Diagnostic Tests for Kidney Disease
For initial evaluation of kidney disease, measure serum creatinine with calculated eGFR using the CKD-EPI equation and obtain a spot urine albumin-to-creatinine ratio (ACR); repeat abnormal results to confirm chronicity over 3 months before diagnosing chronic kidney disease 1.
Core Screening Tests
The diagnostic approach centers on three fundamental laboratory measurements 1, 2:
- Serum creatinine with automated reporting of estimated GFR using creatinine (eGFRcr) via the CKD-EPI 2009 equation
- Spot urine albumin-to-creatinine ratio (ACR) from an untimed urine sample
- Urinalysis with microscopic examination to detect hematuria and assess urine sediment
Critical Confirmation Step
Do not diagnose CKD based on a single abnormal test result 1. A single elevated creatinine or ACR could represent acute kidney injury rather than chronic disease. Repeat testing is mandatory—chronicity requires either:
- Documentation of abnormalities persisting ≥3 months, OR
- Historical evidence from prior lab results, imaging showing small kidneys with cortical thinning, or kidney biopsy showing fibrosis 1
Enhanced Testing When Confirmation Needed
When serum creatinine may be unreliable due to extremes of muscle mass, dietary factors, or assay interference, the KDIGO 2024 guidelines recommend measuring cystatin C and calculating combined eGFRcr-cys 1. This combination provides superior accuracy, particularly in older adults with sarcopenia where creatinine-based estimates may overestimate kidney function 3.
For albuminuria confirmation when spot ACR results are borderline or affected by muscle mass extremes, obtain a timed urine collection to measure albumin excretion rate 2.
Additional Diagnostic Tests to Establish Etiology
Once kidney disease is confirmed, pursue cause-directed testing 1:
Imaging
- Renal ultrasound as first-line to assess kidney size, symmetry, cortical thickness, obstruction, and cystic disease
- CT or MRI reserved for specific indications
Laboratory Tests for Specific Causes
Based on clinical presentation, order targeted serologies:
- Anti-PLA2R antibodies for membranous nephropathy
- ANCA panel for vasculitis
- Anti-GBM antibodies for Goodpasture syndrome
- Serum and urine protein electrophoresis with immunofixation plus serum-free light chains for monoclonal gammopathy (increasingly recognized even without multiple myeloma)
- Complement levels (C3, C4) for glomerulonephritis
Genetic Testing
Consider genetic panels (APOL1, COL4A3/4/5, PKD1/2, NPHS1, UMOD, HNF1B) when family history suggests hereditary disease, though genetic causes may present without classic pedigrees 1.
Kidney Biopsy
Perform kidney biopsy when the diagnosis remains uncertain and results would change management 1. This is particularly valuable for:
- Determining exact glomerular disease type
- Assessing disease activity versus chronicity/fibrosis
- Guiding immunosuppressive therapy decisions
- Evaluating unexplained rapid GFR decline
The biopsy should include light microscopy, immunofluorescence, and electron microscopy, with molecular diagnostics in selected cases 1.
Staging Framework
The KDIGO 2024 classification stages CKD by both GFR category (G1-G5 based on eGFR) and albuminuria category (A1-A3 based on ACR), creating a risk matrix for progression and cardiovascular outcomes 1, 2. Lower levels of albuminuria independently predict adverse renal and cardiovascular events 4.
Common Pitfalls to Avoid
- Assuming chronicity without proof: Always document 3-month persistence or obtain historical data 1
- Relying solely on creatinine in sarcopenic patients: Add cystatin C measurement 3
- Missing monoclonal gammopathy: Screen with serum-free light chains even without obvious myeloma 1
- Delaying nephrology referral: Consult nephrology for eGFR <30 mL/min/1.73m², ACR >300 mg/g, or rapid GFR decline 4