What lab tests are needed before referring patients with Chronic Kidney Disease (CKD) to nephrologists?

Laboratory Testing Before Nephrology Referral in CKD

Before referring CKD patients to nephrologists, obtain serum creatinine with calculated eGFR, urine albumin-to-creatinine ratio (ACR), complete metabolic panel including electrolytes and bicarbonate, complete blood count, and renal ultrasound if indicated. 1, 2

Essential Laboratory Tests

Kidney Function Assessment

• Serum creatinine with eGFR calculation using the CKD-EPI equation is the primary test for assessing kidney function and staging CKD 1, 3
• Report eGFR values below 60 mL/min/1.73 m² as "decreased" rather than providing exact numbers 1
• Cystatin C measurement should be obtained in adults with eGFR 45-59 mL/min/1.73 m² who lack other markers of kidney damage to confirm the CKD diagnosis 1
  • If both eGFR-creatinine and eGFR-cystatin C are <60 mL/min/1.73 m², CKD is confirmed 1

Proteinuria Assessment

• Urine albumin-to-creatinine ratio (ACR) is the preferred first-line test for proteinuria, measured on an early morning urine sample 1
• Alternative acceptable tests in descending order of preference: urine protein-to-creatinine ratio (PCR), reagent strip urinalysis with automated reading 1
• Confirm any ACR ≥30 mg/g (≥3 mg/mmol) detected on random urine with a subsequent early morning sample 1
• Laboratories must report ACR and PCR as ratios, not just albumin or protein concentrations alone 1
• The term "microalbuminuria" should no longer be used 1

Additional Laboratory Studies

• Complete metabolic panel including electrolytes (sodium, potassium, chloride), bicarbonate, calcium, and phosphorus 1, 2
• Complete blood count to assess for anemia, a common CKD complication 1, 4
• Parathyroid hormone (PTH) and vitamin D levels for patients with eGFR <45 mL/min/1.73 m² 1
• Lipid panel as dyslipidemia is a risk factor for CKD progression 1, 4
• Hemoglobin A1c in diabetic patients or those with suspected diabetes 1, 4

Specialized Testing When Indicated

• Serologic tests including hepatitis B and C, complement levels, antinuclear antibody, cryoglobulins, and quantitative immunoglobulins when glomerulonephritis or systemic disease is suspected 1
• Serum and urine protein electrophoresis if monoclonal gammopathy is suspected (particularly with non-albumin proteinuria) 1
• Urine microscopy for casts, red blood cells, and epithelial cells to identify active urinary sediment 1, 2

Imaging Studies

• Renal ultrasound should be obtained when CKD is confirmed to assess kidney size, echogenicity, and rule out obstruction or structural abnormalities 1
• Small kidneys (<9 cm length) suggest advanced irreversible disease 1
• Ultrasound can identify stones, masses, cysts, and hydronephrosis 1

Monitoring Frequency Before Referral

• Annual assessment of eGFR and albuminuria is recommended for all CKD patients 1
• More frequent monitoring (every 3-6 months) is needed for patients at higher risk of progression, including those with:
  • eGFR 30-44 mL/min/1.73 m² (stage 3b): monitor every 3 months 1
  • eGFR 45-59 mL/min/1.73 m² (stage 3a): monitor every 6 months 1
  • Higher albuminuria categories 1
  • Rapid eGFR decline (>5 mL/min/1.73 m² per year) 2, 4

Common Pitfalls to Avoid

• Do not rely on serum creatinine alone without calculating eGFR, as creatinine may appear normal (<1.2 mg/dL in women, <1.4 mg/dL in men) even when eGFR is significantly reduced 5
• Ensure creatinine assays are IDMS-traceable with minimal bias and acceptable imprecision for accurate eGFR calculation 3
• Do not use reagent strip alone for proteinuria diagnosis; confirm positive results with quantitative ACR or PCR 1
• Recognize that small eGFR fluctuations are common and do not necessarily indicate progression 1
• Avoid nephrotoxic agents including NSAIDs and ensure appropriate dose adjustments for renally-cleared medications before referral 4, 6

Specific Referral Triggers Based on Laboratory Results

Immediate nephrology referral is indicated when labs show: 2, 4, 6

• eGFR <30 mL/min/1.73 m² (stage 4-5 CKD)
• Rapid eGFR decline >5 mL/min/1.73 m² per year
• Sustained eGFR decrease >20% after excluding reversible causes
• ACR ≥300 mg/g (≥30 mg/mmol) or PCR ≥500 mg/g (≥50 mg/mmol)
• Persistent proteinuria >1 g/day despite optimal treatment
• Urinary red cell casts or RBC >20 per high-power field
• Persistent hyperkalemia or other refractory electrolyte abnormalities
• Refractory hypertension despite 4 or more antihypertensive agents

Consider nephrology referral for: 1, 2

• eGFR 30-44 mL/min/1.73 m² (stage 3b) with evidence of progression
• Uncertain etiology of kidney disease (absence of diabetic retinopathy in diabetics, heavy proteinuria, active urine sediment, rapid decline)
• Difficult management issues including anemia, secondary hyperparathyroidism, metabolic bone disease, or resistant hypertension