Renal Function Tests . 

Properties of Normal urine .

Volume : 1,000 to 1,500 mL/day 

Reaction : Slightly acidic with pH of 4.5 to 6 

Specific gravity : 1.010 to 1.025 

Osmolarity : 1,200 mOsm/L 

Color : Normally, straw colored 

Odor : Fresh urine has light aromatic odor. If stored for some time, the odor becomes stronger due to bacterial decomposition.

Composition of Urine .

Urine consists of water and solids. Solids include organic and inorganic substances .

Quantity of solids excreted in urine .

 

Renal function tests .

[1]. Renal function tests are the group of tests that are performed to assess the functions of kidney. 

[2]. Renal function tests are of three types: 

A. Examination of urine alone . 

B. Examination of blood alone .

C. Examination of blood and urine. 

Examination of Urine – Urinalysis .

Routine examination of urine or urinalysis is a group of diagnostic tests performed on the sample of urine. Urinalysis is done by: 

1. Physical examination .

2. Microscopic examination .

3. Chemical analysis.

Physical Examination .

1. Volume .

Increase in urine volume indicates increase in protein catabolism and renal disorders such as chronic renal failure, diabetes insipidus and glycosuria. 

2. Color .

[1]. Normally urine is straw colored. 

[2]. Abnormal coloration of urine is due to several causes such as jaundice, hematuria, hemoglobinuria, medications, excess urobilinogen, ingestion of beetroot or color added to food. 

3. Appearance .

[1]. Normally urine is clear. 

[2]. It becomes turbid in both physiological and pathological conditions. 

[3]. Physiological conditions causing turbidity of urine are precipitation of crystals, presence of mucus or vaginal discharge. 

[4]. Pathological conditions causing turbidity are presence of blood cells, bacteria or yeast. 

4. Specific Gravity .

[1]. Specific gravity of urine is the measure of dissolved solutes (particles) in urine. 

[2]. It is low in diabetes insipidus and high in diabetes mellitus, acute renal failure and excess medications. 

5. Osmolarity .

Osmolarity of urine decreases in diabetes insipidus. 

6. pH and Reaction .

[1]. Measurement of pH is useful in determining the metabolic or respiratory acidosis or alkalosis. 

[2]. The pH decreases in renal diseases. 

[3]. In normal conditions, pH of urine depends upon diet. 

[4]. It is slightly alkaline in vegetarians and acidic in non-vegetarians. 

Microscopic Examination .

Microscopic examination of centrifuged sediment of urine is useful in determining the renal diseases. 

1. Red Blood Cells .

Presence of red blood cells in urine indicates glomerular disease such as glomerulonephritis. 

2. White Blood Cells .

[1]. Normally few white blood cells appear in high power field. 

[2]. The number increases in acute glomerulonephritis, infection of urinary tract, vagina or cervix. 

3. Epithelial Cells .

[1]. Normally few tubular epithelial cells slough into urine. 

[2]. Presence of many epithelial cells suggests nephrotic syndrome and tubular necrosis. 

4. Casts .

[1]. Casts are the cylindrical bodies that are casted (molded) in the shape of renal tubule. 

[2]. Casts may be hyaline, granular or cellular in nature. 

[3]. Hyaline and granular casts, which are formed by precipitation of proteins may appear in urine in small numbers. 

[4]. The number increases in proteinuria due to glomerulonephritis. Cellular casts are formed by sticking together of some cells. 

[5]. Red blood cell casts appear in urine during glomerulonephritis and tubular necrosis. 

[6]. White blood cell casts appear in pyelonephritis. Epithelial casts are formed during acute tubular necrosis. 

5. Crystals .

[1]. Several types of crystals are present in normal urine. 

[2]. Common crystals are the crystals of calcium oxalate, calcium phosphate, uric acid and triple phosphate (calcium, ammonium and magnesium). 

[3].  Abnormal crystals such as crystals of cystine and tyrosine appear in liver diseases. 

