Kidney function tests or Renal function tests are comes under organ function tests that are done to investigate the functioning of the kidney.

1. Kidney function tests

6. a. Glomerular function tests
i. Clearance test :
a. Creatinine clearance test
b. Urea clearance test
c. Inulin clearance test.
ii. Blood analysis of urea and creatinine
iii. Test for protein in urine
b. Tubular function tests
i. Urine concentration test (fluid deprivation test)
ii. Urine dilution test (excess fluid intake test)
iii. Acid load test (urine acidification test)
iv. Phenosulfonaphthalein (PSP) test
c. Urine analysis
i. Physical examination
ii. Chemical examination
iii. Microscopic examination

7. Glomerular function tests

8. i. Clearance test
• These tests are performed to assess the glomerular filtration rate
(GFR)
• Clearance is defined as the volume of plasma (in ml) that could be
completely cleared off a substance per minute and is expressed as
as
milliliter per minute
• Renal clearance (C) is calculated by using following formula
C =
U × V
P
where,
C: Renal clearance = GFR of a substance in ml/minute
U: Concentration of substance in urine (mg/100 ml)
V: Volume of urine in ml excreted per minute
P: Concentration of substance in plasma (mg/100 ml).

9. a. Creatinine clearance test
• Based on the rate of excretion of creatinine by the kidneys
• The excretion of creatinine is not influenced by metabolism or
dietary factors.
• Creatinine is freely filtered at the glomerulus and is not
reabsorbed by the tubules.
• A small amount of creatinine is secreted by tubules.
• The creatinine clearance is determined by collecting urine over a
24 hour period and a sample of blood is drawn during the urine
collection period.
• Clearance is calculated by the formula.

10. Clinical interpretation
• The normal range for creatinine clearance is 90 to 120 ml/minute.
• A decreased creatinine clearance is a very sensitive indicator of a
decreased GFR.
• The reduced filtration rate may be caused by acute or chronic
damage to the glomerulus or any of its components.
• Reduced blood flow to the glomeruli may also produce a
decreased creatinine clearance.

11. ii. Urea clearance test
• Urea clearance is not as sensitive as creatinine clearance
• Unlike creatinine, 40–60% of urea is reabsorbed by the
renal tubules after being filtered at glomeruli.
• Hence, its clearance is less than GFR.
• Moreover, urea clearance is influenced by number of
factors, e.g. dietary protein, fluid intake, infection, surgery,
etc.

12. ii. Blood analysis of urea and creatinine
• Clearance determination- helpful in the early stages of
progressive renal disease
• Advanced renal failure - Blood analysis of urea and creatinine
• Impaired glomerular filtration - retention of urea and creatinine
• Elevation of blood urea (normal range 20-40 mg/dl) and
creatinine (normal range 0.5 to 1.5 mg/dl)
• An increase of these end products in the blood is called
azotemia.

13. iii. Test for protein in urine
• First sign of glomerular injury before a decrease in GFR.
• Condition is known as proteinuria.
• Excretion of albumin >300 mg/day - significant damage to
the glomerular membrane.
• Excretion of albumin in the range 30-300 mg/day is termed
microalbuminuria.
• It is the earliest sign of renal damage due to diabetes
mellitus and hypertension.

14. Tubular Function Tests

15. i. Urine concentration test (fluid
deprivation test)
Fluid intake is withheld for 15 hours.
First urine sample in the morning is collected and osmolality
or specific gravity is measured.
Osmolality > 850 mOsmol/kg or specific gravity of 1.025 is
normal

16. ii. Urine dilution test (excess fluid intake test)
After emptying the bladder, 1,000 to 1,200 ml of water is
given to the patient.
Urine specimens are then collected every hour for the next 4
hours.
Osmolality <100 mOsmol/kg or specific gravity of 1.005 is
normal

17. iii. Acid load test (urine acidification test)
Oral administration of Ammonium chloride (100 mg/kg body
weight)
Urine samples are collected hourly for the following 8
hours.
The urine pH falls below 5.5 in at least one sample.
In renal tubular pH remains between 5.7 and 7.0.

18. iv. Phenosulfonaphthalein (PSP) test
Intravenous injection of 6 mg of PSP in 1 ml of saline.
Urine specimen may be collected at 15, 30, 60 and 120
minutes.
15 minute urine - ≥ 25% of the injected PSP
First hour – 40% to 60%
Second hour - 20% to 25%
Excretion of less than 23% of the dye during the 15 minute
urine indicates impaired renal excretory function.

19. A man aged 35 years presenting with loin pain has a serum
creatinine of 1.7 mg/dL. A 24-hour urine of 2160 mL is collected
and found to have a creatinine concentration of 848 mg/L.
i. Calculate the creatinine clearance and comment on the results.
ii. ii. An error in the timed collection was subsequently reported by
the nursing staff, and collection time was reported to be 17
hours. How does this affect the result and its interpretation?