03 September 2016

Lecture #9: G.I. FUNCTION TESTS




GASTRIC ANALYSIS


Analysis of gastric secretions usually implies measurement of total output. However, detection of other components such as blood or lactic acid, which are not normally found in gastric contents, may be of clinical significance.

There are no definite cut–off points for anacidity, normal acidity and hypersecretion. Only at the extremes of secretion, such as anacidity in the patient with pernicious anemia or hypersecretion of acid that occurs in the patient with Zollinger–Ellison syndrome, can any underlying disease be diagnosed with certainty.


Importance of Gastric Analysis

1.      Determination of whether a patient is capable of secreting any gastric acid.
2.      Diagnosis of patients with symptoms of peptic ulcer disease
3.      Diagnosis of Zollinger–Ellison syndrome

Total gastric acid was originally defined as the sum of combined acid and free acid. Combined acid is that fraction complexed with physiologic buffers such as proteins and salts and free acid is that part that exceeds the buffering capacity of gastric fluid.


Collection of specimen

1.      The patient must fast overnight for 12 hours; intake of food and liquid, smoking and physical exertion are prohibited.

2.      The patient is intubated with a nasogastric tube, and the tip of the tube is positioned in the lowest portion of the stomach.

3.      Residual gastric fluid is first aspirated and then discarded and then four samples are collected at 15 minutes intervals.

4.      After the collection of the basal acid secretions, the patient is given a gastric stimulant. The synthetic peptide pentagastrin, which is a strong stimulus for HCl secretion is used.

5.      Gastric secretions are again collected in four 15 minute sampling period.

6.      After the collection of gastric fluid, the volume and pH of each sample are measured and recorded.

7.      Particulate matter, if present, is removed by centrifuging of the sample.

8.      Each sample is next titrated with 0.1 M NaOH with phenolphthalein as the end–point color indicator. The volume of NaOH used to titrate the sample is recorded.

9.      The mEq of acid secreted for each collection period is calculated according to the following formula:

Volume of NaOH (L) x
            Total volume of specimen (mL) x
                        Molarity of NaOH               x          1000 mEq
            Volume of gastric fluid titrated (mL)       mole

            = mEq of acid secreted for each collection period

10.  For basal acid output, the two highest values obtained are used in the following formula to calculate the mEq of acid per hour:

Basal I + Basal II = mEq of acid secreted per hour in basal condition
            0.5

11.  The mEq of acid produced per hour after pentagastrin stimulation is calculated by summing the mEq of acid secreted in each 15 minutes sample (Stim I + Stim II + Stim III + Stim IV).

Normal value     =          20 – 100 ml / 12 hours fast

·         Basal pH values of 6.0 or greater are most likely due to subnormal parietal cell activity and may be associated with pernicious anemia, gastric carcinoma, rheumatoid arthritis and myxedam. The pH of gastric secretion after pentagastrin stimulation should be less than 2.0.

Acid output in basal state and after stimulation:

Subject                                  Basal Acid output                Maximal Acid Output
                                                          (mmol / hr)                                (mmol / hr)

Normal adult male                    2.2 – 2.7                                  14 – 42
      under 30
Over 30                                        2.2 – 2.7                                  3 – 33
Normal adult female                 1.0 – 1.5                                  7 – 20
           

SCHILLING TEST


The Schilling Test for absorption of Vitamin B12 evaluates both gastric and intestinal function. There are three ways of performing Schilling Test. Tests of B12 absorption should first be done with an oral dose of B12 then with intrinsic factor and B12, and finally, if bacterial overgrowth is suspected, after a course of treatment with broad spectrum antibiotics. The urinary excretion and the double isotope techniques are used to perform the test.


