Urea is a metabolic end product from protein and amino acid metabolism (protein metabolism) that is formed in the liver. The toxic ammonia produced in amino acid metabolism is converted into the nontoxic urea via the urea cycle in the mitochondria (power plants of the cells) of the liver. Urea is highly water-soluble and 90 % of it is excreted in the urine by the kidneys (urinary). The laboratory parameter belongs to the renal retention parameters. This is used to assess the performance of the kidneys. An increase indicates impaired kidney function, as the substance is retained in the body (retention). Urea in serum is dependent on the rate of formation, renal perfusion (kidney blood flow) and glomerular filtration rate (GFR). Serum urea does not increase until GFR is significantly decreased (< 25%). When there is an abnormal increase in nitrogenous end products of protein metabolism (residual nitrogen) in the blood, this is called azotemia (see below).
The procedure
Material needed
- Blood serum
Preparation of the patient
- No preparation necessary
Disruptive factors
- None known
Normal values adults
| Gender | Normal value in mg/dl |
| Female,<50th LJ | 15-40 |
| Female,> 50th LJ | 21-43 |
| Male,< 50th LJ | 19-44 |
| Male,> 50th LJ | 18-55 |
Normal values children
| Age | Normal value in mg/dl |
| 1-3 LJ | 11-36 |
| 4-13 LY | 15-36 |
| 14-19 LY | 18-45 |
Conversion factor (urea-nitrogen)
- Urea-N x 2.14 = urea
- Urea x 0.46 = urea-N (English blood urea nitrogen), usually abbreviated BUN; here, only the nitrogen contained in the urea is given, not the urea).
Indications
- Assessment of metabolic state or estimation of metabolic state (catabolism, anabolism).
- Calculation of the osmotic gap
Interpretation
Interpretation of increased values
- High-grade renal insufficiency (renal dysfunction).
- Catabolism in
- Fever
- After trauma (injuries)
- After operations
- Malnutrition
- Hypovolemia (decrease in blood volume).
- Dehydration (lack of fluid)
Interpretation of lowered values
- Liver parenchymal damage
- Malnutrition
- Celiac disease (gluten-induced enteropathy) – chronic disease of the mucosa of the small intestine (small intestinal mucosa) due to hypersensitivity to the cereal protein gluten.
Azotemia
In azotemia, there is an abnormal increase of nitrogenous end products of protein metabolism (residual nitrogen) in the blood. Residual nitrogen substances include urea, uric acid, creatinine, creatine, amino acid, and ammonia. Azotemia is divided by cause into:
- Prerenal azotemia (causes lie before the kidney).
- Renal azotemia (causes lie in the kidney).
- Postrenal azotemia (causes lie after the kidney).
1. prerenal azotemia [urea ↑, creatinine normal (in the absence of renal failure); urea-creatinine quotient ↑]
- Catabolism (too much protein (protein) is broken down); causes:
- Protein breakdown due to disease (chronic obstructive pulmonary disease (COPD), tumor disease).
- Medicinal: therapy with glucocorticoids.
- Excessive protein intake through the diet (= exogenous azotemia).
- Decreased renal perfusion (kidney blood flow): e.g., heart failure (cardiac insufficiency), hypotension, shock; dehydration/fluid deficiency (diarrhea (diarrhea), vomiting, insufficient drinking, sweating).
2. renal azotemia [urea ↑, creatinine ↑; urea-creatinine ratio normal].
- Renal disease Note: Only severe restriction of renal function to about 30% of output will result in an increase in urea, i.e., in renal disease with a mildly to moderately impaired glomerular filtration rate (GFR) and good circulatory function, normal protein intake will not result in an increase in urea
Exceptions to urea creatinine quotient normal:
- Acute renal failure (ANV): urea-creatinine quotient ↑, because urea can rise faster than creatinine.
- Chronic renal failure:
- Urea-creatinine quotient normal or ↓ if protein intake too low.
- Urea-creatinine quotient ↑ if protein intake too high.
3. postrenal azotemia [urea ↑↑↑, creatinine ↑; urea-creatinine quotient ↑]
- Urinary obstruction/urinary retention (see below Urinary obstruction/urinary retention (obstructive uropathy and reflux uropathy)/differential diagnoses).
Reference ranges of the urea-creatinine quotient depending on selected units and laboratory parameters.
| Urea/urea-N (blood urea nitrogen, BUN) [unit]. | Urea [mmol/l) | Urea [mg/dl] | Urea-N (BUN) [mg/dl] |
| Creatinine [unit] | [mmol/l] | [mg/dl] | [mg/dl] |
| Reference ranges of urea-creatinine quotient | 25-40 | 20-35* | 10-16 |
Urea-creatinine quotients* .
- 20-35: normal (with normal diet and glomerular filtration rate (GFR)).
- <20: decreased protein catabolism (malnutrition, liver cirrhosis), decreased renal perfusion (heart failure, exsiccosis/dehydration, hypovolemia), decreased tubular reabsorption,
- > 35: increased protein catabolism (starvation, fever, burning or excessive protein intake).
Interpretation
| Urea | Creatinine | Hanrstoff creatinine quotient | |
| Prerenal azotemia | ↑ | normal | ↑ |
| Renal azotemia | ↑ | ↑ | Normal to decreased (depending on protein intake). |
| Postrenal azotemia | ↑↑↑ | ↑ | ↑ |
| Low protein intake; severe liver disease | ↓ | normal | ↓ |
CONCLUSION: Thus, renal azotemia can usually be distinguished from prerenal or postrenal azotemia. Further notes
- 1 g of excreted urea (in the urine) corresponds to 3 g of protein ingested with food.
