Biochemical Investigation of Kidney Function

The kidneys play a crucial role in maintaining the body’s internal environment, regulating the balance of water, electrolytes, and eliminating waste products. Biochemical tests help assess kidney function by analyzing indicators related to the filtration, reabsorption, and excretion processes of the kidneys.

I. Indicators for Investigating Kidney Function

1. PSP (Phenolsulfonphthalein) Test

This test is used to evaluate the excretory function of the renal tubules. PSP is a colored substance injected intravenously, subsequently filtered by the glomeruli and excreted through the renal tubules.

• Procedure: PSP is injected intravenously, and urine samples are collected 15 minutes, 30 minutes, 1 hour, and 2 hours after injection.

• Analysis: The amount of PSP excreted at each time point is determined and compared to normal values.

• Note:

• PSP can cause allergic reactions.

• This test should not be performed in patients with severe kidney failure.

2. Clearance

Clearance is a measure of the volume of blood cleared of a substance per unit time. The clearance formula is:

C = UV/P

Where:

• C: Clearance (ml/minute)

• U: Concentration of the substance in urine (mg/ml)

• V: Urine flow rate (ml/minute)

• P: Concentration of the substance in plasma (mg/ml)

a. Clearance Adjustment According to Body Surface Area:

Clearance values should be adjusted for body surface area to ensure accuracy.

The adjustment formula is:

Cc = Cx1.73/Sc

Where:

• Cc: Adjusted clearance (ml/minute)

• C: Clearance (ml/minute)

• Sc: Body surface area (m2)

b. Substances Chosen for Investigating Filtration Function:

• Selection Criteria:

• Filtered through the glomeruli without reabsorption or further excretion.

• Not bound to plasma proteins.

• Not rapidly metabolized or stored in the kidneys.

• Non-toxic, not altering kidney function.

• Easily quantifiable and affordable.

• Commonly Used Substances:

• Inulin: An exogenous substance used as the gold standard for assessing glomerular filtration function.

• Creatinine: An endogenous substance, easy to quantify, reflecting long-term glomerular filtration function.

• Urea: An endogenous substance reflecting both filtration and reabsorption functions of the kidneys.

• PAH (para-aminohippuric acid): An exogenous substance reflecting the filtration and excretion functions of the kidneys.

• Cystatin C: An endogenous substance, independent of age, gender, and body mass, providing a good reflection of glomerular filtration function.

• Beta2 microglobulin: An endogenous substance, providing a good reflection of glomerular filtration function in patients with chronic kidney failure.

c. Normal Clearance Values of Substances:

Substance	Clearance (ml/minute)
Urea	60-100
Creatinine	90-120
Inulin	100-130
PAH	400-700

d. Clearance in Kidney Diseases:

Disease	Inulin Clearance (ml/minute)	PAH Clearance (ml/minute)
Acute Glomerulonephritis	15-100	400
Chronic Glomerulonephritis	– Renal impairment: 40-80; End-stage: 0-40	– Renal impairment: 500; End-stage: 100
Chronic Pyelonephritis	50-90	425
Renal Arteriosclerosis	50-150	500

3. Urine Specific Gravity:

Urine specific gravity reflects the kidney’s ability to concentrate urine.

• Urine specific gravity when the kidneys are not working (no absorption and excretion): 1.01.

• In kidney diseases: Urine specific gravity is often lower or higher than normal, depending on the condition.

4. Tests to Investigate Proximal Tubule Function:

• PSP Excretion Test: Evaluates the excretory function of the renal tubules, reflecting proximal tubule function.

• Lithium Clearance Measurement: Evaluates the reabsorption capacity of lithium, reflecting proximal tubule function.

• Fractional Excretion: Evaluates the percentage of a substance filtered through the glomeruli and excreted in urine, reflecting proximal tubule function.

• Maximum PAH Excretion Test: Evaluates the maximum PAH excretion capacity, reflecting proximal tubule function.

• Maximum Glucose Reabsorption Test: Evaluates the maximum glucose reabsorption capacity, reflecting proximal tubule function.

5. Tests to Investigate Distal Tubule Function:

• Kidney’s ability to concentrate urine: Reflects distal tubule function.

• Urine osmolality measurement: Evaluates the ability to concentrate urine, reflecting distal tubule function.

• Dehydration Test: Evaluates the ability to concentrate urine, reflecting distal tubule function.

• Kidney’s ability to dilute urine: Reflects distal tubule function.

• Test to investigate H+ excretion capacity: Evaluates the ability to excrete H+ ions, reflecting distal tubule function.

II. Clinical Applications

1. Evaluating Glomerular Filtration Rate (GFR):

• Gold standard: Inulin clearance (=125 ml/minute).

• Purpose: Assessing the degree of decline in kidney filtration function.

2. Purpose of Measuring Clearance of Other Substances:

• Creatinine: Investigating the filtration and excretion functions of the kidneys.

• Urea: Investigating the filtration and reabsorption functions of the kidneys.

• PAH: Investigating the filtration and excretion functions of the kidneys.

3. Classifying Kidney Failure Severity:

Glomerular Filtration Rate (GFR) (ml/minute)	Kidney Failure Severity
>90	Stage 1 Kidney Failure (Very Mild)
60-89	Stage 2 Kidney Failure (Mild)
30-59	Stage 3 Kidney Failure (Moderate)
15-29	Stage 4 Kidney Failure (Severe)
<15	Stage 5 Kidney Failure (End-Stage)

4. Indications:

• Monitoring patients with chronic kidney disease: Tracking disease progression, adjusting medication dosage.

• Assessing kidney function before surgery: Preventing complications due to kidney failure.

• Monitoring patients with liver, cardiovascular, diabetes, etc.: These conditions can affect kidney function.

• Determining the cause of kidney failure: Assisting in diagnosing the cause of kidney failure.

5. Note:

• Test results should be combined with other factors such as:

• Clinical symptoms.

• Medical history.

• Other test results.

• Biochemical indicators can be influenced by factors such as:

• Dehydration.

• Medication use.

• Dietary habits.

III. Conclusion

Biochemical tests are a valuable tool for assessing kidney function. Biochemical indicators provide information about the kidney’s filtration, reabsorption, and excretion capabilities, helping to monitor disease conditions and adjust treatment effectively.