Urea and creatinine are nitrogenous end products of body metabolism. Urea is generated from dietary protein and tissue protein turnover. Muscle creatine catabolism generates creatinine. In Europe, the whole urea molecule is taken into consideration while in the United States, only the nitrogen component of urea (the blood urea nitrogen or serum urea nitrogen, i.e., BUN or SUN) is measured. The BUN is roughly one-half (28/60 or 0.446) of the blood urea.
This is the ratio of blood urea nitrogen to serum creatinine. Creatinine is a break-down product of creatine phosphate in muscle. It is usually produced at a constant rate by the body, however, it can vary according to the muscle mass. Men have more muscle mass than women and hence, tend to have higher levels of creatinine than women. Vegetarians usually have low creatinine levels. Kidneys play an important role in the filtration or removal of creatinine from the blood. When the kidneys do not filter the blood properly, elevated creatinine levels are observed. If more or complete estimation of renal function is to be done, then the blood (plasma) concentration of creatinine along with that of urea has to be taken into consideration. Measurement of BUN and creatinine helps detect other problems besides those related to the kidney. For instance, a disproportionate rise in urea level when compared with the creatinine may indicate a prerenal problem such as volume depletion. Creatinine levels in urine can help one calculate the creatinine clearance, which reflects the glomerular filtration rate (GFR). Glomeruli are the small capillaries which filter the blood during urine formation. The GFR is clinically important as renal function can be assessed with the help of the same.
The normal range of urea nitrogen in blood is around 5 to 20 mg/dl or 1.8 to 7.1 mmol urea per liter. The range is wide pertaining to normal variations due to protein intake, endogenous protein catabolism, state of hydration, hepatic urea synthesis, and renal urea excretion. For example, a BUN of 15 mg/dl would indicate 'significantly affected function' for a woman in the third trimester of pregnancy whereas a tough person who consumes 125 g of excess protein each day may have a normal BUN of 20 mg/dl. The pregnant woman's higher glomerular filtration rate (GFR), expanded extracellular fluid volume and anabolism in the developing fetus are responsible for her relatively low BUN of 5 to 7 mg/dl.
An elevated BUN can occur in case of diabetes, congestive heart failure, gastrointestinal bleeding (GI), dehydration, kidney dysfunction, hypertension, etc. Dry-itchy skin, insomnia, nausea, fatigue, etc. are some of the commonly noticed elevated BUN symptoms.
The normal serum creatinine (Cr) may vary according to the person's body muscle mass and the methods used to measure it. For the adult male, the normal creatinine levels range from 0.6 to 1.2 mg/dl, or from 53 to 106 μmol/L by the kinetic or enzymatic method and 0.8 to 1.5 mg/dl, or 70 to 133 μmol/L by the older manual Jaffé reaction. For an adult female, the normal creatinine levels in blood ranges from 0.5 to 1.1 mg/dl, or from 44 to 97 μmol/L by the enzymatic method.
Normal BUN and Creatinine Levels
Normally, BUN and creatinine should show a ratio of 10:1 to 20:1, if the age of the affected person is more than twelve months. In case of babies, younger than twelve months, the ratio can be around 30:1. The normal BUN values should be 5-15 mg/dL for children, 8-23 mg/dL for the adults, and 5-12 mg/dL during pregnancy.
A BUN-creatinine ratio greater than 20:1, is referred to as elevated BUN and creatinine levels which is usually noticed when the glomerular filtration rate (GFR) decreases. Low BUN creatinine levels, where the BUN-creatinine ratio is less than 10:1, indicates reduced reabsorption of BUN due to renal damage. Owing to the decreased muscle mass, the elderly affected people may have an elevated BUN:Cr ratio at baseline.
Low BUN creatinine levels indicate inadequate protein intake, reduced urea synthesis as in advanced liver disease, excessive excretion of urea as in sickle cell anemia, increased creatinine production as in rhabdomyolysis, or more effective removal of urea than creatinine, during dialysis.
The external factors affecting the levels of blood urea nitrogen and creatinine are high protein intake, gastrointestinal bleeding, fever, burns, excessive use of steroids, starvation, malnutrition, urinary tract obstruction, dehydration, etc. Measuring the levels of BUN and creatinine, periodically, helps monitor the functioning of kidneys and other organs.
Disclaimer: This HealthHearty article is for informative purposes only, and should not be used as a replacement for expert medical advice.