1.      Urinary Excretion Technique

a.      Vitamin B12 radiolabeled with Cobalt–57 or Cobalt–58 is taken orally. A test dose of 1 mg is most frequently used since this dose is within the physiologic range of Vitamin B12 absorption capacity of the gastrointestinal tract.

b.      One to two hours after ingestion of the radiolabeled B12, 100 mg of non–radioactive B12 is given via intramuscular injection. This non–radioactive B12 given saturates the body’s binding sites for B12 preventing any of the radiolabeled B12 that might be absorbed from being stored. Any of the radiolabeled B12 that is absorbed is thus excreted into the urine.

c.       Urine is collected for 24 hours after the oral dose of B12 and the radioactivity counted. The amount of radioactive B12 excreted in the urine is determined as a percentage of the original dose. Since the test relies on urinary excretion of B12 that is absorbed is thus excreted into the urine.

d.     In patients whom radiolabeled B12 is not absorbed, the Schilling test may be repeated except this time radiolabeled B12 is given in conjunction with intrinsic factor. Urine is collected as before and the percentage of the original dose of B12 excreted of B12 is now normal. A deficiency of intrinsic factor is the cause of B12 deficiency. If the renal excretion of B12 is still abnormal after the administration of B12 and intrinsic factor, an absorptive defect for vitamin B12 may be present. This may be seen in patients with bacterial overgrowth in the small intestine. If bacterial overgrowth is suspected, patients are given a course of broad spectrum antibiotic therapy and the Schilling test is repeated.


2.      Double Isotope Technique

a.      A dual isotope test has been devised that uses two isotopes of cobalt (Cobalt–57 and Cobalt–58), each of which is used to make a different preparation of radiolabeled B12.

b.      One preparation contains free radiolabeled B12; the other contains intrinsic factor that is bound to radiolabeled B12 in vitro.

c.       One hour after the administration of the two preparations, a loading dose of B12 is administered intramuscularly as the conventional Schilling test.

d.     Urine is collected over the next 24 hours, and the ratio of the two isotopes in urine is measured.

e.      Use of the two preparations allows one to discriminate between failure to produce endogenous intrinsic factor (associated with pernicious anemia) and failure to absorb B12 bound to intrinsic factor (associated with ileal disease) as a cause of B12 deficiency.

Sensitivity     = for pernicious anemia = 83%
                        = for ileal disease = 67%

D–XYLOSE ABSORPTION TEST


D–xylose is a pentose sugar not normally present in blood. Taken orally, D–xylose is passively absorbed in the proximal small intestine and is not metabolized by the liver. Most of that which is absorbed is eliminated via the kidneys. Thus, the amount of D–xylose excreted into the urine over a specified time period after ingestion is directly correlated with the amount absorbed in the gastrointestinal tract.

Procedure of the test

1.      Patient should fast before starting the test

2.      Immediately before receiving D–xylose, the patient should avoid and discard any urine. Although the amount of D–xylose is given is variable, a 25 g dose appears to be adequate.

3.      After administration of the sugar, all urine voided over the next 5 hours is collected.

4.      Blood is also collected.

5.      Blood samples are usually collected at one hour in children and 2 hours after administration in adults.

6.      The amount of D–xylose present in the urine and blood is measured by a quantitative assay

a.      Performed using a urine or PFF specimen

b.      These samples are mixed with p–bromoaniline in acid medium

c.       In the presence of acid, D–xylose is dehydrated to furforol, which in turn condenses with p–bromoaniline, forming a pink complex that has a maximum absorption of 520 nm.

d.     Reaction of non–specific chromogens with p–bromoaniline is minimized by the addition of thiourea to the reaction mixture, which is an antioxidant and helps prevent the formation of interfering colored compounds.

Sources of error:

a.      Patients with renal insufficiency who cannot excrete D–xylose into the urine may be falsely classified as having malabsorption. This problem can be overcome by measurement of D–xylose in blood.

b.      In addition to malabsorption, low concentrations of urine and plasma D– xylose may be seen in patients with ascites, thyroid disease, vomiting and delayed gastric emptying.

c.       Incomplete urine collection and metabolism of D–xylose by microorganisms in urine.

d.     Increase in urine D–xylose is seen if urine contains increased galactose or glucose concentrations.

