LABORATORY TESTS TO EVALUATE TUBULAR FUNCTION
Tests to Assess Proximal Tubular Function
In renal glycosuria, glucose is excreted in urine, while blood glucose level is normal. This is because of a specific tubular lesion which leads to impairment of glucose reabsorption. Renal glycosuria is a benign condition. Glycosuria can also occur in Fanconi syndrome.
2. Generalized aminoaciduria
In proximal renal tubular dysfunction, many amino acids are excreted in urine due to defective tubular reabsorption.
3. Tubular proteinuria (Low molecular weight proteinuria)
Normally, low molecular weight proteins (β2 –microglobulin, retinol-binding protein, lysozyme, and α1 –microglobulin) are freely filtered by glomeruli and are completely reabsorbed by proximal renal tubules. With tubular damage, these low molecular weight proteins are excreted in urine and can be detected by urine protein electrophoresis. Increased amounts of these proteins in urine are indicative of renal tubular damage.
4. Urinary concentration of sodium
If both BUN and serum creatinine are acutely increased, it is necessary to distinguish between prerenal azotemia (renal underperfusion) and acute tubular necrosis. In prerenal azotemia, renal tubules are functioning normally and reabsorb sodium, while in acute tubular necrosis, tubular function is impaired and sodium absorption is decreased. Therefore, in prerenal azotemia, urinay sodium concentration is < 20 mEq/L while in acute tubular necrosis, it is > 20 mEq/L.
5. Fractional excretion of sodium (FENa)
Measurement of urinary sodium concentration is affected by urine volume and can produce misleading results. Therefore, to avoid this, fractional excretion of sodium is calculated. This refers to the percentage of filtered sodium that has been absorbed and percentage that has been excreted. Measurement of fractional sodium excretion is a better indicator of tubular absorption of sodium than quantitation of urine sodium alone.
This test is indicated in acute renal failure. In oliguric patients, this is the most reliable means of early distinction between pre-renal failure and renal failure due to acute tubular necrosis. It is calculated from the following formula:
(Plasma sodium × Urine creatinine)
In pre-renal failure this ratio is less than 1%, and in acute tubular necrosis it is more than 1%. In pre-renal failure (due to reduced renal perfusion), aldosterone secretion is stimulated which causes maximal sodium conservation by the tubules and the ratio is less than 1%. In acute tubular necrosis, maximum sodium reabsorption is not possible due to tubular cell injury and consequently the ratio will be more than 1%. Values above 3% are strongly suggestive of acute tubular necrosis.
Tests to Assess Distal Tubular Function
1. Urine specific gravity
Normal specific gravity is 1.003 to 1.030. It depends on state of hydration and fluid intake.
- Causes of increased specific gravity:
a. Reduced renal perfusion (with preservation of concentrating ability of tubules),
e. Urinary tract obstruction.
- Causes of reduced specific gravity:
a. Diabetes insipidus
b. Chronic renal failure
c. Impaired concentrating ability due to diseases of tubules.
As a test of renal function, it gives information about the ability of renal tubules to concentrate the glomerular filtrate. This concentrating ability is lost in diseases of renal tubules.
Fixed specific gravity of 1.010, which cannot be lowered or increased by increasing or decreasing the fluid intake respectively, is an indication of chronic renal failure.
2. Urine osmolality
The most commonly employed test to evaluate tubular function is measurement of urine/plasma osmolality. This is the most sensitive method for determination of ability of concentration. Osmolality measures number of dissolved particles in a solution. Specific gravity, on the other hand, is the ratio of mass of a solution to the mass of water i.e. it measures total mass of solute. Specific gravity depends on both the number and the nature of dissolved particles while osmolality is exact number of solute particles in a solution. Specific gravity measurement can be affected by the presence of solutes of large molecular weight like proteins and glucose, while osmolality is not. Therefore measurement of osmolality is preferred.
When solutes are dissolved in a solvent, certain changes take place like lowering of freezing point, increase in boiling point, decrease in vapor pressure, or increase of osmotic pressure of the solvent. These properties are made use of in measuring osmolality by an instrument called as osmometer.
Osmolality is expressed as milliOsmol/kg of water.
Urine/plasma osmolality ratio is helpful in distinguishing pre-renal azotemia (in which ratio is higher) from acute renal failure due to acute tubular necrosis (in which ratio is lower). If urine and plasma osmolality are almost similar, then there is defective tubular reabsorption of water.
3. Water deprivation test
If the value of baseline osmolality of urine is inconclusive, then water deprivation test is performed. In this test, water intake is restricted for a specified period of time followed by measurement of specific gravity or osmolality. Normally, urine osmolality should rise in response to water deprivation. If it fails to rise, then desmopressin is administered to differentiate between central diabetes insipidus and nephrogenic diabetes insipidus. Urinary concentration ability is corrected after administration of desmopressin in central diabetes insipidus, but not in nephrogenic diabetes insipidus.
If urine osmolality is > 800 mOsm/kg of water or specific gravity is ≥1.025 following dehydration, concentrating ability of renal tubules is normal. However, normal result does not rule out presence of renal disease.
False result will be obtained if the patient is on low-salt, low-protein diet or is suffering from major electrolyte and water disturbance.
4. Water loading antidiuretic hormone suppression test
This test assesses the capacity of the kidney to make urine dilute after water loading.
After overnight fast, patient empties the bladder and drinks 20 ml/kg of water in 15-30 minutes. The urine is collected at hourly intervals for the next 4 hours for measurements of urine volume, specific gravity, and osmolality. Plasma levels of antidiuretic hormone and serum osmolality should be measured at hourly intervals.
Normally, more than 90% of water should be excreted in 4 hours. The specific gravity should fall to 1.003 and osmolality should fall to < 100 mOsm/kg. Plasma level of antidiuretic hormone should be appropriate for serum osmolality. In renal function impairment, urine volume is reduced (<80% of fluid intake is excreted) and specific gravity and osmolality fail to decrease. The test is also impaired in adrenocortical insufficiency, malabsorption, obesity, ascites, congestive heart failure, cirrhosis, and dehydration.
This test is not advisable in patients with cardiac failure or kidney disease. If there is failure to excrete water load, fatal hyponatremia can occur.
5. Ammonium chloride loading test (Acid load test)
Diagnosis of renal tubular acidosis is usually considered after excluding other causes of metabolic acidosis. This test is considered as a ‘gold standard’ for the diagnosis of distal or type 1 renal tubular acidosis. Urine pH and plasma bicarbonate are measured after overnight fasting. If pH is less than 5.4, acidifying ability of renal tubules is normal. If pH is greater than 5.4 and plasma bicarbonate is low, diagnosis of renal tubular acidosis is confirmed. In both the above cases, further testing need not be performed. In all other cases in which neither of above results is obtained, further testing is carried out. Patient is given ammonium chloride orally (0.1 gm/kg) over 1 hour after overnight fast and urine samples are collected hourly for next 6-8 hours. Ammonium ion dissociates into H+ and NH3. Ammonium chloride makes blood acidic. If pH is less than 5.4 in any one of the samples, acidifying ability of the distal tubules is normal.
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