Q.1- What is the normal range of blood urea?
Answer- Blood urea in normal health ranges between 20-40 mg/dl.
Q.2- How is urea formed in the body?
Answer- Urea is the end product of protein metabolism. It is formed in the liver from ammonia and carbon dioxide;both of them are considered to be waste products of the body. Ammonia is highly toxic, it is detoxified through conversion to urea, which is non toxic and water-soluble and is excreted through urine by the kidneys. In higher concentration urea is also toxic. Hence formation and excretion of urea is dependent on liver and kidney functions. In liver disorders urea formation is impaired hence blood urea decreases while in disorders of kidney; excretion of urea is impaired resulting in high blood urea levels. Hence blood urea level can be considered a predictor of hepatic or renal functional status.
Q. 3-What is the difference between Blood Urea and Blood Urea Nitrogen (BUN)?
Answer-Blood urea is sometimes expressed in terms of nitrogen. Such expression is very common in clinical practice. Molecular weight of urea is 60 and each gram mol of urea contains 28 gram of nitrogen. Thus a serum concentration of 28 mg/dl of BUN is equivalent to 60mg/dl of urea. Any value of BUN can be converted to urea by multiplying the figure by 2.14.
Blood urea= BUN x2.14
For example if BUN for a patient is 30 mg/dl,
Blood urea = 30x 2.14=64.20mg/dl.
Q.4- What is meant by NPN (Non protein Nitrogen)?
Answer- NPN includes Urea, uric acid and Creatinine. The major route for excretion of these compounds is urine. In kidney dysfunction the levels of these compounds are elevated in plasma. Of the three, creatinine estimation is the most specific index of renal function. Urea level depends on the protein intake and protein catabolism and also on age of an individual. It is also affected by volume of plasma.
Other minor components of NPN are urobilinogen, Indican, ammonia and amino acids.
Q.5- What is Uremia? What are the conditions causing uremia?
Answer- The clinical state with blood urea higher than normal is called Uremia. The conditions causing high urea level are as follows-
A) Physiological – Advancing age and high protein diet.
B) Pathological-Classified in to three categories-
a) Pre Renal– Primarily there is reduction of plasma volume, leading to lowering of blood pressure with consequent reduction of renal blood flow and Glomerular filtration rate(GFR). This leads to urea retention. Reduced plasma volume is seen in
i) Dehydration- as in severe vomiting, intestinal obstruction, pyloric stenosis, severe prolonged diarrhea, fluid depletion associated with Diabetic keto acidosis, shock, burns and hemorrhages.
ii) Increased protein catabolism- as in High fever, toxic state, metabolic response to injury, hemorrhage in to the alimentary tract, digestion of protein passing along the intestine and later deamination of amino acids.
b) Renal Uremia-In renal diseases there is reduction of GFR resulting in urea retention. Acute renal failure, acute glomerulonephritis, Malignant Hypertension and Pyelonephritis, all of them produce increase in the blood urea levels.
c) Post Renal Uremia– Obstruction to the out flow of urine after it leaves the kidney leads to back pressure on the renal pelvis,diminished glomerular filtration of urea with consequent increase in the blood urea level.
Renal stone, stricture,Enlargement of prostate and malignant tumors may produce post renal uremia.
Q.6- Under what conditions blood urea is lower than normal?
Answer- Blood urea level is low in liver diseases (Since urea is synthesized in liver),in pregnancy, growing period ,recovery from illness and in tissue healing, since in all these later conditions, there is positive nitrogen balance and amino groups of the amino acids are not available for urea formation. In disorders of urea cycle also there is impaired urea formation, hence blood urea is low.
Q.7-What is nitrogen balance?
Answer- A normal healthy adult is said to be in nitrogen balance, because the dietary intake equals the loss through urine, feces and skin. When the excretion exceeds intake, it is negative nitrogen balance. When the intake exceeds excretion, it is said to be positive nitrogen balance.
a)Positive nitrogen balance is observed in- Pregnancy- due to enlargement of uterus and fetal growth, growing period, during convalescence(recovery from illness or surgery due to active regeneration of tissues) and under the influence of Growth hormone, Insulin and androgens.
b) Negative nitrogen Balance is observed in- acute illness,surgery, trauma, burns, malignancy, diabetes mellitus and chronic, debilitating diseases and in protein energy malnutrition. Corticosteroids cause a negative nitrogen balance.
