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ORGAN FUNCTION TEST
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Lecture notes of 6 pages for the course MBBS at ALL INDIA INSTITUTE OF MEDICAL SCIENCE (ORGAN FUNCTION TEST)
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196 Question Bank of BiochemistryBACHELOR OF SCIENCE IN NURSING (BSN)
B.SC NURSING
ORGAN FUNCTION TEST
Q. What are Renal Function Tests? List the Renal function tests? Describe in detail any
one RFT.
Ans. Kidney plays an important role in the maintenance of acid base balance and volume of water
in the body. It serves an important function of excretion of products of metabolism and other
harmful substances. The assessment of renal functions involves urine analysis, blood chemistry,
urography and some special renal function tests.
Each kidney contains about 1 million nephrons. A nephron has a glomerulus and a long tubule that
has three parts
(1) The proximal convoluted tubule (PCT)
(2) The thin loop of Henle (LH)
(3) The distal convoluted tubule (DCT)
The glomeruli are the ultra filter and the filtrate produced is like plasma except that it has almost
no protein, 180 litres of this filtrate is produced in 24 hrs, of which 178 litres of water and most of
the organic and inorganic solutes are reabsorbed. Normally, some components of the filtered solutes
are actively absorbed; glucose, phosphate, amino acids, sodium etc. for some solutes, the maximum
reabsorptive capacity of the tubule is limited and filtered material in excess of this limit is passed on
in the urine. Normal renal threshold for glucose is 180 mg m%; if excess is presented to the
nephrons, it would result in glycosuria. Other solutes are not reabsorbed or are only passively and
partially reabsorbed or are actively secreted by the tubule. Inulin, a carbohydrate used for renal
function studies is not at all reabsorbed by the tubules. Some urea is passively reabsorbed but most
of the filtered urea escapes reabsorption. Exogenous creatinine, H+, K+, phenol red (PSP), para
aminohippurate and penicillin are actively secreted by the tubule cells, thus increasing excretion over
the amount filtered.
Classification of Renal function Tests
On the basis of different functions of kidney the renal function test can be classified as follows
I. Test based on glomerular filtration
Urea Clearance Test
Endogenous Creatinine Clearance Test
Inulin Clearance Test
Cr51 EDTA Test
II. Test based on tubular function
Concentration and Dilution Test
, PSP excretion test
Measurement of tubular secretary mass
III. Tests to measure Renal Plasma Flow (RPF)
Para amino hippurate test (PAH)
Filtration fraction
Maximal Tubular Capacity (Tm)
I. Tests based on Glomerular Filtration
Clearance Tests
Urea clearance, creatinine clearance and inulin clearance test are used to examine for impairment
of glomerular filtration. Clearance is means of expressing quantitatively the rate of excretion of a given
substance by the kidney. This is defined as a volume of blood or plasma, which contains the amount
of substance, which is excreted in the urine in one minute or alternatively, the clearance of a
substance, may be defined as that volume of blood or plasma cleared of the amount of substance
found in one minute excretion of urine.
Urea Clearance Test
The normal blood urea level is in the range of 20 – 40 mg/dl. Uraemia donates a high level of
blood urea and is often due to retention caused by kidney damage. It can also rise in dehydration and
it catabolic states. Urea clearance is defined as the volume of blood or plasma cleared of urea per
minute. If the urine volume exceeds 2 ml/min, the rate of urea elimination is at a maximum and is
directly proportional to the concentration of urea in the blood.
Volume of blood-cleared urea per minute can be calculated from the formula
U×V
B
Where U = concentration of urea in urine (mg).
V = volume of urine in ml/min.
B = the concentration of urea in blood (mg).
Substituting average values, the number of ml of blood cleared of urea per minute
100 × 2.1
= = 75
2.8
It means that the amount of urea excreted in the urine in one minute is equal to the amount found
in 75 ml of blood.
The clearance which occurs when the urinary volume exceeds 2 ml/min is termed as Maximum
Urea Clearance (Cm) and averages normal value is 75.
Cm = 75 ml (normal range 75 ± 10)
Standard Clearance
When the urinary volume is less than 2 ml/min, the rate of urea elimination is reduced, because
relatively more urea is reabsorbed in the tubules, and is proportional to the square root of the urinary
volume. Such clearance is termed as Standard Clearance of Urea (Cs), and average normal value
is 54.
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