Kidney function tests types, classification, Normal and Abnormal values

This article is to illustrate the Kidney function tests with their normal values, classifications, causes of increased and decreased values of tests .
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.

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