Oxalate, Urine

Synonym/acronym: N/A.

Common use

To identify patients who are at risk for renal calculus formation or hyperoxaluria related to malabsorption.

Specimen

Urine (25 mL) from a timed specimen collected in a clean plastic collection container with hydrogen chloride (HCl) as a preservative.

Normal findings

(Method: Spectrophotometry)

	Conventional Units	SI Units (Conventional Units × 11.1)
Children and adults	0–40 mg/24 hr	0–444 micromol/24 hr

Description

Oxalate is derived from the metabolism of oxalic acid, glycine, and ascorbic acid. Some individuals with malabsorption disorders absorb and excrete abnormally high amounts of oxalate, resulting in hyperoxaluria. Hyperoxaluria may be seen in patients who consume large amounts of animal protein, certain fruits and vegetables, or megadoses of vitamin C (ascorbic acid). Hyperoxaluria is also associated with ethylene glycol poisoning (oxalic acid is used in cleaning and bleaching agents). Patients who absorb and excrete large amounts of oxalate may form calcium oxalate kidney stones. Simultaneous measurement of serum and urine calcium is often requested.

This procedure is contraindicated for

N/A

Indications

Assist in the evaluation of patients with ethylene glycol poisoning
Assist in the evaluation of patients with a history of kidney stones
Assist in the evaluation of patients with malabsorption syndromes or patients who have had jejunoileal bypass surgery

Potential diagnosis

Increased in

Conditions that result in malabsorption for any reason can lead to increased levels. Chronic diarrhea results in excessive loss of calcium to bind oxalate. Increased oxalate is absorbed by the intestine and excreted by the kidneys.
Bacterial overgrowth
Biliary tract disease
Bowel disease
Celiac disease
Cirrhosis
Crohn’s disease
Diabetes
Ethylene glycol poisoning (ethylene glycol is metabolized to oxalate and excreted by the kidneys; crystals are present in urine)
Ileal resection
Jejunal shunt
Pancreatic disease
Primary hereditary hyperoxaluria (rare)
Pyridoxine (vitamin B₆) deficiency (pyridoxine is a cofactor in an enzyme reaction that converts glyoxylic acid to glycine; deficiency results in an increase in oxalate)
Sarcoidosis

Decreased in

(related to formation of calcium oxalate crystals)

(related to oxalate kidney stone disease)

Critical findings

N/A

Interfering factors

Drugs and vitamins that may increase oxalate levels include ascorbic acid, calcium, and methoxyflurane.
Drugs that may decrease oxalate levels include nifedipine and pyridoxine.
Failure to follow dietary restrictions before the procedure may cause the procedure to be canceled or repeated.
Failure to collect sample in proper preservative may cause the procedure to be repeated. HCl helps keep oxalate dissolved in the urine. If the pH rises above 3.0, oxalate may precipitate from the sample, causing falsely decreased values. The acid also prevents oxidation of vitamin C to oxalate in the sample, causing falsely increased values.

Nursing Implications and Procedure

Pretest

Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
Patient Teaching: Inform the patient this test can assist in evaluating risk for kidney stones.
Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
Obtain a history of the patient’s gastrointestinal and genitourinary systems, symptoms, and results of previously performed laboratory tests and diagnostic and surgical procedures.
Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values).
Review the procedure with the patient. Provide a nonmetallic urinal, bedpan, or toilet-mounted collection device. Address concerns about pain and explain that there should be no discomfort during the procedure.
Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
Usually a 24-hr time frame for urine collection is ordered. Inform the patient that all urine must be saved during that 24-hr period. Instruct the patient not to void directly into the laboratory collection container. Instruct the patient to avoid defecating in the collection device and to keep toilet tissue out of the collection device to prevent contamination of the specimen. Place a sign in the bathroom to remind the patient to save all urine.
Instruct the patient to void all urine into the collection device and then to pour the urine into the laboratory collection container. Alternatively, the specimen can be left in the collection device for a health-care staff member to add to the laboratory collection container.
Note that there are no fluid or medication restrictions unless by medical direction.
Instruct the patient to abstain from calcium supplements, gelatin, rhubarb, spinach, strawberries, tomatoes, and vitamin C for at least 24 hr before the test. High-protein meals should also be avoided 24 hr before specimen collection. Protocols may vary among facilities.

Intratest

Potential complications: N/A
Ensure that the patient has complied with dietary restrictions; assure that restricted foods have been avoided for at least 24 hr prior to the procedure.
Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
Instruct the patient to cooperate fully and to follow directions.
Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection.

Random Specimen (Collect in Early Morning)

Clean-Catch Specimen
Instruct the male patient to (1) thoroughly wash his hands, (2) cleanse the meatus, (3) void a small amount into the toilet, and (4) void directly into the specimen container.
Instruct the female patient to (1) thoroughly wash her hands; (2) cleanse the labia from front to back; (3) while keeping the labia separated, void a small amount into the toilet; and (4) without interrupting the urine stream, void directly into the specimen container.

Indwelling Catheter

Put on gloves. Empty drainage tube of urine. It may be necessary to clamp off the catheter for 15 to 30 min before specimen collection. Cleanse specimen port with antiseptic swab, and then aspirate 5 mL of urine with a 21- to 25-gauge needle and syringe. Transfer urine to a sterile container.

Timed Specimen

Obtain a clean 3-L urine specimen container, toilet-mounted collection device, and plastic bag (for transport of the specimen container). The specimen must be refrigerated or kept on ice throughout the entire collection period. If an indwelling urinary catheter is in place, the drainage bag must be kept on ice.
Begin the test between 6 and 8 a.m. if possible. Collect first voiding and discard. Record the time the specimen was discarded as the beginning of the timed collection period. The next morning, ask the patient to void at the same time the collection was started and add this last voiding to the container. Urinary output should be recorded throughout the collection time.
If an indwelling catheter is in place, replace the tubing and container system at the start of the collection time. Keep the container system on ice during the collection period, or empty the urine into a larger container periodically during the collection period; monitor to ensure continued drainage, and conclude the test the next morning at the same hour the collection was begun.
At the conclusion of the test, compare the quantity of urine with the urinary output record for the collection; if the specimen contains less than what was recorded as output, some urine may have been discarded, invalidating the test.
Include on the collection container’s label the amount of urine, test start and stop times, and ingestion of any foods or medications that can affect test results.

General

Promptly transport the specimen to the laboratory for processing and analysis.

Post-Test

Inform the patient that a report of the results will be made available to the requesting health-care provider (HCP), who will discuss the results with the patient.
Instruct the patient to resume usual diet, as directed by the HCP.
Nutritional Considerations: Consideration may be given to lessening dietary intake of oxalate if urine levels are increased. Encourage patients with abnormal results to seek advice regarding dietary modifications from a trained nutritionist. Magnesium supplementation may be recommended for patients with GI disease to prevent the development of calcium oxalate kidney stones.
Recognize anxiety related to test results, and be supportive of fear of shortened life expectancy. Discuss the implications of abnormal test results on the patient’s lifestyle. Provide teaching and information regarding the clinical implications of the test results, as appropriate. Educate the patient regarding access to counseling services.
Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

Related tests include calcium, calculus kidney stone panel, UA, urine uric acid, and vitamin C.
Refer to the Gastrointestinal and Genitourinary systems tables at the end of the book for related tests by body system.