| Core Lab |
Synonym/Acronym
Cr, SCr
Rationale
To assess kidney function found in acute kidney injury and chronic kidney disease (CKD), related to drug reaction and disease such as diabetes.
This Core Lab Study is included in the basic metabolic panel (BMP), comprehensive metabolic panel (CMP), general health panel, and renal function panel. Panels are used as general health and targeted screens to identify or monitor conditions such as bone disease, diabetes, hypertension, kidney disease, liver disease, or malnutrition. Serum creatinine (SCr) is also ordered with the urine creatinine clearance test.
Patient Preparation
There are no food, fluid, or medication restrictions unless by medical direction. Instruct the patient to refrain from excessive exercise for 8 hr before the test.
Normal Findings
Method: Spectrophotometry for Cr; immunoturbidometry for cystatin C.
| Creatinine | ||
|---|---|---|
| Age | Conventional Units | SI Units (Conventional Units × 88.4) |
| Newborn–11 mo | 0.17–0.42 mg/dL | 15–37 mmol/L |
| 1–5 yr | 0.19–0.49 mg/dL | 17–43 mmol/L |
| 6–10 yr | 0.26–0.61 mg/dL | 23–54 mmol/L |
| 11–14 yr | 0.35–0.86 mg/dL | 31–76 mmol/L |
| 15 yr–Adult male | 0.74–1.35 mg/dL | 65–119 mmol/L |
| 15 yr–Adult female | 0.59–1.04 mg/dL | 52–92 mmol/L |
| Estimated Glomerular Filtration Rate (eGFR) Many laboratories now report blood Cr and cystatin C values with the corresponding eGFR. | ||
| Age | Conventional Units | |
| Less than 18 yr | Note: The eGFR equation generally recommended for individuals under 18 yr of age is the Bedside Schwartz formula and is based on Cr levels and height in centimeters. | |
| 18–70 yr | 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. | |
| Cystatin C | ||
| Age | Conventional Units | SI Units (Conventional Units × 74.9) |
| Adult | 0.56–1.2 mg/L | 41.9–89.9 mmol/L |
| BUN/Cr Ratio | 10:1 to 20:1 | |
Adults
Potential critical finding is greater than 7.4 mg/dL (SI: 654.2 micromol/L) (patient not on dialysis).
Children
Potential critical finding is greater than 3.8 mg/dL (SI: 336 micromol/L) (patient not on dialysis).
Timely notification to the requesting health-care provider (HCP) of any critical findings and related symptoms is a role expectation of the professional nurse. A listing of these findings varies among facilities.
Consideration may be given to verification of critical findings before action is taken. Policies vary among facilities and may include requesting immediate recollection and retesting by the laboratory or retesting using a rapid point-of-care testing instrument at the bedside, if available.
Chronic renal insufficiency is identified by Cr levels between 1.5 and 3 mg/dL (SI: 132.6 and 265.2 micromol/L); CKD is present at levels greater than 3 mg/dL (SI: 265.2 micromol/L).
Possible interventions may include renal or peritoneal dialysis and organ transplant, but early discovery of the cause of elevated Cr levels might avoid such drastic interventions.
(Study type: Blood collected in a red-, green-, or red/gray-top tube; related body system: .)
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, which can result from increased glomerular permeability to proteins, is considered an independent risk factor predictive of kidney or cardiovascular disease. The National Kidney Foundation and American Society for Clinical Pathology recommend using timed or random urine albumin (formerly microalbumin) and eGFR together to screen for CKD.
Creatinine
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. 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.
Creatinine, a longstanding marker for kidney function, is now routinely evaluated in combination with other markers of kidney health in order to identify kidney disease and provide clinical interventions sooner. Kidney function, as with other bodily functions, normally diminishes with age; estimated loss of filtration occurs at the rate of 1% per year beginning at age 40 yr. Many kidney-related diseases (e.g., diabetic nephropathy, hypertension) progress without signs or symptoms until a significant amount of kidney function is lost (30% to 40%).
When one kidney becomes damaged, diseased, or removed (by nephrectomy), the remaining healthy kidney can compensate; no change in kidney function is expected. Diseases such as diabetes affect both kidneys over time. Kidney transplantation of a single healthy kidney can restore a normal level of renal function; the second nonfunctioning kidney is usually left in place.
Screening for CKD is recommended for high-risk individuals with:
BUN and BUN/Cr ratio
BUN and BUN/Cr ratio BUN is another marker for kidney function often ordered with blood Cr for comparison and mentioned here because it is required to calculate the BUN/Cr ratio. While Cr levels remain fairly constant over time, BUN levels can vary due to normal circumstances including fluctuations in protein intake, protein catabolism, and fluid levels (second- and third-trimester pregnancy), making it a less valuable marker than Cr, in some ways. BUN can also vary due to pathological conditions other than kidney disease (e.g., liver disease). For additional information regarding BUN, refer to the study titled “Urea Nitrogen, Blood and Urine.” The BUN/Cr ratio is a useful indicator of kidney disease. The normal BUN/Cr ratio is 10:1 to 20:1 (e.g., if a patient has a BUN of 10 mg/dL and a Cr of 0.91 mg/dL, the ratio is calculated:10/0.91or BUN/Cr ratio = 11 or 11:1).
