Creatinine, Urine, and Creatinine Clearance, Urine, and Glomerular Filtration Rate (GFR)
Synonym/Acronym
CrCl, eGFR
Rationale
To assess and monitor kidney function related to acute kidney injury or chronic kidney disease.
Patient Preparation
There are no fluid or medication restrictions unless by medical direction. Instruct the patient to refrain from eating meat during the test and to refrain from excessive exercise for 8 hr before the test. Protocols may vary among facilities. Usually, a 24-hr urine collection is ordered. As appropriate, provide the required urine collection container and specimen collection instructions.
Normal Findings
Method: Spectrophotometry.
| Normal Urine Volume per 24 hr | These ranges are very general averages and were not calculated on the basis of normal average body weights. Literature shows that expected urinary output can be estimated by a formula in which the expected output is as follows:
Infants: 1–2 mL/kg/hr Children and adolescents: 0.5–1 mL/kg/hr Adults and older adults: 1 mL/kg/hr | | Newborns | 15–60 mL | | Infants | | | 3–10 days | 100–300 mL | | 11–59 days | 250–450 mL | | 2–12 mo | 400–500 mL | | Children and adolescents | | | 13 mo–4 yr | 500–700 mL | | 5–7 yr | 650–1,000 mL | | 8–14 yr | 800–1,400 mL | | Adults and older adults | 800–2,500 mL (average 1,200 mL) |
|
| Timed Urine Creatinine |
|---|
| Age | Conventional Units | SI Units = (Conventional Units × 0.00884) | | 3–8 yr | 140–700 mg/24 hr | 1–6 mmol/day | | 9–12 yr | 300–1300 mg/24 hr | 3–12 mmol/day | | 13–17 yr | 500–2300 mg/24 hr | 4–20 mmol/day | | Adult male | 1000–2500 mg/24 hr | 9–22 mmol/day | | Adult female | 700–1600 mg/24 hr | 6–14 mmol/day | | Adult male | 600–2100 mg/24 hr | 5–19 mmol/day | | Adult female | 400–1400 mg/24 hr | 4–12 mmol/day | | Random Urine Creatinine | | Note: Laboratories may not report an established normal range | Conventional Units | SI Units = (Conventional Units × 0.0884) | | Adult male | 20–320 mg/dL | 1.77–28.3 mmol/L | | Adule female | 20–275 mg/dL | 1.77–24.3 mmol/L |
|
| Creatinine Clearance | SI Units = (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 | 1.42–2.09 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 |
The 24-hr urine volume is recorded and provided with the results of the creatinine measurement. |
| Estimated Glomerular Filtration Rate (eGFR) Many laboratories now report eGFR with creatinine clearance. | | Age | Conventional Units | | Less than 18 yrs | Note: The eGFR equation generally recommended for individuals under 18 yrs of age is the Bedside Schwartz formula and is based on Cr levels and height in centimeters. | | 18–70 yrs | Greater than or equal to 60 mL/min/body surface area | | Greater than 70 yr | Values in older adults remain relatively stable, after a period of decline related to loss of muscle mass during the transition from adult to older adult. |
|
Study type: Urine from an unpreserved random or timed specimen collected in a clean plastic collection container; related body system: Urinary system.
Chronic kidney disease (CKD) is a significant health concern worldwide. International research has been undertaken to evaluate the risk factors common to cardiovascular disease, diabetes, and hypertension; these three diseases are all associated with CKD. Albuminuria (formerly referred to as microalbuminuria), which can result from increased glomerular permeability to proteins, is considered an independent risk factor predictive of kidney or cardiovascular disease.
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, creatine is irreversibly converted to Cr, which then circulates to the kidneys and is excreted into the urine at a relatively consistent rate. A consistent rate of excretion is the reason urine creatinine measurements have been used to evaluate kidney health for many years. The amount of Cr 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. Cr values normally decrease with age owing to diminishing muscle mass. Conditions involving degenerative muscle wasting or massive muscle trauma from a crushing injury will also result in decreased Cr levels.
For additional information regarding other significant markers of kidney health, refer to the study titled “Albumin, Urine and Albumin to Creatinine Ratio” and “Creatinine, Blood, Core Lab Study and Estimated Glomerular Filtration Rate (eGFR).”
