What other tests can be used to diagnose Impaired renal function?

Diagnostic Tests for Impaired Renal Function

Serum creatinine and estimated glomerular filtration rate (eGFR) combined with urine albumin measurement are the cornerstone tests for diagnosing impaired renal function, with cystatin C-based eGFR recommended as an additional confirmatory test when available. 1

Primary Diagnostic Tests

First-Line Testing

• Serum creatinine with eGFR calculation

  • Recommended as the initial test for all patients 1
  • Should be combined with urine albumin measurement for comprehensive assessment
  • Limitations: affected by muscle mass, age, and nutritional status

• Urine albumin-to-creatinine ratio (ACR)

  • Essential complement to eGFR for complete kidney function assessment 1
  • Detects early kidney damage before GFR decline
  • Should be measured in random spot urine samples

Confirmatory Testing

• Cystatin C-based eGFR (eGFRcr-cys)
  • Recommended when available to improve GFR staging accuracy 1
  • Less influenced by muscle mass, age, and diabetes than creatinine
  • Combined creatinine-cystatin C equations provide the least bias in GFR estimation

Specialized Diagnostic Tests

Biomarkers of Tubular Injury

• Neutrophil gelatinase-associated lipocalin (NGAL)

  • Useful for differentiating acute tubular necrosis (ATN) from other causes of AKI 1
  • Urinary NGAL performs better than serum NGAL
  • Cutoff value of 220-244 μg/g creatinine helps differentiate ATN from prerenal azotemia
  • May predict 90-day mortality in certain conditions

• Kidney injury molecule-1 (KIM-1)

  • Biomarker upregulated by tubular injury 1
  • Helps identify tubular damage

• Other tubular injury markers

  • N-acetyl-β-D-glucosaminidase
  • α-glutathione S-transferase
  • Interleukin-18

Functional Assessment Tests

• Fractional excretion of sodium (FENa)

  • Reflects sodium handling more accurately than urinary sodium alone 1
  • FENa <1% suggests prerenal causes (including hepatorenal syndrome)
  • FENa >1% suggests structural causes like ATN
  • Limited specificity (only 14%) in patients with cirrhosis

• Fractional excretion of urea (FEUrea)

  • Better discriminator between functional and structural causes of AKI 1
  • Not modified by diuretic use
  • FEUrea <28.16% has sensitivity of 75% and specificity of 83% in separating hepatorenal syndrome from other causes

Imaging Studies

• Renal ultrasound

  • Evaluates kidney size, echogenicity, and structural abnormalities 1
  • Small echogenic kidneys suggest chronic kidney disease
  • Helps exclude post-renal obstruction

• Cross-sectional imaging (CT/MRI)

  • With and without IV contrast (when not contraindicated) 1
  • Evaluates for structural abnormalities, masses, or vascular issues
  • Particularly important in trauma cases or suspected renovascular disease 1

Establishing Chronicity

To determine if kidney disease is chronic (present for >3 months), consider:

1. Review of past GFR measurements
2. Review of past albuminuria/proteinuria measurements
3. Imaging findings (reduced kidney size, cortical thinning)
4. Kidney pathological findings (fibrosis, atrophy)
5. Medical history of conditions known to cause CKD 1

Clinical Application Algorithm

1. Initial screening:

   • Measure both serum creatinine (with eGFR calculation) AND urine albumin-to-creatinine ratio 1
   • If abnormal, repeat to confirm findings

2. If abnormalities confirmed:

   • Add cystatin C measurement for more accurate GFR assessment 1
   • Perform renal ultrasound to assess structure and exclude obstruction

3. For acute changes:

   • Measure tubular biomarkers (NGAL, KIM-1) to differentiate between functional and structural causes 1
   • Calculate FENa and FEUrea to help determine etiology

4. For suspected specific etiologies:

   • Consider additional testing based on clinical presentation (e.g., renovascular imaging for suspected renovascular hypertension) 1

Important Caveats

• Single abnormal eGFR or ACR values should not be used to diagnose CKD - confirmation with repeat testing is essential 1
• Establishing chronicity (>3 months duration) is crucial for distinguishing CKD from AKI 1
• Biomarkers like NGAL are not yet widely available for clinical use in many settings 1
• Renal vein renin assays have high rates of false-positive and false-negative results and are not recommended as screening tests for renovascular hypertension 1

By systematically applying these diagnostic tests, clinicians can accurately identify impaired renal function, determine its chronicity, and guide appropriate management to improve patient outcomes.