiplies the Plasma Sodium (140) by the Urine Creatinine (100), which equals 14,000. Finally, it divides 100 by 14,000 and multiplies by 100.

FENa = 0.71%

This result of 0.71% is below the typical 1% mark. This suggests the kidneys are working hard to keep sodium in the body. This pattern is often seen when the kidney problem is due to reduced blood flow rather than damage to the kidney cells themselves.

Frequently Asked Questions

Who is this FENa Calculator for?

This tool is for healthcare providers, such as doctors and nurses, and for students learning about kidney function. It can also be used by patients who want to understand their lab results better, but results should always be reviewed with a professional.

What is a normal FENa value?

In most healthy people, the FENa is usually around 1% to 2%. However, doctors look at the number in the context of kidney health. A value less than 1% often suggests one type of issue, while a value greater than 2% often suggests another.

Does this calculator work for children?

The formula itself works the same way for children, but the reference ranges may be slightly different. Pediatricians interpret the results based on standards for specific age groups. It is best to consult a specialist for pediatric results.

Can I use this calculator if I have chronic kidney disease?

This calculator is most often used for sudden (acute) kidney problems. In long-term (chronic) kidney disease, the FENa result can sometimes be unclear or misleading. A doctor can tell you if this test is useful for your specific condition.