6. Bacteria .

[1]. Bacteria are common in urine specimens because of normal microbial flora of urinary tract, urethra and vagina and because of their ability to multiply rapidly in urine. 

[2]. Culture studies are necessary to determine the presence of bacteria in urine. 

Chemical Analysis .

[1]. Chemical analysis of urine helps to determine the presence of abnormal constituents of urine or presence of normal constituents in abnormal quantity. 

[2]. Both the findings reveal the presence of renal abnormality. 

[3]. Following are the common chemical tests of urine :-

1. Glucose .

[1]. Glucose appears in urine when the blood glucose level increases above 180 mg/dL. 

[2]. Glycosuria (presence of glucose in urine) may be the first indicator of diabetes mellitus. 

2. Protein .

[1]. Presence of excess protein (proteinuria) particularly albumin (albuminuria) in urine indicates renal diseases. 

[2]. Urinary excretion of albumin in a normal healthy adult is about 30 mg/day. 

[3]. It exceeds this level in glomerulonephritis. It also increases in fever and severe exercise. 

3. Ketone Bodies .

Ketonuria (presence of ketone bodies in urine) occurs in pregnancy, fever, diabetes mellitus, prolonged starvation and glycogen storage diseases. 

4. Bilirubin .

Bilirubin appears in urine (bilirubinuria) during hepatic and post-hepatic jaundice. 

5. Urobilinogen .

Normally, about 1 to 3.5 mg of urobilinogen is excreted in urine daily. Excess of urobilinogen in urine indicates hemolytic jaundice. 

6. Bile Salts .

Presence of bile salts in urine reveals jaundice. 

7. Blood .

[1]. Presence of blood in urine (hematuria) indicates glomerulonephritis, renal stones, infection or malignancy of urinary tract. 

[2]. Hematuria must be confirmed by microscopic examination since chemical test fails to distinguish the presence of red blood cells or hemoglobin in urine. 

8. Hemoglobin .

Hemoglobin appears in urine (hemoglobinuria) during excess hemolysis. 

9. Nitrite .

Presence of nitrite in urine indicates presence of bacteria in urine since some bacteria convert nitrate into nitrite in urine.

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 are : 

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 & Urine .

Plasma Clearance .

[1]. Plasma clearance is defined as the amount of plasma that is cleared off a substance in a given unit of time. 

[2]. It is also known as renal clearance. 

[3]. It is based on Fick principle. 

[4]. 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. 

[5]. 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

 C  =  UV / 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 .

[1]. A substance that is completely filtered but neither reabsorbed nor secreted should be used to measure glomerular filtration rate (GFR). 

[2]. Inulin is the ideal substance used to measure GFR. 

[3]. It is completely filtered and neither reabsorbed nor secreted. So, inulin clearance indicates GFR. 

Inulin clearance .

[1]. 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,

Glomerular filtration rate =  UV / P  = 125 x 1 / 1 = 125 mL/min 

                                                             

Creatinine clearance .

[1]. Creatinine clearance is also used to measure GFR accurately. 

[2]. It is easier than inulin clearance, because, creatinine is already present in body fluids and its plasma concentration is steady throughout the day. 

[3]. It is completely filtered and being a metabolite it is neither reabsorbed nor secreted. 

[4]. The normal value of GFR by this method is approximately the same as determined by inulin clearance. 

2. Measurement of Renal Plasma Flow .

[1]. To measure renal plasma flow, a substance, which is filtered and secreted but not reabsorbed, should be used. Such a substance is para­-aminohippuric acid (PAH). 

[2]. PAH clearance indicates the amount of plasma passed through kidneys. 

[3]. 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. 

[4]. 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: 

1. Renal plasma flow .

2. Percentage of plasma volume in blood. 

1. Renal plasma flow .

Renal plasma flow is measured by using PAH clearance. 

2. 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. 

 

Urea Clearance Test  .

[1]. Urea clearance test is a clinical test to assess renal function by using clearance of urea from plasma by kidney every minute. 

[2]. 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. 

[3]. Normal value of urea clearance is 70 mL/min. 

[4]. 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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