Creatinine, Urine, and Creatinine Clearance, Urine

Synonym/acronym: N/A.

Common use

To assess and monitor kidney function related to acute or chronic nephritis.

Specimen

Urine (5 mL) from an unpreserved random or timed specimen collected in a clean plastic collection container.

Normal findings

(Method: Spectrophotometry)

Age	Conventional Units	SI Units
		*Urine Creatinine (Conventional Units × 8.84)*
2–3 yr	6–22 mg/kg/24 hr	53–194 micromol/kg/24 hr
4–18 yr	12–30 mg/kg/24 hr	106–265 micromol/kg/24 hr
Adult male	14–26 mg/kg/24 hr	124–230 micromol/kg/24 hr
Adult female	11–20 mg/kg/24 hr	97–177 micromol/kg/24 hr
		*Creatinine Clearance (Conventional Units × 0.0167)*
Children	70–140 mL/min/1.73 m²	1.17–2.33 mL/s/1.73 m²
Adult male	85–125 mL/min/1.73 m²	1.42–2.08 mL/s/1.73 m²
Adult female	75–115 mL/min/1.73 m²	1.25–1.92 mL/s/1.73 m²
For each decade after 40 yr	Decrease of 6–7 mL/min/1.73 m²	Decrease of 0.06–0.07 mL/s/1.73 m²

Description

Creatinine is the end product of creatine metabolism. Creatine resides almost exclusively in skeletal muscle, where it participates in energy-requiring metabolic reactions. In these processes, a small amount of creatine is irreversibly converted to creatinine, which then circulates to the kidneys and is excreted. The amount of creatinine generated in an individual is proportional to the mass of skeletal muscle present and remains fairly constant, unless there is massive muscle damage resulting from crushing injury or degenerative muscle disease. Creatinine values decrease with advancing age owing to diminishing muscle mass. Although the measurement of urine creatinine is an effective indicator of renal function, the creatinine clearance test is more precise. The creatinine clearance test measures a blood sample and a urine sample to determine the rate at which the kidneys are clearing creatinine from the blood; this reflects the glomerular filtration rate (GFR) and is based on an estimate of body surface.

Chronic kidney disease (CKD) is a significant health concern worldwide. An international effort to standardize methods to identify and monitor CKD has been undertaken by the National Kidney Disease Education Program (NKDEP), the International Confederation of Clinical Chemistry and Laboratory Medicine, and the European Communities Confederation of Clinical Chemistry. International efforts have resulted in development of an isotope dilution mass spectrometry (IDMS) reference method for standardized measurement of creatinine. The National Kidney Foundation (NKF) has recommended use of an equation to estimate glomerular filtration rate (eGFR). The equation is based on factors identified in the NKF Modification of Diet in Renal Disease (MDRD) study. The equation includes four factors: serum or plasma creatinine value, age in years, gender, and race. The equation is valid only for patients between the ages of 18 and 70. A correction factor is incorporated in the equation if the patient is African American because CKD is more prevalent in African Americans; results are approximately 20% higher. It is very important to know whether the creatinine has been measured using an IDMS traceable test method because the values will differ; results are lower. The equations have not been validated for pregnant women (GFR is significantly increased in pregnancy); patients younger than 18 or older than 70; patients with serious comorbidities; or patients with extremes in body size, muscle mass, or nutritional status. eGFR calculators can be found at the NKDEP (www.nkdep.nih.gov/professionals/gfr_calculators/index.htm).

Creatinine clearance can be estimated from a blood creatinine level: Creatinine clearance = [1.2 × (140 − age in years) × (weight in kg)]/blood creatinine level.

The result is multiplied by 0.85 if the patient is female; the result is multiplied by 1.18 if the patient is African American.

This procedure is contraindicated for

N/A

Indications

Determine the extent of nephron damage in known renal disease (at least 50% of functioning nephrons must be lost before values are decreased)
Determine renal function before administering nephrotoxic drugs
Evaluate accuracy of a 24-hr urine collection based on the constant level of creatinine excretion
Evaluate glomerular function
Monitor effectiveness of treatment in renal disease

Potential diagnosis

Increased in

Acromegaly (related to increased muscle mass)
Carnivorous diets (related to increased intake of creatine, which is metabolized to creatinine and excreted by thekidneys)
Exercise (related to muscle damage; increased renal blood flow)
Gigantism (related to increased muscle mass)

Decreased in

Conditions that decrease GFR, impair kidney function, or reduce renal blood flow will decrease renal excretion of creatinine

(related to abnormal creatinine production; decreased production reflected in decreased excretion)

(related to reduced GFR)

(evidenced by diets that exclude intake of animal muscle, the creatine source metabolized to creatinine and excreted by the kidneys)

Critical findings

Degree of impairment:

Borderline: 62.5–80 mL/min/1.73 m² (SI: 1—1.3 mL/s/1.73 m²)
Slight: 52–62.5 mL/min/1.73 m² (SI: 0.9–1 mL/s/1.73 m²)
Mild: 42–52 mL/min/1.73 m² (SI: 0.7–0.9 mL/s/1.73 m²)
Moderate: 28–42 mL/min/1.73 m² (SI: 0.5–0.7 mL/s/1.73 m²)
Marked: Less than 28 mL/min/1.73 m² (SI: Less than 0.5 mL/s/1.73 m²)

