EXAMINATION OF BLOOD
1. Estimation of
Plasma Proteins
Normal values of
plasma proteins:
Total proteins : 7.3
g/dL (6.4 to 8.3 g/dL)
Serum albumin : 4.7 g/dL
Serum globulin : 2.3
g/dL
Fibrinogen : 0.3 g/dL
Level of plasma
proteins is altered during renal failure.
2. Estimation of
Urea, Uric Acid and Creatinine
Normal values :
Urea : 25 to 40 mg/dL
Uric acid : 2.5 mg/dL
Creatinine : 0.5 to 1.5 mg/dL
The blood level of
these substances increases in renal failure.
EXAMINATION OF BLOOD AND URINE
Plasma Clearance
Plasma clearance is defined
as the amount of plasma that is cleared off a substance in a given unit of
time. It
is also known as renal clearance. It is based on Fick principle. Determination
of clearance value for certain substances helps in assessing the following renal functions:
1. Glomerular
filtration rate
2. Renal plasma flow
3. Renal blood flow.
Value of following factors is required
to determine the plasma clearance of a particular substance:
1. Volume of urine excreted
2. Concentration of
the substance in urine
3. Concentration of the substance in
blood.
Formula to calculate
clearance value
U V
C =
P
Where, C = Clearance
U = Concentration of the substance in
urine
V = Volume of urine
flow
P = Concentration of the substance in
plasma.
1. Measurement of
Glomerular Filtration Rate
A substance that is completely
filtered but neither reabsorbed
nor secreted should be used to measure
glomerular filtration rate
(GFR). Inulin is the ideal substance used to measure GFR. It is completely
filtered and neither
reabsorbed nor secreted. So, inulin clearance indicates GFR.
Inulin clearance
A known amount of inulin is injected
into the body. After sometime, the concentration of inulin in plasma and urine
and the volume of urine excreted are estimated.
For example,
Concentration of
inulin in urine = 125 mg/dL
Concentration of inulin in plasma = 1
mg/dL
Volume of urine output = 1 mL/min Thus,
U V 125 × 1
Glomerular filtration
rate = =
P 1
= 125 mL/min
Creatinine clearance is also used to
measure GFR accurately. It is easier than inulin clearance, because,
creatinine is already
present in body fluids and its plasma concentration is steady throughout the day. It
is completely filtered
and being a metabolite it is neither reabsorbed nor secreted. The normal value
of GFR by this
method is approximately the same as determined by inulin clearance.
2. Measurement of
Renal Plasma Flow
To measure renal plasma
flow, a substance, which is filtered and secreted but not reabsorbed, should be
used. Such a substance is paraaminohippuric acid
(PAH).
PAH clearance indicates the amount of plasma passed through kidneys. A known amount of PAH
is injected into the body. After sometime, the concentration of PAH in plasma and urine
and the volume of urine excreted are estimated.
For example,
Concentration of PAH in urine = 66
mg/dL
Concentration of PAH in plasma = 0.1
mg/dL
Volume of urine output = 1 mL/min
Thus, U V Renal
plasma flow =
P 66 × 1 = 0.1
= 660 mL/min
Diodrast clearance also can be used to
measure this.
3. Measurement of
Renal Blood Flow
Values of factors necessary to
determine renal blood
flow are:
i. Renal plasma flow
ii. Percentage of
plasma volume in blood.
i. Renal
plasma flow
Renal plasma flow is
measured by using PAH clearance.
ii. Percentage of plasma volume in
blood
Percentage of plasma volume
is indirectly determined by using packed cell volume (PCV).
For example,
If PCV = 45%
Plasma volume in the
blood = 100 – 45 = 55%
That is 55 mL of
plasma is present in every 100 mL of blood.
Calculation
of renal blood blow
Renal blood flow is
calculated with the values of renal plasma volume and percentage of plasma in
blood by using a formula given below.
Renal plasma flow
Renal blood flow = % of plasma in blood
For example,
Renal plasma flow =
660 mL/min
Amount of plasma in blood = 55%
660
Renal blood flow =
55/100
= 1,200 mL/min
Urea Clearance Test
Urea clearance test is a clinical test
to assess renal function by using clearance of urea from plasma by
kidney every minute.
This test requires a blood sample to determine urea level in blood and two
urine sample collected at 1 hour interval to determine the urea cleared by
kidneys into urine. Normal value of urea clearance is 70 mL/min. Urea is a waste
product formed during protein metabolism and excreted in urine. So,
determination of urea clearance forms a specific test to
assess renal function
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