EXAMINATION OF BLOOD

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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