Essentials of Clinical Nephrology (Shorouk Press, Cairo, 2000, ISBN ...

Where
C= creatinine clearance
U= urine concentration of creatinine
V= urine flow rate (minute or second)
P= plasma concentration of creatinine
Normal creatinine clearance in adult male is 90 -150 ml/minute. To
estimate creatinine clearance, the patient should collect 24 hours urine from
which V and U could be estimated then, blood is withdrawn for P estimation.
The major disadvantage of clearance study is the possible faulty
collection of 24 hours urine, especially in females.
99mTc-DTPA or 151Cr-labeled EDTA or iothalamate isotope renal
scan is an alternative method which does not require urine collection. The
199mTc-DTPA is injected I.V. and multiple images of the kidney are obtained
over 30 minutes. This study provides the total and split (right and left) kidney
GFR.
B. TESTS FOR TUBULAR FUNCTIONS:
1. Urine Acidification Test: This is indicated to test for the ability of the
kidney to acidify urine (excrete H+). This is done by decreasing
plasma pH (i.e. inducing acidosis) by giving gelatin-coated ammonium
chloride capsules 0.1 g/kg with water and checking the urine pH hourly
for 6 hours. Normally, it should drop < 5.4. Otherwise, it will indicate
renal tubular acidosis (RTA). If blood pH is already low (acidosis),
there will be no need for giving ammonium chloride and check urine
pH directly. As a screening test we can look for urine pH of the first
morning voided urine (the highest acidic urine), which should be < 5.4.
Presence of urinary infection with urea- splitting organism makes the
urine alkaline and interferes with these tests. Therefore, irradication of
this infection is mandatory before doing this test.
2. Urine Concentration Test: for screening purpose examine early
morning specimen for osmolality. If it is > 700 mosmol/L, concentrating
capacity is considered normal and there would be no need for further
investigation. Otherwise, we may either do water deprivation test or
vasopressin (ADH) test.
In water deprivation test the patient is asked to stop fluid intake
completely. This results in a progressive increase in plasma osmolality
(normal 290 mosmol/L). In a normal person this should be followed by a