Note and immediately report to the health-care provider (HCP) any critically increased values and related symptoms.
It is essential that a critical finding be communicated immediately to the requesting health-care provider (HCP). A listing of these findings varies among facilities.
Timely notification of a critical finding for lab or diagnostic studies is a role expectation of the professional nurse. Notification processes will vary among facilities. Upon receipt of the critical value the information should be read back to the caller to verify accuracy. Most policies require immediate notification of the primary HCP, Hospitalist, or on-call HCP. Reported information includes the patient’s name, unique identifiers, critical value, name of the person giving the report, and name of the person receiving the report. Documentation of notification should be made in the medical record with the name of the HCP notified, time and date of notification, and any orders received. Any delay in a timely report of a critical finding may require completion of a notification form with review by Risk Management.

Interfering factors

Drugs that may increase urine creatinine levels include ascorbic acid, cefoxitin, cephalothin, corticosteroids, fluoxymesterone, levodopa, methandrostenolone, methotrexate, methyldopa, nitrofurans (including nitrofurazone), oxymetholone, phenolphthalein, and prednisone.
Drugs that may increase urine creatinine clearance include enalapril, oral contraceptives, prednisone, and ramipril.
Drugs that may decrease urine creatinine levels include anabolic steroids, androgens, captopril, and thiazides.
Drugs that may decrease the urine creatinine clearance include acetylsalicylic acid, amphotericin B, carbenoxolone, chlorthalidone, cimetidine, cisplatin, cyclosporine, guancidine, ibuprofen, indomethacin, mitomycin, oxyphenbutazone, probenecid (coadministered with digoxin), puromycin, and thiazides.
Excessive ketones in urine may cause falsely decreased values.
Failure to follow proper technique in collecting 24-hr specimen may invalidate test results.
Failure to refrigerate specimen throughout urine collection period allows decomposition of creatinine, causing falsely decreased values.
Consumption of large amounts of meat, excessive exercise, and stress should be avoided for 24 hr before the test. Protocols may vary among facilities.
Failure to follow dietary restrictions before the procedure may cause the procedure to be canceled or repeated.

Nursing Implications and Procedure

Pretest

Positively identify the patient usingat least two unique identifiers before providing care, treatment, or services.
Patient Teaching: Inform the patient this test can assist in assessing kidney function.
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 genitourinary system, 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 online at DavisPlus).
Review the procedure with the patient. Provide a nonmetallic urinal, bedpan, or toilet-mounted collection device. Address concerns about pain and explain to the patient that there should be no discomfort during the urine collection procedure. Inform the patient that a blood sample for creatinine will be required on the day urine collection begins or at some point during the 24 hr collection period (see monograph titled “Creatinine, Blood” for additional information).
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 refrain from eating meat during the test. Protocols may vary among facilities.

Intratest

Potential complications: N/A
Ensure that the patient has complied with dietary and activity restrictions for 24 hr prior to the procedure; assure that ingestion of meat has been restricted during the test.
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. Perform a venipuncture as appropriate.

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.

Pediatric Urine Collector

Put on gloves. Appropriately cleanse the genital area and allow the area to dry. Remove the covering over the adhesive strips on the collector bag and apply over the genital area. Diaper the child. When specimen is obtained, place the entire collection bag in a sterile urine 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.

Urinary Catheterization

Place female patient in lithotomy position or male patient in supine position. Using sterile technique, open the straight urinary catheterization kit and perform urinary catheterization. Place the retained urine in a sterile specimen container.

Suprapubic Aspiration

Place the patient in a supine position. Cleanse the area with antiseptic and drape with sterile drapes. A needle is inserted through the skin into the bladder. A syringe attached to the needle is used to aspirate the urine sample. The needle is then removed and a sterile dressing is applied to the site. Place the sterile sample in a sterile specimen container.
Do not collect urine from the pouch from the patient with a urinary diversion (e.g., ileal conduit). Instead, perform catheterization through the stoma.

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 time 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 any foods or medications that can affect test results.
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 HCP, who will discuss the results with the patient.
Instruct the patient to resume usual diet, medications, and activity, as directed by the HCP.
Recognize anxiety related to test results and be supportive of impaired activity related to 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. Help the patient to cope with long-term implications. Recognize that anticipatory anxiety and grief related to potential lifestyle changes may be expressed when someone is faced with a chronic disorder. Provide contact information, if desired, for the NKF (www.kidney.org) or the NKDEP (www.nkdep.nih.gov).
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 anion gap, antimicrobial drugs, antibodies antiglomerular basement membrane, ANF, biopsy kidney, biopsy muscle, blood gases, BNP, BUN, calcium, calculus kidney stone analysis, C4, CT abdomen, CT renal, CK and isoenzymes, creatinine, culture urine, cytology urine, cystoscopy, echocardiography, echocardiography transesophageal, electrolytes, EMG, ENG, EPO, gallium scan, glucagon, glucose, haptoglobin, insulin, IVP, KUB studies, lung perfusion scan, microalbumin, osmolality, phosphorus, renogram, retrograde ureteropyelography, TSH, thyroxine, US kidney, uric acid, and UA.
Refer to the Genitourinary System table at the end of the book for related tests by body system.