What is the initial test to order for suspected kidney disease?

Initial Testing for Suspected Kidney Disease

The initial test to order for suspected kidney disease should be both a urine albumin-to-creatinine ratio (uACR) and serum creatinine with estimated glomerular filtration rate (eGFR). 1

Rationale for Dual Testing Approach

Urine Albumin-to-Creatinine Ratio (uACR)

• Preferred method for detecting kidney damage as it provides a sensitive marker for chronic kidney disease due to diabetes, glomerular disease, and hypertension 1
• More sensitive than standard dipstick tests, especially at lower levels of proteinuria 1
• Can detect microalbuminuria (30-300 mg/g), which represents early kidney damage before progression to overt proteinuria 2
• Should be performed on a spot urine sample, preferably first morning void 1, 2

Serum Creatinine with eGFR

• Essential for assessing kidney function 1
• Should be calculated using a GFR estimating equation rather than relying on serum creatinine alone 1
• Provides classification of kidney function according to GFR categories (G1-G5) 1

Testing Algorithm

1. Initial Assessment:

   • Order both uACR and serum creatinine with eGFR calculation 1
   • Use first morning void urine sample when possible 1
   • Ensure laboratory reports eGFR in addition to serum creatinine 1

2. Interpretation of Results:

   • Normal: uACR <30 mg/g and eGFR ≥60 ml/min/1.73m² 2
   • Kidney damage: uACR ≥30 mg/g regardless of eGFR 1
   • Reduced kidney function: eGFR <60 ml/min/1.73m² regardless of uACR 1
   • CKD diagnosis: Abnormalities persisting for >3 months 1

3. Confirmation:

   • If uACR is elevated (≥30 mg/g), repeat within 3 months to confirm persistence 1
   • For borderline eGFR (45-59 ml/min/1.73m²) without albuminuria, consider cystatin C measurement for confirmation 1

Special Considerations

High-Risk Populations

More vigilant screening is recommended for:

• Patients with diabetes 1
• Patients with hypertension 1, 2
• African American individuals 1, 2
• Patients with hepatitis C virus coinfection 1
• Older adults 1

Technical Aspects

• The ratio of albumin to creatinine corrects for variations in urine concentration 1
• uACR is more accurate than total protein-to-creatinine ratio (uPCR) for early detection of kidney disease 2, 3
• Point-of-care testing for uACR can facilitate immediate clinical decision-making 4

Common Pitfalls to Avoid

• Don't rely on standard dipstick alone: Standard dipsticks have lower sensitivity for detecting microalbuminuria 1, 5
• Don't use isolated serum creatinine: Creatinine alone without eGFR calculation is insufficient 1
• Don't ignore borderline results: Values near cutoff points should be confirmed with repeat testing 1
• Don't forget to assess chronicity: CKD requires persistence of abnormalities for >3 months 1

By following this dual testing approach of uACR and eGFR, clinicians can effectively screen for, diagnose, and stage kidney disease, enabling earlier intervention to prevent disease progression and complications.