Q.8- Define clearance, what is its significance?
Answer- Clearance is defined as the volume of plasma completely cleared of a substance per unit time and is expressed in ml/minute. In other words it is the ml of plasma which contains the amount of a substance excreted by the kidney in one minute. For example Urea clearance of 75 ml/minute means, 75 ml of plasma gets completely cleared of urea in one minute by excretion of urea through urine by kidney.
Q.9- If a person has blood urea as 54 mg/dl and urea clearance 25 ml/minute, comment on the functional status of the kidney.
Answer- Urea clearance is affected by volume of urine excreted per minute. If the excretion of urine is at the rate of 2 ml or more the clearance is designated as Maximum urea clearance (Cm) and if the rate of excretion is less than 2ml/minute, the clearance is designated as Standard urea clearance (Cs). The average normal values are 75 ml/minute for Maximum urea clearance (Cm)and 54 ml/minute for Standard urea clearance (Cs).
Now in the given patient, the urea clearance is 25 ml/minute means only 25 ml of plasma is getting completely cleared of urea per minute, this is very low in comparison to the normal clearance values, that means kidney is failing to excrete urea in urine , hence it is getting accumulated in blood to cause high urea level, as is apparent from the high level of 54 mg/dl in this patient. So overall interpretation is that there is functional impairment of the kidney, this deficit seems to be a mild to moderate functional deficit.
Q.10- Calculate the urea clearance for the following three patients from the values given below and comment on the functional status of the kidney.
Patient Blood urea Urinary Urea Volume of urine excreted/ minute
(mg/dl) (G/L) (ml/24 hours)
1) 40 12 1500
2) 80 10 1000
3) 64 18 1200
Blood urea (P) =40 mg/dl
Urinary urea (U) = 12 G/L =12,000mg/L = 1200 mg/dl
( Value should be in mg/dl)
Volume of urine excreted/day – 1500 ml/24 hours
Convert it into ml/minute= 1500/24×60 ml/minute
Or 1500/1440= 1.04ml/minute
Since V is less than 2 ml/minute, hence standard urea clearance is to be considered
Cs = U√V
= 1200 x√1.04
= 30.6 ml/min (approx)
The value of urea clearance is expressed in terms of percentage of the normal (Which is 54 ml/min– normal Cs)
Hence Percentage urea clearance = 30.6 x 100
= 56.6 %
It is mild renal functional deficit (The value between 50-70% of the normal is considered mild renal deficit)
(Calculate for the other two patients- self exercise)
Q.11- If blood urea is high and serum Creatinine is normal what is the probable diagnosis?
Answer- In renal failure both blood urea and creatinine should be high, but in the given case normal creatinine rules out the possibility of renal failure. High blood urea can be due to physiological factors like high protein diet or advancing age or it could be pre renal uremia. In post renal uremia if the obstruction is not relieved renal damage does occur and in that case creatinine also rises after a time.
Q.12- In a patient suffering from cirrhosis of liver, what results do you expect from the following biochemical parameters-
i) Blood Urea ii) Blood Ammonia iii) CSF Glutamine iv)Urinary Urea
Answer- In Cirrhosis of liver, blood urea is low ( since urea is formed in the liver), blood ammonia is high(since ammonia is not getting converted to urea), CSF glutamine is high (Ammonia couples with glutamate to form Glutamine) and urinary urea is low since blood urea is also low.
Q.13- A known diabetic patient has been brought to emergency with nausea, vomiting, extreme weakness, puffiness of face and mental confusion. His biochemical profile is as follows
Blood urea-80 mg/dl
Serum Creatinine- 3.4 mg/dl
Random blood glucose- 234 mg/dl
Serum total proteins- 4.5 G/dl
Serum Total Cholesterol- 335mg/dl
Urine analysis – Proteins and glucose are present
Make a probable diagnosis.
Answer- The patient is suffering from Diabetic nephropathy which has progressed to renal failure, High blood glucose,cholesterol, low serum total proteins,proteinuria and glycosuria go in favor of diagnosis of diabetic nephropathy and high blood urea and creatinine signify Renal failure.Please help "Biochemistry for Medics" by CLICKING ON THE ADVERTISEMENTS above!