What tests can be run to assess kidney function?

Tests for Assessing Kidney Function

The most accurate approach to assess kidney function is to use serum creatinine with a GFR estimating equation as initial assessment, followed by additional confirmatory tests such as cystatin C when more precision is needed. 1

Primary Assessment Methods

Estimated Glomerular Filtration Rate (eGFR)

• Serum creatinine-based equations:
  • CKD-EPI creatinine equation (2021 race-free version) is preferred 2
  • MDRD-4 equation is an acceptable alternative 1
  • These equations incorporate age, sex, and other variables to improve accuracy
  • Should be reported rounded to the nearest whole number in ml/min/1.73 m² 1

Albuminuria/Proteinuria Assessment

• Urine albumin-to-creatinine ratio (ACR) - preferred method 1
• Urine protein-to-creatinine ratio - acceptable alternative 1
• Reagent strip urinalysis - less accurate but useful for screening 1
• Early morning urine samples are preferred for consistency 1

Confirmatory Tests

Cystatin C-Based Assessment

• Recommended when eGFRcr may be inaccurate 1, 2
• Particularly useful in patients with:
  • Extremes of muscle mass
  • Unusual dietary intake (vegetarian, high protein)
  • eGFRcr of 45-59 ml/min/1.73 m² without other markers of kidney damage 1
• Combined creatinine-cystatin C equation (eGFRcr-cys) provides the most accurate GFR estimation 2

Direct GFR Measurement

• Gold standard but not practical for routine clinical use 1, 3
• Methods include:
  • Inulin clearance (historical gold standard) 4
  • Iothalamate clearance (radioactive or non-radioactive) 4
  • Iohexol clearance 4, 5
  • EDTA clearance 4
• Indications for measured GFR:
  • Critical clinical decisions where eGFR may be inaccurate 2
  • Potential kidney donors 5
  • Patients requiring precise medication dosing of nephrotoxic drugs 5
  • Extreme body compositions where estimating equations are unreliable 5

Special Considerations

Monitoring Kidney Function

• Frequency of assessment:
  • At least annual testing for patients with CKD 1
  • More frequent monitoring for those at higher risk of progression 1
• Defining progression:
  • Decline in GFR category plus ≥25% drop in eGFR from baseline 1
  • Rapid progression defined as sustained decline of ≥5 ml/min/1.73 m²/year 1

Point-of-Care Testing

• Valuable for underserved populations and remote locations 1
• Should be standardized and calibrated similar to laboratory tests 1
• Ideally measures both creatinine and urine albumin/creatinine 1

Limitations to Consider

• Small fluctuations in GFR are common and not necessarily indicative of progression 1
• Serum creatinine alone should not be used to assess kidney function 6
• Creatinine is affected by muscle mass, diet, and certain medications 1
• 24-hour urine collections are prone to collection errors but may be useful in specific situations 6

Clinical Algorithm for Kidney Function Assessment

1. Initial Assessment:

   • Measure serum creatinine and calculate eGFR using CKD-EPI equation 1
   • Assess albuminuria using ACR in morning urine sample 1

2. If eGFR is 45-59 ml/min/1.73 m² without albuminuria:

   • Consider measuring cystatin C for confirmation 1
   • If eGFRcys or eGFRcr-cys is also <60 ml/min/1.73 m², confirm CKD diagnosis 1

3. If more accurate GFR assessment is needed:

   • Use combined creatinine-cystatin C equation (eGFRcr-cys) 2
   • Consider measured GFR using exogenous filtration marker if critical decisions depend on precise GFR 1

4. For monitoring:

   • Track trends in eGFR and albuminuria over time 1
   • Assess for factors associated with CKD progression 1

By following this evidence-based approach to kidney function assessment, clinicians can accurately diagnose kidney disease, monitor progression, and make appropriate treatment decisions to improve patient outcomes.