Clearance Tests and Micturition
Clearance Tests
Clearance of a substance may be defined as the volume of blood or plasma that is completely cleared of that substance per minute through the excretion of urine. Clearance tests help assess the kidney’s filtering capacity and are used to estimate glomerular filtration rate (GFR).
Salient Features of Substances Used for Clearance Tests
- It must be non-toxic.
- It should be small enough to pass through the glomerular membrane.
- It should neither be secreted nor absorbed by the renal tubules.
- It should not be metabolized in the body.
Formula
The clearance (C) of a substance can be calculated using the formula:
C = (U × V) / P
- U = Concentration of the substance in urine
- P = Concentration of the substance in plasma or serum
- V = Volume of urine excreted per minute
Clearance is expressed in mL/minute.
Types of Clearance Tests
- Creatinine Clearance Test
- Urea Clearance Test
- Inulin Clearance Test
1. Creatinine Clearance Test
The creatinine clearance test is one of the most widely used methods to evaluate kidney function and estimate the glomerular filtration rate (GFR).
Formula
Creatinine Clearance = (Urine creatinine in mg/dL × Total volume of urine) / (Serum creatinine in mg/dL × 1440)
Why Creatinine is Used
- Creatinine is produced endogenously from muscle metabolism.
- It is released into body fluids at a constant rate.
- Plasma creatinine levels remain within narrow limits and are not affected by diet.
Advantages
- Extrarenal factors rarely interfere with results.
- Conversion of creatine phosphate to creatinine is spontaneous and non-enzymatic.
- Blood can be collected at any time as creatinine levels remain stable.
- Not affected by diet or exercise.
Disadvantages
- Creatinine is both filtered and actively secreted by renal tubules (around 10% tubular secretion), leading to a slight overestimation of GFR (by about 10–20 mL/min).
- In severe renal impairment, extra-renal excretion increases due to bacterial degradation in the intestine.
- Early decreases in GFR may not be detected (“creatinine blind area”).
- Other pre-renal, renal, and post-renal factors may influence clearance results.
Significance
A decreased creatinine clearance is a sensitive indicator of reduced glomerular filtration rate, often seen in acute or chronic kidney disease.
2. Urea Clearance Test
The urea clearance test measures the volume of blood that contains the amount of urea excreted per minute by the kidneys. It helps assess both glomerular and tubular functions.
Formula for Maximum Urea Clearance
Maximum Urea Clearance = (U × V) / P
- U = mg of urea per mL of urine
- P = mg of urea per mL of plasma
- V = mL of urine excreted per minute
Normal value: 75 mL/min
Interpretation of Urea Clearance Value
- Values below 75% of normal indicate impaired kidney function.
- Clearance values decrease progressively with renal failure.
- Abnormal clearance may appear even when plasma urea levels are normal.
- Plasma urea rises only when clearance falls below 50% of normal.
- As urea is reabsorbed in both PCT and DCT, tubular function affects clearance.
Hence, creatinine clearance or cystatin C estimation are preferred for more accurate assessment. Urea clearance is typically less than GFR.
Normal serum urea value: 20–40 mg/dL
3. Inulin Clearance Test
The inulin clearance test is the most accurate method for determining the glomerular filtration rate (GFR).
Principle
Inulin is an inert polysaccharide that passes freely through the glomeruli and is neither reabsorbed nor secreted by the renal tubules, making it the gold standard for GFR measurement.
Disadvantages
- Requires intravenous administration of inulin.
- Procedure is technically complex and costly.
- Difficult analysis limits its routine clinical use.
Micturition
Micturition refers to the process of emptying the urinary bladder, also known as urination or voiding. It involves both involuntary and voluntary control mechanisms.
Reflex Micturition (Infants)
- In infants, bladder filling activates stretch receptors in the bladder wall.
- This triggers sensory impulses to the spinal cord, initiating a spinal reflex.
- The reflex causes contraction of the detrusor muscle and relaxation of the internal urethral sphincter, leading to urination.
Voluntary Control (Adults)
- As bladder fills (around 300–400 mL in adults), stretch receptors send signals to the brain.
- Awareness of the need to urinate develops, but voluntary contraction of the external urethral sphincter and pelvic floor muscles can delay urination until convenient.
- Urination can be assisted by increasing intra-abdominal pressure via contraction of abdominal muscles and lowering of the diaphragm.
Clinical Note
- Overdistension of the bladder is painful and may cause involuntary leakage if the external sphincter relaxes.
- Incontinence refers to involuntary urination after bladder control has been established.
Detailed Notes:
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