Urine Creatinine (Timed)
Measurement of urine creatinine is an effective indicator of kidney function and is ordered with almost every timed, quantitative urine study (e.g., 5-hydroxyindoleacetic acid, calcium, drug screen, lead, potassium, sodium, protein).
Timed urine creatinine collections are used to validate the completeness of a 24 hr collection in patients with normal kidney function. For example if a 24 hr urine sodium is requested, two tests will be performed: a urine sodium and a urine creatinine. The measured urine creatinine value is compared to the established 24 hr urine creatinine normal range. If the urine creatinine value falls within the 24 hr urine creatinine reference range, the collection is validated as complete. Circumstances that may indicate an invalid collection include incomplete collection (e.g., not adding all voided urine into the collection container) or inaccurate recording of urine volume or collection times.
The same concept of validation applies to urine drug screen samples where the patient sample findings might not match up to expected standard results (e.g., the drug levels are very low and the creatinine is abnormally low—the specimen may be rejected for analysis possibly due to dilution of the patient sample with water in an effort to falsely decrease the drug test measurements).
Urine Creatinine (Random)
While timed urine collections are preferred, random or spot urine collections are often used for a quick screen (e.g., electrolytes, albumin).
Creatinine clearance
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. eGFR formulas are now routinely used to evaluate kidney function; their use has become more common than other tests that reflect GFR, such as the creatinine clearance test. The CrCl is more complicated and time consuming to perform; it has become impractical for routine use given the near instantaneous availability of mathematically derived algorithms. Many laboratories include the eGFR with the results of the creatinine clearance.
- Creatinine clearance can be roughly estimated from a blood creatinine level.: Creatinine clearance = [(1.2 for males or 0.95 for females) × (140 - age in years) × (weight in kg)]/blood creatinine level.The result is multiplied by 1.2 if the patient is male (average normal filtration rate is 120 mL/min); the result is multiplied by 0.95 if the patient is female (average normal filtration rate is 95 mL/min).
- Measured GFR using an injected marker is still the gold standard and most accurate way to determine GFR; however, it is complex to perform. Measured GFR should be used in the rare situations where improved accuracy would influence treatment decisions.
eGFR
There are multiple formulas in use for calculation of the eGFR. Either Cr or cystatin may be used. The most common formulas use SCr value, age, and correction factors for gender and race. In September 2021, the National Kidney Foundation and American Society of Nephrology joint task force released its final report, which outlines a race-free approach for the diagnosis of kidney disease and recommends the use of the CKD-EPI Cr calculation (2021) for eGFR. The recommendation was endorsed by the United States Pathology and Laboratory Society Leadership in February 2022. Laboratories have begun to transition to race-free eGFR calculations. For additional information regarding eGFR and eGFR for pregnant females and children under 18 yr of age, refer to the study titled “Creatinine, Blood, Core Lab Study and Estimated Glomerular Filtration Rate (eGFR).”
Recommendations for measuring and reporting creatinine values and eGFR (formulas are available that use Cr and/or cystatin to calculate eGFR):- At this time, all laboratories should be using Cr methods calibrated to be isotope dilution mass spectrometry (IDMS) traceable. IDMS is the international reference method used to standardize measurement of Cr.
- Serum/plasma Cr levels should be reported to the nearest hundredth (two decimal places) in conventional units (mg/dL).
- Serum/plasma Cr levels should be reported to the nearest whole number in standard international units (mmol/L).
- Lab reports that include SCr for adults should also include the corresponding eGFR.
- The National Kidney Foundation recommends the use of the CKD-EPI (2021) race-free calculation for Cr to arrive at the eGFR.
- The National Kidney Foundation kidney disease outcome quality initiative (KDOQI) and kidney disease improving global outcome (KDIGO) guidelines recommend confirming Cr-based eGFR values between 45 and 59 mL/min/body surface, with an ACR less than 30 mg/g, using an eGFR calculation based on both creatinine and cystatin C.
- Lab reports that include eGFR should specify the equation used (e.g., CKD-EPI creatinine equation [2021], CKD-EPI creatinine and cystatin C equation [2012], CKD-EPI creatinine and cystatin C equation [2021], CKD-EPI cystatin C equation [2012], CKD-EPI creatinine equation [2009], etc.).