Cystatin C
Cystatin C, also known as cystatin 3 and CST3, is now recognized as a useful biomarker for kidney damage. It is also used to monitor function of transplanted kidneys. It is a low-molecular-weight molecule belonging in the family of proteinase inhibitors. Cystatin C is produced by all nucleated cells in the body and is freely filtered by the glomerular membrane in the kidney. It is broken down in the tubules; it does not reenter circulation, small amounts may be excreted in the urine. Cystatin is believed to be a better marker of kidney function than Cr in some ways because levels are independent of variations in age, gender, or muscle mass.
eGFR
GFR is a measure of kidney function. The kidney’s primary function is to filter waste products and excess water from the blood. Other important functions include maintaining pH and electrolyte balance and production of hormones that regulate critical feedback loops over functions involving acid-base balance, blood pressure, bone health, electrolyte balance, hydration, and production of red blood cells. Each kidney contains more than a million glomeruli; GFR refers to the amount of blood that is filtered by the glomeruli per minute.
GFR can be measured by calculating the rate at which an injected marker (e.g., inulin) is excreted into the urine by the kidneys. Measured GFR is still the gold standard and most accurate way to determine GFR. Measured GFR should be used in situations where improved accuracy would influence treatment decisions, but it is more complicated to perform and impractical for routine use given the near instantaneous availability of mathematically derived algorithms.
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. Studies have shown that people of specific descent are at increased risk of developing CKD (African, Native American, and Pacific Islander); Hispanics are 1.5 times more likely than non-Hispanics to develop CKD. Continuing to use race in estimating eGFR is being reevaluated; incorporating race does not consider the (genetic) diversity found in communities of color or take into account the issue of requiring multiracial individuals to choose a single category for race. In September 2021, the National Kidney Foundation and American Society of Nephrology joint task force released its final report that 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 U.S. Pathology and Laboratory Society Leadership in February 2022. Laboratories have begun to transition to race-free eGFR calculations.
The eGFR calculation based on blood cystatin C level is recommended for confirmatory testing in circumstances when eGFR based on serum creatinine is less accurate, such as:
Recommendations for measuring and reporting creatinine values and eGFR (formulas are available that use Cr and/or cystatin to calculate eGFR):
eGFR for Pregnant Patients and Children Under 18 Years of Age
The 2009 Bedside Schwartz formula, is recommended for estimating GFR for children under 18 yr of age. It is an IDMS-traceable equation using serum Cr results from a method that has a calibration traceable to the IDMS. eGFR = 0.413 × (height/Scr) if height is expressed in centimeters or 41.3 × (height/Scr) if height is expressed in meters. The formula uses the patient’s height in centimeters and the serum Cr value where the GFR (mL/min/ 1.73 m2) = (0.41 × height cm)/serum Cr mg/dL (SI Units: GFR (mL/min/ 1.73 m2) = (36.2 × height cm)/serum Cr micromol/L. For consistency in the interpretation of test results, it is important to know whether the Cr has been measured using IDMS-traceable test methods and equations.
The equations have not been validated for pregnant patients (GFR is significantly increased in pregnancy); patients older than 70 yr; patients with serious comorbidities; or patients with extremes in body size, muscle mass, or nutritional status. eGFR calculators can be found at the National Kidney Disease Education Program (NKDEP) website (www.niddk.nih.gov/health-information/communication-programs/nkdep); links are provided for various calculators (e.g., adult and pediatric patients), select the appropriate link to access the desired calculator.
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 use of other tests that reflect GFR, such as the urine creatinine or inulin clearance tests. For additional information regarding the creatinine clearance and marker injection tests, refer to the study titled “Creatinine, Urine, and Creatinine Clearance, Urine, and Glomerular Filtration Rate (GFR).”
Other Considerations
Cr
Increased In
Decreased In
eGFR
eGFR is used to classify the five stages of CKD
BUN/Cr Ratio
The ratio can provide a general indication of an underlying cause; it should not be evaluated in isolation because it does not always point to a clear or accurate finding. For example, the individual BUN and Cr levels are both elevated in chronic kidney disease, but the ratio is often normal.
Increased In
Decreased In
Potential Problems: Assessment & Nursing Diagnosis/Analysis
| Problems | Signs and Symptoms | ||
|---|---|---|---|
| Cardiac output (decreased—related to excess fluid volume, pericarditis, electrolyte imbalance, toxin accumulation) | Weak peripheral pulses, slow capillary refill, decreased urinary output, cool clammy skin, tachypnea, dyspnea, altered level of consciousness, abnormal heart sounds, fatigue, hypoxia, loud holosystolic murmur, ECG changes, increased jugular venous distention (JVD) | ||
| Fluid volume (excess—related to excess fluid and sodium intake, compromised renal function) | Edema, shortness of breath, increased weight, ascites, rales, rhonchi, diluted laboratory values, JVD, tachycardia, restlessness |
Before the Study: Planning and Implementation
Teaching the Patient What to Expect
After the Study: Implementation & Evaluation Potential Nursing Actions
Treatment Considerations
Cardiac Output
Fluid Volume
Nutritional Considerations
Clinical Judgement
Follow-Up Evaluation and Desired Outcomes