Reference ranges for D–xylose in urine and blood

Specimen                                          Concentration
                                                             mg/dl                        mmol / L

Blood

            Child (dose, 0.5 g/lb)

            1 hour                                     >30                              >2.00

            Adult (dose, 25 grams)

            1 hour                                     21 – 57                        1.4 – 3.8
            2 hours                                   32 – 58                        2.13 – 3.86
            3 hours                                   19 – 42                        1.27 – 2.8
            4 hours                                   11 – 29                        0.73 – 1.93
            5 hours                                   6 – 18                          0.40 – 1.20

Urine (5 hours collection)

                        Child, 16 – 33% of ingested dose

                        Adult (dose, 25 grams)        >4                                >26.64
                        >65 years old                         >3.5                             >23.31


FECAL FAT


The determination of fecal fat is performed in the evaluation of malabsorption due to pancreatic or intestinal dysfunction. The fat content of feces in normal individuals consists primarily of fatty acids, fatty acid salts (soap) and neutral fats. Tests of fecal fat excretion are affected by disorders that influence digestion in the intestinal lumen as well as processes affecting absorption of fats by the mucosa. Although determination of fecal fat is useful for identifying the presence of steatorrhea, it does not reveal its cause.

1.      Screening test

Average number of fat droplets               Approximate fat content

            10                                                                    5
            20                                                                    10
            30                                                                    15
            35                                                                    20
            >40                                                                  >30


2.      Quantitative measurement

a.      Collection of specimen is done for three consecutive days.

b.      Two days before starting the test, the patient consumes a diet containing approximately 100 g of fat per day. Ingestion of other triglyceride– containing compounds, such as castor oil or cod liver oil, should be avoided during this time.

c.       Ingestion of capsules containing markers such as chromic oxide, congo red, charcoal or barium sulfate may be used to keep track of the transit time. Contamination of feces with urine should be avoided.

d.     Van de Kamer titration method

(1)   Fats are converted to soaps by boiling a preweighed fecal sample in alcoholic potassium hydroxide

(2)   The soap are next converted to fatty acids by the addition of hydrochloric acid and then extracted into petroleum ether.

(3)   An aliquot is evaporated and the remaining residue dissolved in ethanol.

(4)   After a standard dietary intake of 100 grams of fat per day, normal individuals should excrete less than 6 grams of fat per 24 hours period.


SWEAT CHLORIDE


The determination of chloride concentrations in sweat of patients with cystic fibrosis is an extremely reliable tool for the diagnosis of the disease when performed correctly. Sweat for chloride determination is collected after iontophoretic delivery of pilocarpine to the skin.

Pilocarpine is a drug that stimulates sweating when introduced to the skin. At least 100 mg of sweat is required for quantitation. Lower yields of sweat may result in unreliable results because chloride concentration varies with rate of sweating and accurate measurement may be difficult owing to insufficient chloride amounts. Infants less than 3 weeks of age may show abnormally increased sweat chloride concentration and testing should therefore be delayed in these individuals.

Procedure of the test:

1.      Sweat is collected by iontophoresis.

2.      Pilocarpine is introduced into the skin by applying gauze square to the anterior surface of the forearm, which has been moistened with a 4 mg/dl solution of pilocarpine nitrate.

3.      A second gauze square, moistened with a solution of potassium sulfate, is attached to the posterior surface of the forearm.

4.      The positive electrode of the iontophoresis power supply is fastened to the gauze moistened with pilocarpine and the negative electrode fastened to the gauze moistened with potassium sulfate. A 2mA current is applied for 5 minutes.

5.      After iontophoresis, the gauze squares are removed and the skin thoroughly cleaned with distilled water and dried.

6.      The actual sweat collection process begins with the placement of dry, preweighed gauze square over each of the pilocarpine–treated areas.

7.      Each square is covered with parafilm or a plastic sheet and sealed to the skin with tape.

8.      After approximately 30 minutes of sweat collection, the gauze is removed with forceps and placed in a preweighed vial.

9.      The vial and gauze are weighed and the tare is subtracted to obtain the actual weigh of sweat collected.

10.  The concentration of chloride in sweat is determined with the use of approximate titrating equipment.

Sweat chloride concentration:

            Normal                      0 – 35 mmol/L
            Ambiguous               35 – 60 mmol/L
Cystic fibrosis           60 – 200 mmol/L

Sources of error

1.      Falsely decreased value

a.      Patients with cystic fibrosis who are salt depleted as a result of vomiting, diarrhea or gastric suction.

2.      False positive results

a.      Patients without cystic fibrosis who have electrolyte imbalances associated with meconium ileus, hypothyroidism, congestive heart failure and some types of renal disease that result in increased sweat chloride concentration.

