What is Urea (BUN) and Why is it Measured?

Urea is often marked as BUN (Blood Urea Nitrogen) in medical practice. Measuring urea is a common routine lab test, providing essential information about kidney and liver function.

When proteins are broken down in the liver, they produce amino acids. Further breakdown of these amino acids generates ammonia, a toxic gas. The body removes ammonia by converting it, along with CO₂, into urea in the Krebs cycle in the liver. Urea then moves from the liver to the bloodstream, reaching the kidneys, where it is filtered and largely excreted through urine. The amount of urea in the urine is directly proportional to diuresis (total urine output in 24 hours). Additionally, small amounts of urea can be eliminated through the skin via sweating (this can increase by about 15% with intense sweating) and through the intestines (about 40% of urea enters the intestines but only trace amounts are found in feces, as bacteria break it down into ammonia and CO₂). Thus, urea is a waste product resulting from protein and amino acid breakdown in the liver.

Our body synthesizes urea, and the amount produced depends on protein intake. Higher protein intake leads to more urea production. Physical stress, such as intense muscle work, also stimulates urea formation, as do hormones that promote protein breakdown (e.g., thyroxine and glucocorticoids).

Elevated Blood Urea Levels

Increased urea in the blood is called azotemia, classified into three types based on origin:

1. Prerenal Azotemia: Occurs due to decreased glomerular filtration (with normal kidney function), found in conditions like shock, dehydration, and congestive heart disease.
2. Renal Azotemia: Linked to major kidney diseases (acute or chronic), where reduced glomerular filtration leads to urea retention in the blood. Blood urea concentration rises when glomerular filtration drops by half.
3. Postrenal Azotemia: Due to urinary tract obstruction, causing reabsorption of urea back into the bloodstream. Causes include kidney stones, tumors, or blockages.

Low Blood Urea Levels

Lower urea levels are generally less diagnostically significant and may occur in cases of:

• Prolonged fasting
• Reduced protein intake
• Increased protein synthesis
• High fluid intake (by drinking or intravenously)

Testing Urea in Blood

Urea concentration in blood is commonly measured to evaluate kidney function, typically alongside creatinine levels. The blood sample is venous, collected in the morning after fasting for 12 hours. Another commonly requested parameter is urea clearance, which measures the kidneys’ ability to excrete a substance over time. To measure urea clearance, a 24-hour urine sample is collected, and on the same day, a venous blood sample is taken. Urea concentration is measured in both samples, and clearance is then calculated. The clearance value depends on diuresis, as higher urine output in 24 hours results in greater urea clearance.