What are the initial tests to assess renal function in patients with impaired renal function?

Initial Tests for Assessing Renal Function in Patients with Impaired Renal Function

The initial tests to assess renal function in patients with impaired renal function should be serum creatinine with estimated glomerular filtration rate (eGFRcr) and urine albumin-to-creatinine ratio (ACR). 1

Primary Assessment Tests

1. Serum Creatinine with eGFRcr

• Serum creatinine should be measured and used with an estimating equation for initial assessment of GFR 1
• The KDIGO guidelines specifically recommend creatinine-based estimated GFR (eGFRcr) as the initial test for evaluation of kidney function 1
• Clinical laboratories should report an estimate of GFR using a prediction equation in addition to reporting the creatinine measurements 2
• Common equations include:
  • MDRD (Modification of Diet in Renal Disease) equation
  • CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equation
  • Cockcroft-Gault equation (particularly useful for drug dosing)

2. Urine Albumin-to-Creatinine Ratio (ACR)

• ACR should be measured alongside eGFRcr for comprehensive assessment 1, 3
• Random spot urine samples are acceptable for ACR measurement 3
• Normal ACR is <30 mg/g (<3 mg/mmol) 3
• ACR categories:
  • A1 (normal to mildly increased): <30 mg/g
  • A2 (moderately increased): 30-299 mg/g
  • A3 (severely increased): ≥300 mg/g

Follow-up Testing Algorithm

If the initial assessment indicates abnormal kidney function, follow this algorithm:

1. If eGFRcr is thought to be accurate:

   • Continue using eGFRcr for monitoring

2. If eGFRcr is thought to be inaccurate (due to factors like abnormal muscle mass, diet, or medications):

   • Measure cystatin C and calculate eGFRcr-cys (combined creatinine and cystatin C-based eGFR) 1

3. If even more accurate assessment is needed (for critical clinical decisions):

   • Measure GFR using plasma or urinary clearance of an exogenous filtration marker 1
   • Options include iohexol, iothalamate, or DTPA clearance tests

Important Considerations

• Confirm chronicity: Abnormal kidney function should be present for at least 3 months to diagnose CKD 1
• Repeat testing: Single measurements may have >20% biological variability; confirm with repeat testing 3
• First morning samples: Preferred for ACR/PCR to avoid orthostatic proteinuria 3
• Avoid exercise before testing: Patients should refrain from vigorous exercise 24 hours before collection 3

Special Situations

• When 24-hour urine collections are warranted:

  • Patients with exceptional dietary intake (vegetarian diet, creatine supplements)
  • Abnormal muscle mass (amputation, malnutrition, muscle wasting)
  • When eGFRcr-cys is thought to be inaccurate 1, 2
  • For assessment of diet and nutritional status 2

• When to consider kidney biopsy:

  • When clinical decisions about treatment require knowledge of the cause of kidney disease 1
  • When histological findings would guide therapy 1

Common Pitfalls to Avoid

• Do not use serum creatinine alone to assess kidney function 2
• Do not rely solely on dipstick testing for proteinuria diagnosis, as standard dipsticks have lower sensitivity for detecting low-grade proteinuria 3
• Do not assume chronicity based on a single abnormal measurement, as it could result from acute kidney injury 1
• Do not overlook factors affecting measurements:
  • Medications affecting creatinine secretion
  • Extreme body sizes
  • Dietary factors (high protein, creatine supplements)
  • Inflammation (affecting cystatin C)

By following this systematic approach to renal function assessment, clinicians can accurately diagnose kidney dysfunction, appropriately stage chronic kidney disease, and make informed decisions about patient management to improve outcomes related to morbidity, mortality, and quality of life.