FEUrea Calculator — Acute Kidney Injury Cause Differentiation

In the presence of Acute Kidney Injury (AKI), determining the underlying cause is a crucial thing, as the cause may be reversible. Among the wide range of potential examinations, laboratory tests, and imaging methods, a simple general urine test remains an inevitable part. Parameters like creatinine, electrolytes, or urea give an overall picture of the kidney's function, and, if wisely used, can provide a clue about the cause of acute kidney injury — whether it is prerenal or intrinsic renal pathology.

To find out the reason for kidney malfunction, clinicians use two fractional excretion parameters: Fractional Excretion of Sodium (FENa) and Fractional Excretion of Urea (FEUrea). Fractional excretion means that the result represents the ratio of sodium/urea filtration to the overall glomerular filtration rate - GFR (check the GFR calculator to estimate it). In a state of transient hypovolemia (an example of prerenal pathology), the healthy kidneys react by raising urine osmolarity and diminishing the sodium and/or urea excretion. The resulting FEUrea will be relatively small. On the other hand, while the organ itself is not well (intrinsic renal pathology), it cannot condense urine properly, so the excretion parameters will be bigger.

Currently, the FEUrea is considered better at differentiating types of AKI from FENa. This is because the fractional excretion of sodium ratio is sensitive to common clinical states and drugs. Examples include use of diuretics, radiocontrast nephropathy, glucosuria, or AKI with underlying liver or heart failure. Thus, the role of the FENa role is diminishing.

Look at the panel to your side. All you have to know to count the FEUrea are four biochemical parameters, two measured in the patient's serum and two from their urine.

1. Fill in the serum creatinine field. You can choose between units — mg/dL or μmol/L.

2. Fill in the serum urea or blood urea nitrogen parameter. There are also two units available — mg/dL and mmol/L.

3. Do the same with the urine-measured parameters.

4. The calculator will count the fractional excretion of urea using the following formula:

   FEUrea = [(urine urea × serum creatinine)/(serum urea × urine creatinine)] × 100

5. The result will be given as a percentage (%)

Fractional excretion of urea should be used together with other values (creatinine, electrolytes); if you wish to assess kidney function properly, the FEUrea calculator is only the beginning. But what actually does the result mean? FEUrea can be interpreted by where it places in three ranges:

• FEUrea <35% indicates that the cause of the kidney malfunction is prerenal;
• FEUrea in a range of 35-50% suggests intrinsic renal pathology; and
• FEUrea >50% indicates intrinsic renal disease.

Acute kidney injury is a sudden (hours-days) decrease of glomerular filtration rate (GFR) with or without reduced urine output. We can see it clinically by an increase in serum kidney parameters such as creatinine, urea, electrolytes, changes in a general urine test, changes in diagnostic imaging, and, of course, just seeing the patient and taking their medical history. Currently, definitions regarding the precise numbers of the parameters vary and depend on the classification used.

The causes of acute kidney injury are numerous. They can be divided, regarding the point of injury, into three categories: prerenal (most frequent — resulting, e.g., from ), intrinsic renal, and postrenal. The most common states leading to AKI are:

1. Prerenal — hypovolemia, heart failure, liver failure, renal artery stenosis, or renal vein thrombosis.
2. Intrinsic renal — glomerulonephritis, acute tubular necrosis, rhabdomyolysis, or use of nephrotoxic drugs (tacrolimus).
3. Postrenal — urinary tract obstruction, e.g., prostate hyperplasiaor prostate cancer, kidney or bladder stones, or cancer.

Urea is a waste product created in the final stages of protein metabolism. It's how the body excretes waste nitrogen. Its level reflects the state of the kidneys, liver, and patient's nutritional status. A patient will have a high urea concentration when they have kidney disease, a diet with high meat intake, or tissue damage (like massive burns). Low urea levels tell us about liver malfunction or about very low protein intake.

Urea can be measured in blood and urine.

Normal ranges:

• Serum urea/blood urea nitrogen: 8-20 mg/dL (2.9-7.1 mmol/L)

• Urine urea: 350-700 mg/dL (125-250 mmol/L)

The urea level can also be helpful in assessing the severity of pneumonia in the CURB-65 calcualator.

Creatinine is a metabolite from muscle breakdown. In a healthy state, its level depends on muscle mass. Diminished creatinine levels can be seen in malnourished persons, in states leading to muscle atrophy, or with severe liver disease. Raised creatinine levels usually reflect poor kidney function.

There are two main creatinine laboratory tests used: serum creatinine and urine creatinine. Those two values can be used to count creatinine clearance to evaluate the kidneys' function.

Normal ranges:

• Serum creatinine: 0.7-1.3 mg/dL (62-115 µmol/L)

• Urine creatinine: 15-25 mg/kg of the body mass per 24h (133-221 mmol/kg per 24h)