- The eGFR (adult) should be reported to the nearest whole number in mL/min/1.73m2 (based on a body surface area of 1.73 m2).
- The CKD-EPI (2009) equation for adults is valid only for patients between the ages of 18 and 70.
Factors That May Alter the Results of the Study
- Drugs and other substances that may increase urine creatinine levels include cefoxitin, cephalosporins, corticosteroids, levodopa, methotrexate, methyldopa, and nitrofurans.
- Drugs and other substances that may increase urine creatinine clearance include aminoglycosides, cephalosporins, cimetidine, cisplatin, plicamycin, and ramitidine.
- Drugs and other substances that may decrease urine creatinine levels include anabolic steroids, androgens, captopril, and thiazides.
- Drugs and other substances that may decrease the urine creatinine clearance include acetylsalicylic acid, amphotericin B, cimetidine chlorthalidone, cisplatin, cyclosporine indomethacin, and thiazides.
- Excessive ketones in urine may cause falsely decreased values.
- Failure to refrigerate specimen throughout urine collection period allows decomposition of creatinine, causing falsely decreased values.
Other Considerations
- All urine voided for the timed collection period must be included in the collection or else falsely decreased values may be obtained. Compare output records with volume collected to verify that all voids were included in the collection.
Increased In
- Acromegaly(related to increased muscle mass)
- Carnivorous diets (related to increased intake of creatine, which is metabolized to creatinine and excreted by the kidneys)
- 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.
- Acute or chronic glomerulonephritis
- Chronic bilateral pyelonephritis
- Kidney disease, acute kidney injury, and CKD (related to decreased urinary excretion)
- Leukemia
- Muscle wasting diseases (related to abnormal creatinine production; decreased production reflected in decreased excretion)
- Paralysis (related to abnormal creatinine production; decreased production reflected in decreased excretion)
- Polycystic kidney disease
- Pregnancy-induced hypertension (related to reduced GFR)
- Shock
- Urinary tract obstruction (e.g., from calculi)
- Vegetarian diets(evidenced by diets that exclude intake of animal muscle, the creatine source metabolized to creatinine and excreted by the kidneys)
eGFR
- Degree of renal impairment:
- Stage 1 (normal or increased GFR with or without early kidney disease): GFR greater than 90 mL/min/1.73 m2
- Stage 2 (damage with mild reduction in GFR): GFR between 60 and 89 mL/min/1.73 m2
- Stage 3 (moderate reduction in GFR): GFR between 30 and 59 mL/min/1.73 m2
- Stage 4 (severe reduction in GFR): GFR between 15 and 29 mL/min/1.73 m2
- Stage 5 (kidney failure/end stage renal disease): GFR less than 15 mL/min/1.73 m2
Before the Study: Planning and Implementation
Teaching the Patient What to Expect
- Discuss how this test can assist in assessing kidney function.
- Explain that a urine sample is needed for the test. Information regarding urine specimen collection is presented with other general guidelines in Appendix A: Patient Preparation and Specimen Collection.
- Explain 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 study titled “Creatinine, Blood” for additional information).
Potential Nursing Actions
- Include on the collection container’s label urine total volume, test start and stop times/dates, and any medications that may interfere with test results.
- Remind the patient that every drop of urine must be collected for the specified time period regardless of how little the amount.
- Explain that failure to collect all urine for the specified period of time can affect results and may necessitate restarting the collection process.
After the Study: Implementation & Evaluation Potential Nursing Actions
Treatment Considerations
- Discuss symptoms of chronic renal disease, such as increased or decreased urination, nausea, vomiting, hypertension, appetite loss, insomnia, dry skin, itchy skin, weakness, decreased mental acuity, edema.
- Facilitate management of the underlying cause of chronic renal disease: diabetes, hypertension, obesity, smoking, cardiovascular disease,
Clinical Judgement
- Consider which cultural nuances will be needed to have a successful urine collection.
Follow-Up and Desired Outcomes
- Acknowledges contact information provided for the American Diabetes Association (www.diabetes.org), NKF (www.kidney.org), or NKDEP (www.nkdep.nih.gov).
- Recognizes the importance of counseling services to assist in coping with long-term disease implications associated with a chronic disorder.