What labs check renal function?

Laboratory Tests for Evaluating Renal Function

The primary laboratory tests for evaluating kidney function are serum creatinine with estimated glomerular filtration rate (eGFR), urinalysis with albumin-to-creatinine ratio (ACR), and blood urea nitrogen (BUN). 1

Core Renal Function Tests

First-Line Tests

• Serum Creatinine and eGFR

  • Creatinine-based eGFR (eGFRcr) is the recommended initial assessment of glomerular filtration 1
  • Should be calculated using validated equations rather than relying on serum creatinine alone 1
  • For higher accuracy, combined creatinine and cystatin C-based eGFR (eGFRcr-cys) is recommended when clinical decisions depend on precise GFR 1

• Urinalysis

  • Complete urinalysis including dipstick and microscopic examination 1, 2
  • First morning void sample is preferred 2
  • Evaluates for:
    • Protein/albumin
    • Blood/hematuria
    • Leukocytes
    • Casts
    • Specific gravity

• Quantitative Protein Assessment

  • Urine albumin-to-creatinine ratio (ACR) is the preferred test 1
  • Urine protein-to-creatinine ratio (PCR) when total protein assessment is needed 2
  • ACR ≥30 mg/g requires confirmation with a subsequent sample 2

• Blood Urea Nitrogen (BUN)

  • Complements creatinine assessment 2
  • BUN/creatinine ratio helps distinguish prerenal from intrarenal causes 2

Second-Line Tests

• Serum Electrolytes

  • Sodium, potassium, chloride, bicarbonate 1
  • Calcium, phosphorus, magnesium 2

• Acid-Base Status

  • Serum bicarbonate level 1
  • Helps assess metabolic acidosis associated with kidney disease 2

CKD Staging and Monitoring

CKD is defined by GFR and categorized into stages 1:

Stage	Description	GFR (mL/min per 1.73m²)
I	Kidney damage with normal or increased GFR	≥90
II	Kidney damage with mildly decreased GFR	60-89
III	Moderately decreased GFR	30-59
IV	Severely decreased GFR	15-29
V	Kidney failure	<15 (or dialysis)

Additional Tests for Specific Situations

• Complete Blood Count

  • Assesses for anemia, which may indicate chronic kidney disease 2

• Advanced Testing When Indicated

  • Cystatin C measurement when more accurate GFR assessment is needed 1
  • Measured GFR using exogenous filtration markers when highest accuracy is required 1
  • Timed urine collections for measured creatinine clearance if eGFRcr-cys is thought to be inaccurate 1

Special Considerations

• Pediatric Patients

  • Use pediatric-specific equations (e.g., Schwartz formula) 2
  • Enzymatic creatinine assay is preferred over Jaffe method 2
  • eGFRcr <90 mL/min/1.73m² should be flagged as "low" in children over 2 years 2

• Laboratory Quality Control

  • Labs measuring creatinine in children should ensure quality control processes include the lowest end of expected values 2
  • Standardization of creatinine measurement is critical for accurate GFR estimation 3

When to Consider Kidney Biopsy

Kidney biopsy should be considered when 1:

• Exact diagnosis is needed
• Treatment planning requires histological confirmation
• Assessment of disease activity and chronicity is needed
• Evaluation of genetic disease is required

Common Pitfalls

• A single abnormal eGFR or ACR value should not be assumed to indicate chronic kidney disease; confirmation over at least 3 months is required 1
• Creatinine-based eGFR is less accurate in the normal and slightly increased range of serum creatinine concentrations 3
• Dietary intake can affect serum creatinine levels 1
• Non-standardized creatinine assays can lead to significant variability in GFR estimation 3
• Certain medications and conditions can affect creatinine levels independent of kidney function 2

By following this systematic approach to laboratory evaluation of kidney function, clinicians can accurately assess renal status, diagnose kidney disease, and monitor treatment effectiveness.