Measuring Creatinine is Necessary for Assessing Kidney Function
Serum creatinine measurement is necessary for assessing kidney function, but should not be used alone as it must be incorporated into GFR estimation equations for accurate evaluation of kidney function. 1
Why Creatinine Measurement is Essential
Creatinine serves as the foundation for estimating glomerular filtration rate (GFR), which is the gold standard for kidney function assessment. However, the relationship between creatinine and kidney function is complex:
- Serum creatinine alone is insufficient to assess kidney function accurately 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 doesn't reflect age-related GFR decline due to decreased muscle mass 1
Proper Use of Creatinine in Clinical Practice
GFR Estimation Using Creatinine
- Clinical laboratories should report an estimate of GFR using prediction equations in addition to reporting serum creatinine measurement 1
- The Modification of Diet in Renal Disease (MDRD) equation using 4 variables (serum creatinine, age, sex, and race) is recommended for adults 18 years or older 1
- For children under 18, the Schwartz or Counahan-Barratt equation should be used 1
When Timed Urine Collections Are Needed
Timed urine collections for creatinine clearance are generally not necessary for routine GFR estimation but may be helpful in specific circumstances 1:
- Cachexia
- Muscle atrophy
- Progressive weight loss
- Extreme obesity
- Diseases of skeletal muscle
- Paraplegia or quadriplegia
- Vegetarian diet
- Rapidly changing kidney function
- Calculation of doses for potentially toxic drugs excreted by kidneys 1
Standardization and Quality Considerations
Laboratory Standardization
- Laboratories should calibrate serum creatinine assays using an international standard 1
- Differences in calibration between laboratories can account for errors in GFR estimates as high as 20% 1
Reporting Standards
Laboratory reports of GFR estimates should include 1:
- Confidence interval for the estimate
- Reference values for age and sex
- Explanation for body surface area adjustment
- Caution about conditions affecting creatinine measurements
- Explanation of indications for timed clearance measurements
Kidney Disease Classification
The National Kidney Foundation classification system for Chronic Kidney Disease (CKD) uses GFR derived from creatinine measurements 1:
| Stage | Description | GFR (mL/min/1.73 m²) |
|---|---|---|
| 1 | Kidney damage with normal or increased GFR | ≥90 |
| 2 | Mild decrease in GFR | 60-89 |
| 3 | Moderate decrease in GFR | 30-59 |
| 4 | Severe decrease in GFR | 15-29 |
| 5 | Kidney failure | <15 or dialysis |
Common Pitfalls in Creatinine Interpretation
- Serum creatinine production is affected by body mass, muscle mass, diet, drugs, age, and sex 1
- Among cancer patients with normal serum creatinine, one in five may have asymptomatic renal insufficiency 1
- 40% of individuals with decreased GFR may have serum creatinine levels within the normal range 1
Screening Recommendations
- Universal screening is not recommended due to large numbers needed to screen to identify progressive kidney disease 1
- Case finding is recommended for high-risk individuals:
- Hypertension or diabetes
- Vascular disease
- Before major surgery or potential AKI exposure
- Before nephrotoxic medications
- Family history of kidney disease
- High-risk ethnic groups 1
In conclusion, while creatinine measurement has limitations, it remains the cornerstone of kidney function assessment when properly incorporated into GFR estimation equations. The key is understanding its limitations and using it as part of a comprehensive approach to kidney function evaluation.