This article is to illustrate the Kidney function tests with their normal values, classifications, causes of increased and decreased values of tests .
• The level of urea depends both on the G.F.R & its production rate. Also it is heavily influenced by protein intake, increased tissue catabolism and GIT bleeding (++ production). So it ++ with protein diet, tissue breakdown (provided that there is renal insufficiency).
• Also it decreases with chronic advanced liver disease (as it is produced in the liver) and with reduced catabolism in old age (-- production).
• Example:- (in patient with the serum creatinine 10 mg/dl + mild ++ urea, associated liver disease must be considered).
It is similar to blood urea as regard the influencing factors affecting its blood level.
Then,, Urea = BUN x 2
• The level of the serum creatinine is much less dependent on the diet but is more related to the age, sex & muscular mass.
• So S. creatinine is a better guide to (GFR) than urea.
• It starts elevation when kidney Function (GFR) is reduced > 70% .
The Kidney function tests include :
Blood urea, Blood urea nitrogen (BUN), BUN : Creatinine Ratio, Creatinine Clearance , Cystatin-C level in addition to serum levels of Na and K .
Illustration is supported by guiding examples .
1-Blood urea
(Normal = 20 -40 mg/dl)• The level of urea depends both on the G.F.R & its production rate. Also it is heavily influenced by protein intake, increased tissue catabolism and GIT bleeding (++ production). So it ++ with protein diet, tissue breakdown (provided that there is renal insufficiency).
• Also it decreases with chronic advanced liver disease (as it is produced in the liver) and with reduced catabolism in old age (-- production).
• Example:- (in patient with the serum creatinine 10 mg/dl + mild ++ urea, associated liver disease must be considered).
2- Blood urea nitrogen (BUN)
(Normal = 10-20 mg/dl)It is similar to blood urea as regard the influencing factors affecting its blood level.
Then,, Urea = BUN x 2
3- Serum creatinine
(Normal = 0.7-1.4 mg/dl)• The level of the serum creatinine is much less dependent on the diet but is more related to the age, sex & muscular mass.
• So S. creatinine is a better guide to (GFR) than urea.
• It starts elevation when kidney Function (GFR) is reduced > 70% .
4- BUN : Creatinine Ratio
(normally it is 10: 1)
Causes of Increased ratio
Example : (urea 400 mg/dl, S.cr. 4 mg/dl)
'.' BUN = 200 :. ratio 50 : 1
1. High protein diet.
2. GIT bleeding.
3. Hypercatabolic patient e.g. trauma, tissue breakdown
4. Corticosteroid therapy.
6. Sepsis
Causes of Decreased ratio
Example : (urea 120 mg/dl, S.cr. 10 mg/dl)
'.' BUN = 60 :. ratio 6 : 1
1. Associated liver disease.
3. Haemodialysis patients (urea more dialyzable).
4. Low protein diet .
5- Creatinine Clearance
6- Cystatin-C level :
correlates better with radioisotope estimation of GFR than creatinine and more sensitive. It is not secreted by tubules and is unaffected by gender.
Classification of Kidney function tests
A- Glomerular function assessment :
- Blood urea.
- Serum creatinine, cytatin -C Level.
- GFR measurement by renal scan or by creatinine clearance.
B- Tubular function assessment :
- Proximal tubular function is assessed by serum potassium and phosphorus, also by detection of Glycosuria in absence of hyperglycemia, amino aciduria and B2 microglobulin in urine.
- Distal tubular function assessed by measurement of urinary acidification by ammonium chloride test and concentrating capacity by water deprivation test.