Which laboratory tests should be ordered to assess renal insufficiency?

Laboratory Tests for Assessing Renal Insufficiency

Order serum creatinine with calculated eGFR and urine albumin-to-creatinine ratio (ACR) as the essential initial tests for evaluating renal insufficiency. 1

Core Laboratory Tests

Serum Creatinine and eGFR

• Serum creatinine alone is inadequate—always use a validated GFR estimating equation rather than relying on serum creatinine values alone 1
• Use the CKD-EPI equation (or regionally validated equation) to calculate eGFR from serum creatinine, which automatically adjusts for age, sex, and race 1
• The MDRD equation is an alternative but CKD-EPI is preferred for accuracy across different GFR ranges 1
• Avoid using race in eGFR computation when possible, per current KDIGO recommendations 1

Urine Albumin Assessment

• First morning void urine sample is preferred for initial testing 1
• Order tests in this priority: 1
  1. Urine albumin-to-creatinine ratio (ACR) - most preferred
  2. Reagent strip urinalysis for albumin with automated reading
  3. Urine protein-to-creatinine ratio (PCR) if measuring total protein
• Confirm any positive result with quantitative laboratory measurement 1
• ACR ≥30 mg/g (≥3 mg/mmol) on random urine should be confirmed with subsequent first morning void 1

Additional Tests When eGFR is Abnormal

For eGFR 45-59 mL/min/1.73 m² (Stage G3a)

• Consider serum cystatin C if confirmation of CKD is needed, particularly in patients without albuminuria or other markers of kidney damage 1
• Use combined creatinine-cystatin C equation (eGFRcr-cys) for more accurate GFR estimation when creatinine-based eGFR may be inaccurate 1

For eGFR <30 mL/min/1.73 m² (Stage 4-5)

• Electrolytes panel: sodium, potassium, chloride, bicarbonate 1
• Blood urea nitrogen (BUN) 1
• Complete blood count to assess for anemia 1, 2
• Serum calcium, phosphorus, and intact parathyroid hormone (iPTH) for mineral-bone disorder screening 3, 4
• 25-hydroxyvitamin D if iPTH >100 pg/mL 4
• Lipid panel (total cholesterol, LDL, HDL, triglycerides) for cardiovascular risk assessment 4

Clinical Context Considerations

When eGFRcr May Be Inaccurate

Consider cystatin C-based or combined equations in: 1

• Extremes of muscle mass: very low (malnutrition, amputation, muscle wasting) or very high (bodybuilders)
• Dietary extremes: vegetarian, low-protein, ketogenic, or high-protein diets with creatine supplements
• Chronic illness: cancer, heart failure, cirrhosis
• Medication effects: anabolic steroids, drugs decreasing tubular secretion (trimethoprim, cimetidine), broad-spectrum antibiotics

Avoid Common Pitfalls

• Do not order 24-hour urine creatinine clearance—it is less reliable than eGFR from serum creatinine alone due to variability in collection and overestimation from tubular secretion 1, 5
• Do not rely on "normal" serum creatinine—substantial GFR reduction can occur before creatinine becomes abnormal 1, 5
• Serum creatinine must be measured using standardized methods (IDMS-traceable) for accurate eGFR calculation 1, 6

Monitoring Frequency

• Annual assessment minimum for all CKD patients: eGFR and ACR 1
• Every 3 months for Stage 4 CKD (eGFR 15-29): creatinine, eGFR, potassium, urinary albumin 3, 4
• More frequent monitoring if rapid progression, new medications, or clinical status changes 3

When to Measure GFR Directly

Consider measured GFR using exogenous filtration markers (inulin, iohexol, ⁵¹Cr-EDTA) when: 1

• More accurate GFR assessment will impact treatment decisions (e.g., kidney donation evaluation, chemotherapy dosing)
• eGFR equations are expected to be highly inaccurate
• Discrepancy exists between clinical presentation and estimated GFR