Why check creatinine levels?

Why Check Creatinine Levels

Serum creatinine should be checked to estimate glomerular filtration rate (GFR) and assess kidney function, as serum creatinine alone is an inadequate measure of kidney function and must be incorporated into prediction equations for accurate assessment. 1

Limitations of Serum Creatinine Alone

Serum creatinine has significant limitations when used in isolation:

• It is affected by factors other than GFR, including creatinine secretion, generation, and extrarenal excretion 1
• There is a wide normal range for serum creatinine values 1
• GFR must decline to approximately half the normal level before serum creatinine rises above the upper limit of normal 1
• In elderly patients, serum creatinine does not reflect age-related decline in GFR due to concomitant age-related decline in muscle mass 1

Proper Use of Creatinine Measurements

The National Kidney Foundation guidelines strongly recommend:

1. Not using serum creatinine concentration as the sole means to assess kidney function (level A recommendation) 1
2. Using GFR estimation equations that account for serum creatinine plus variables such as age, sex, race, and body size 1
3. Clinical laboratories should report an estimate of GFR using a prediction equation in addition to reporting the serum creatinine measurement 1

Clinical Applications of Creatinine Testing

Creatinine testing is essential for:

• Diagnosing and staging chronic kidney disease (CKD) based on the presence of kidney damage and level of kidney function 1
• Adjusting medication dosages for drugs excreted by the kidneys 1, 2
• Monitoring kidney function in patients at risk for kidney disease or those with established kidney disease 1
• Detecting early stages of kidney dysfunction before symptoms develop 1

Recommended Equations for GFR Estimation

For adults, the guidelines recommend:

• The Modification of Diet in Renal Disease (MDRD) Study equation 1
• The Cockcroft-Gault equation (less desirable but preferable to using serum creatinine alone) 1

For children:

• The Schwartz formula or Counahan-Barratt equation 1

Common Pitfalls to Avoid

• Relying solely on serum creatinine values: This can lead to missed diagnoses of kidney dysfunction, especially in elderly patients or those with reduced muscle mass 3
• Ignoring laboratory variability: Differences in creatinine assay calibration between laboratories can account for errors in GFR estimates as high as 20% 1
• Using 24-hour urine collections unnecessarily: The guidelines state that measurement of creatinine clearance using timed urine collections does not provide more accurate estimates of GFR than prediction equations 1
• Failing to standardize creatinine measurements: Laboratories should calibrate serum creatinine assays using an international standard 1

Clinical Implications

Regular monitoring of creatinine levels is crucial for:

• Detecting CKD early when interventions can prevent or delay progression to kidney failure 1
• Identifying patients who need dose adjustments for medications eliminated by the kidneys 2
• Monitoring for nephrotoxicity from medications or other causes 1
• Evaluating patients with hypertension, heart failure, or myocardial infarction 2

By properly utilizing creatinine measurements within GFR estimation equations, clinicians can more accurately assess kidney function and provide appropriate care to prevent adverse outcomes related to kidney disease.