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Anion Gap
Anion Gap
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Anion Gap
Category: Blood gases and electrolytes.
Subcategory: Electrolytes.
Material studied: Blood.
Estimated cost of test: $20.00 to $30.00.
Patient time for test: 5-10 minutes.
Reliability of test results: Good.
Available as home self-test? No.
BEFORE THE TEST
Purpose of test:
Differentiates between types of metabolic acidosis.
Monitors kidney function.
Monitors hyperalimentation*.
Where is test performed?
Commercial laboratory, hospital, doctor's office.
Who performs test?
Lab technician, nurse, doctor.
Risks and precautions:
If tourniquet is applied on arm too long (over 1 minute), it may cause an inaccurate test result. Request another sample to be collected to ensure accuracy.
Patient preparation:
Activity--No changes necessary.
Diet--No changes necessary.
Medicines--Inform the person performing the test if you have recently taken any medications listed under Taking these drugs may affect test results. You may be asked not to take this medication before the test.
Disrobing--None required. Roll up sleeve only.
THE TEST
Sensory factors:
Touching--You will feel mild discomfort when the needle is inserted into the vein or when the lancet pricks a finger, heel or ear lobe.
Seeing--You will see the technician, nurse or doctor, the basket or tray to hold the equipment, the needles, syringes, collecting tubes and bandages.
Feeling--Some degree of apprehension or fear is normal and should be expected. Discomfort disappears when the test is finished.
Other senses (taste, smell, hearing)--Not affected.
Equipment used:
Needles, syringes and heparinized* collecting tubes. Sterile, disposable equipment prevents contamination or spread of infection. There is no risk of becoming infected with the hepatitis virus, AIDS virus or any other infecting germ.
Description of test:
The technician, doctor or nurse applies a tourniquet or blood-pressure cuff to the upper arm if blood is collected from a vein.
Skin over the vein to be stuck is cleaned with alcohol or other antiseptic on a piece of cotton.
When blood is drawn from a vein, the operator feels the vein to be used, then punctures both the skin and vein in one quick stroke. The needle used is a sterile, disposable needle attached to a sterile, disposable syringe.
The operator withdraws the needle and transfers the sample from the collecting syringe into sterile tubes (identified with your name) before sending the sample to the laboratory for analysis. The tubes are treated with an anti-coagulant chemical to prevent clotting.
If blood is collected from a finger, heel or ear lobe, skin over the selected site is cleaned with an antiseptic. The operator quickly pierces the skin to a shallow depth, using a sterile, disposable metal lancet. The drop or two of blood produced is collected into a capillary pipette.
AFTER THE TEST
Immediate post-test care:
Apply pressure to the puncture site.
Apply tape or gauze pad over the puncture site. Don't bandage the entire circumference of any puncture site.
Activity after test:
Resume taking any medications that were withheld before the test.
Return to pretest activities right away.
Use warm compresses if blood collects under the puncture site or if the area becomes tender, red or painful.
Time before results are available:
Test requires only a few minutes in the laboratory. Time before results are reported to the doctor or patient varies from a few minutes to a few days.
TEST RESULTS
Test values:
Test results are determined by calculations based on measurements of sodium, chloride and bicarbonate.
Normal values:
8 to 14mEq/liter.
What HIGH or INCREASED anion gap may indicate:
Ketoacidosis*.
Kidney failure.
Lactic acidosis*.
Toxic effects of ingesting excessive amounts of aspirin, salicylates, methanol, ethylene glycol (antifreeze) or paraldehyde.
What LOW or DECREASED anion gap may indicate:
Hypermagnesemia*.
Multiple myeloma*.
Waldenstrom's macroglobulinemia*.
Taking these drugs may affect test results:
ACTH.
Acetazolamide.
Ammonium chloride.
Antacids in excessive amounts.
Anti-hypertensives.
Aspirin.
Bicarbonates.
Boric acid.
Chlorpropamide.
Cholestyramine.
Corticosteroids.
Cortisone.
Dimercaprol.
Diuretics.
Ethacrynic acid.
Furosemide.
I.V. solution--saline or dextrose.
Lithium.
Methicillin.
Methyl alcohol.
Oxyphenbutazone.
Paraldehyde.
Phenylbutazone.
Salicylates.
Thiazide diuretics.
Vasopressin.
Other factors that may affect test results:
Methanol, ethylene glycol (antifreeze) or paraldehyde.
Eating too much licorice.
Rough handling of the sample.
Wounds wrapped or packed with povidone-iodine may cause a low anion gap.
*See Glossary.
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From Complete Guide to Medical Tests by H. Winter Griffith, M.D. Copyright by Fisher Books. Electronic rights by Medical Data Exchange.
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