Precaution in specimen collection

1.      Patients who are malnourished or dehydrated may not produce adequate amounts of sweat for analysis.

2.      All sweat chloride measurements should be done in duplicate.

3.      Patients who have a positive initial finding should be repeated.


SERUM GASTRIN

Several commercial kit preparations are available for measurement of serum gastrin. Most use a double–antibody separation technique that does not measure the same components of gastrin. Thus, the biologic equivalence of the different immunoassay cannot be ensured. The ideal antibody would detect all gastrin components. This point is well illustrated by measurement of gastrin in gastrinoma patients in whom G–34 is the predominant form of gastrin. Since most assays use antibody directed toward G–17, measurement of gastrin in patients with gastrinoma should be expressed in terms of G–17 equivalents.


CLINICAL SIGNIFICANCE OF G.I. FUNCTION TEST


1.      Lactase deficiency is the most common derangement of carbohydrate digestion and may be inherited as a congenital defect or acquired in later life. Infants with congenital lactase deficiency present with profuse watery diarrhea soon after the introduction of milk. Unless the disease is quickly identified and lactose withdrawn from the diet; infants may die of dehydration.

Lactose that remains in the intestinal lumen of patients with lactase deficiency increases the osmotic pressure of the luminal contents. Fermentation of lactose by bacteria in the lower small intestine and colon results in the production of gas as well as lactic acid and fatty acid which also add to the osmotic effect. The osmotic retention of water in the intestinal lumen leads to diarrhea. Thus, lactase deficiency results in abdominal discomfort, cramps flatulence, diarrhea–associated dehydration and electrolyte imbalances.

2.      Malabsorption occurs as a result of maldigestion of foodstuffs. This form of malabsorption usually results from pancreatic diseases such as chronic pancreatitis or fibrocystic disease of the pancreas. Malabsorption resulting from normal digestion but inadequate assimilation of foodstuffs may result from competition by bacteria or altered bacterial flora, from obstruction to the flow of lymph, from diminished mucosal surface area, or from rapid transit of small bowel contents.

The presence of fat in stool, steatorrhea, is a major sign of malabsorption. Patients with severe forms of fat malabsorption frequently develop calcium oxalate kidney stones because of enhanced absorption of dietary oxalate. The increased absorption of oxalate occurs as a result of increased fatty acid concentrations, which bind calcium and thus prevent the precipitation of oxalate by free calcium, which normally occurs. Another mechanism contributing to increased absorption of oxalate is the result of solubilization of oxalate by dihydroxy bile acids. In those forms of steatorrhea in which large amounts of dihydroxy bile acids reach the colon, passive absorption of oxalate is greatly increased.

3.      Zollinger–Ellison syndrome, also known as gastrinoma occurs as the result of gastrin–secreting tumors of the pancreas or rarely the upper small intestine. Gastrin is released from these tumors at a high continuous rate that is not altered by intake of food. The high rate of gastrin secretion results in hypersecretion of gastic acid by the parietal cells. The low duodenal pH that results causes inactivation of pancreatic enzymes, leading to maldigestion and erodes the intestinal mucosa, producing ulceration. When secreted in very large amounts, gastrin may inhibit absorption of fluid and electrolytes by the intestine. The resulting large volume of fluid in the intestine, coupled with the decreased intestinal transit time, contributes to the diarrhea seen in these patients.

4.      Menetrier disease is an uncommon condition characterized by hyperplasia of the surface cells of gastric mucosa. The disease is most often encountered in men in the fourth to sixth decade of life and is of unknown cause. The hyperplasia of the surface mucosal cells is associated with gastric hyposecretion and anacidity, and there is excessive loss of gastric protein.

5.      Cystic fibrosis is among the most lethal genetic pediatric disorders. The incidence of heterozygote carriers is estimated to be approximately 1 in 20. The symptomatology may range from mild to severe, and onset may be at birth or may not become evident until years later. Usually, the disorder is discovered between the second and twelfth months of life. Affected individuals usually present with malodorous steatorrhea and chronic pulmonary infection.

6.      Crohn’s disease is a chronic inflammatory disorder of the intestine of unknown etiology. It most commonly affects the ileocecal area, although it may involve virtually any part of the gastrointestinal tract.







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