Why Check GFR: Essential for Monitoring Kidney Function and Clinical Decision-Making
Checking glomerular filtration rate (GFR) is essential because it is the best overall measure of kidney function and helps identify chronic kidney disease (CKD) early, enabling interventions that can prevent or delay progression to kidney failure and reduce cardiovascular complications. 1
Key Reasons to Check GFR
1. Early Detection of Kidney Disease
- GFR below 60 mL/min/1.73 m² represents loss of half or more of normal adult kidney function 1
- Early stages of CKD can be detected through routine laboratory measurements before symptoms develop 1
- Allows for early intervention when treatment is most effective at preventing progression 1
2. Risk Stratification and Disease Classification
- GFR is used to stage CKD severity (G1-G5) which guides management decisions 1
- G1: ≥90 mL/min/1.73 m²
- G2: 60-89 mL/min/1.73 m²
- G3a: 45-59 mL/min/1.73 m²
- G3b: 30-44 mL/min/1.73 m²
- G4: 15-29 mL/min/1.73 m²
- G5: <15 mL/min/1.73 m²
3. Medication Management
- Critical for appropriate dosing of medications that are cleared by the kidneys 2
- Helps identify when medications may become nephrotoxic at certain GFR levels 3
- Determines when to hold medications during acute illness or dehydration 3
- Essential for adjusting doses of renally cleared drugs to prevent toxicity 1
4. Monitoring Disease Progression
- Rapid progression (decline >5 mL/min/1.73 m²/year) predicts adverse outcomes 1
- Regular monitoring helps identify reversible causes of kidney function decline 1
- Total GFR slope over time is strongly associated with clinical kidney failure endpoints 4
5. Cardiovascular Risk Assessment
- Decreased GFR is an independent predictor of cardiovascular disease and mortality 1
- Patients with CKD should be considered in the highest risk group for cardiovascular events 1
When to Check GFR
General Population
- Case finding (not universal screening) in high-risk individuals 1:
- Hypertension
- Diabetes
- Vascular disease
- Family history of kidney disease
- First Nations/high-risk ethnic groups
- Before starting nephrotoxic medications
Specific Recommendations
- Diabetes: At least annual GFR assessment 1
- Type 1 diabetes: After 5 years of disease duration
- Type 2 diabetes: From time of diagnosis
Monitoring Frequency
- For established CKD: Monitor 1-4 times per year based on CKD stage 1
- More frequent monitoring with:
- Lower GFR levels
- Rapid progression
- Acute changes in clinical status
- After starting medications that affect kidney function
Methods of GFR Assessment
Estimated GFR (eGFR)
- Most practical for routine clinical use 3
- Based on serum creatinine with equations that account for:
- Age
- Sex
- Race
- Body size
Common Equations
- MDRD Study equation: GFR = 186 × [serum creatinine]^-1.154 × [age]^-0.203 × [0.742 if female] × [1.21 if African American] 3
- CKD-EPI equation: More accurate, especially at higher GFR levels 5
Limitations of eGFR
- Less accurate in certain populations:
- Elderly (decreased muscle mass)
- Extreme body sizes
- Malnutrition
- Advanced liver disease
- Pregnancy
When to Consider Measured GFR
- Critical clinical decisions when eGFR may be unreliable 3, 6
- Using exogenous filtration markers (inulin, iohexol) for gold standard measurement 3
- For precise drug dosing in high-risk scenarios 6
Clinical Implications
Treatment Decisions Based on GFR
- ACE inhibitors or ARBs strongly recommended for:
- Severely increased albuminuria (≥300 mg/g creatinine)
- eGFR <60 mL/min/1.73 m² 1
- SGLT2 inhibitors recommended when eGFR ≥30 mL/min/1.73 m² 1
- Specialist referral thresholds based on GFR level 1
Common Pitfalls to Avoid
- Relying solely on serum creatinine without calculating eGFR 3
- Failing to recognize non-renal factors affecting creatinine (muscle mass, diet, medications) 3
- Overreacting to small fluctuations in GFR that may not indicate disease progression 3
- Not adjusting medication doses when GFR declines 2
Conclusion
GFR assessment is fundamental to kidney disease management, enabling early detection, appropriate staging, treatment selection, medication dosing, and monitoring of disease progression. Regular GFR monitoring helps prevent complications, reduce cardiovascular risk, and improve patient outcomes.