Urea Nitrogen 24 Hour Test

When your body uses protein, whether from the food you eat or from your own muscle tissue, the leftover nitrogen gets packaged into a small molecule called urea and filtered out through your kidneys. A 24-hour UUN (urinary urea nitrogen) test collects all of your urine over a full day and measures how much of that nitrogen waste you excreted. The result is a window into your protein metabolism: how much protein you are eating, how much your body is breaking down, and whether you are building tissue or losing it.

This test is most valuable in two situations. First, if you have chronic kidney disease and are following a low-protein diet, it tells you (and your care team) whether you are actually hitting your protein targets. Second, if you are critically ill, recovering from surgery, or healing from a major burn or trauma, it reveals whether your body is in a catabolic state, meaning it is tearing down more protein than it is building up.

For most healthy adults eating a typical diet, 24-hour UUN falls between 6 and 17 grams per day, with an average around 10 to 12 grams. But the number shifts meaningfully based on how much protein you eat and how stressed your body is. That responsiveness is exactly what makes it useful.

What UUN Actually Tells You

Urea nitrogen accounts for roughly 80 to 90% of all the nitrogen your body excretes in urine. Because of that dominance, measuring UUN gives a reliable estimate of your total nitrogen output. And once you know your nitrogen output, you can calculate something called nitrogen balance: the difference between the nitrogen going into your body (from protein you eat) and the nitrogen leaving it.

A positive nitrogen balance means you are retaining more protein than you are losing. Your body is in a building state. A negative nitrogen balance means you are breaking down protein faster than you are replacing it. This is common during serious illness, after surgery, or with inadequate nutrition. The standard formula is straightforward: subtract your UUN plus 2 grams (to account for nitrogen lost through skin, stool, and other non-urine routes) from your dietary nitrogen intake.

If you have kidney disease, UUN serves a different but equally practical purpose. Using the Maroni-Mitch formula, your actual daily protein intake can be estimated from your UUN result. The calculation is: protein intake in grams per day equals your UUN plus 0.031 times your body weight in kilograms, multiplied by 6.25. This is the standard method recommended for checking whether you are sticking to a prescribed low-protein diet.

Who Benefits Most From This Test

This is not a routine screening test. It is most informative in specific clinical scenarios where protein metabolism is a central question.

• People with chronic kidney disease: if you are following a restricted protein diet (typically 0.6 to 0.8 g/kg/day), this test checks whether your actual intake matches the target. Dietary recall is unreliable; UUN provides an objective number.
• People recovering from burns: burn injuries drive intense protein breakdown. UUN rises in proportion to the size of the burn. In one study, people with burns covering less than 10% of body surface area excreted an average of 11.1 g/day, while those with burns covering 31 to 60% excreted 25.3 g/day. This tells clinicians how aggressively to supplement protein.
• Critically ill or post-surgical individuals: UUN values in ICU settings typically run between 14.7 and 15.7 g/day, well above the healthy range, reflecting the accelerated protein breakdown that accompanies severe illness.
• People with liver cirrhosis: the liver is where urea is made. In cirrhosis, the liver's maximum capacity to produce urea drops sharply, averaging 27 mg urea nitrogen per hour per kg of metabolic body weight compared to 65 in healthy individuals. Lower capacity predicts higher risk of ammonia buildup and the confusion that can follow.

One important nuance: if your kidney function is significantly impaired, the reliability of UUN-based estimates changes. When kidney filtration rate (creatinine clearance) is above 50 mL/min, UUN correlates very well with more sophisticated modeling methods. When filtration drops below 20 mL/min, that correlation weakens considerably, and alternative approaches may be needed.

What Moves This Biomarker

Because UUN directly reflects protein turnover, the most powerful influences are dietary protein intake and the body's catabolic or anabolic state.

• Dietary protein: the single largest modifiable factor. Eating more protein increases UUN; eating less lowers it. This linear relationship is exactly why the test works as a protein intake estimator.
• Catabolic stress: sepsis, major trauma, burns, and severe infections all drive UUN up by accelerating the breakdown of muscle and other tissues. In severely burned individuals, urinary urea excretion rises significantly within 24 hours of injury.
• Corticosteroids: these medications enhance protein breakdown and will raise UUN independent of dietary intake.
• Anabolic states: when your body is actively building tissue (for example, during recovery with adequate nutrition), more nitrogen is retained and less appears in the urine.
• Liver disease: because the liver converts nitrogen waste into urea, advanced liver disease reduces UUN even when protein breakdown is elevated. The nitrogen simply accumulates as ammonia instead.
• Gastrointestinal bleeding: blood in the gut is digested and its protein absorbed, which raises UUN as the nitrogen from those blood proteins is processed.

What this means for you: if you are tracking UUN to monitor your diet, keep in mind that illness, medications, and liver or kidney function all shift the number independently of what you eat. A rising UUN does not always mean you ate more protein; it may mean your body is under stress.

Practical Considerations for Collection

The accuracy of this test depends entirely on collecting all of your urine over a full 24-hour period. Missed samples will underestimate your true nitrogen excretion. For people receiving continuous nutrition support (such as tube feeding or IV nutrition), a 12-hour collection can be doubled to estimate the 24-hour value with reasonable accuracy. Shorter collections of 4 or 8 hours are less reliable.

While UUN captures the large majority of urinary nitrogen, there are situations where it may underestimate total losses. In highly catabolic surgical or trauma patients, the ratio of UUN to total urinary nitrogen can vary widely, sometimes introducing errors as large as 12 grams per day in nitrogen balance calculations. Certain lab factors can occasionally affect the result, so confirming proper collection technique matters more than with most urine tests.

Questions This Biomarker Can Answer

• Am I actually eating the amount of protein my kidney diet requires?
• Is my body breaking down more protein than it is building up right now?
• How severe is my catabolic response to this illness, injury, or surgery?
• Is my liver still able to process nitrogen waste effectively?
• Am I getting enough nutritional support to move toward a positive nitrogen balance?