What lab values are used to confirm kidney disease?

Laboratory Values to Confirm Chronic Kidney Disease

To confirm chronic kidney disease, you need two essential lab values: serum creatinine to calculate estimated glomerular filtration rate (eGFR) and urine albumin-to-creatinine ratio (ACR), with abnormalities persisting for at least 3 months. 1

Core Laboratory Tests Required

1. Serum Creatinine and eGFR Calculation

• Measure serum creatinine and use a validated GFR estimating equation (such as CKD-EPI) rather than relying on serum creatinine concentration alone 1
• CKD is confirmed when eGFR is <60 mL/min/1.73 m² for 3 months or more, irrespective of cause 1, 2
• The CKD-EPI equation is preferred over the MDRD equation because it has less bias, especially at GFR ≥60 mL/min/1.73 m², and greater accuracy 1
• Laboratories should use enzymatic assays instead of Jaffe method for creatinine measurements to avoid interference from drugs and other substances 1

2. Urine Albumin-to-Creatinine Ratio (ACR)

• Measure ACR in untimed (spot) urine samples as the preferred method for detecting kidney damage 1, 3
• Normal ACR is ≤30 mg/g; values >30 mg/g indicate abnormal albumin excretion and kidney damage 1, 3
• ACR is more sensitive than total protein for detecting early kidney damage, particularly in diabetic nephropathy 3, 4
• Confirm abnormal results with repeat testing within 3 months—persistent albuminuria requires two of three specimens collected within 3-6 months to be abnormal 3, 4

Confirmatory Testing in Specific Circumstances

When to Add Cystatin C

• Measure serum cystatin C in adults with eGFR 45-59 mL/min/1.73 m² who lack other markers of kidney damage (no albuminuria) if confirmation of CKD is required 1
• This applies to approximately 41% of persons in the United States estimated to have CKD based on eGFR and ACR alone 1
• Two-thirds of persons with eGFRcreat <60 mL/min/1.73 m² have CKD confirmed by eGFRcys <60 mL/min/1.73 m² and show markedly elevated risks for death, cardiovascular disease, and end-stage renal disease 1

Establishing Chronicity

Review past history and previous measurements to determine duration of kidney disease 1:

• If eGFR <60 mL/min/1.73 m² or markers of kidney damage persist for >3 months, CKD is confirmed 1
• If duration is not >3 months or unclear, CKD is not confirmed—the patient may have acute kidney injury, CKD, or both, and tests should be repeated 1

Classification System

Once CKD is confirmed, classify by:

• GFR category: G1 (≥90), G2 (60-89), G3a (45-59), G3b (30-44), G4 (15-29), G5 (<15 mL/min/1.73 m²) 1
• Albuminuria category: A1 (<30 mg/g), A2 (30-299 mg/g), A3 (≥300 mg/g) 1, 4
• These categories determine prognosis and guide treatment decisions 1

Common Pitfalls to Avoid

• Do not rely on serum creatinine alone without calculating eGFR, as creatinine levels are influenced by muscle mass, age, and sex 1
• Do not use the outdated term "microalbuminuria"—use "moderately increased albuminuria" or "category A2" instead 1, 5
• Do not diagnose CKD based on a single abnormal test—chronicity (≥3 months) must be established 1
• Avoid testing during transient conditions that elevate proteinuria: vigorous exercise within 24 hours, urinary tract infection, fever, acute illness, or menstruation 3, 4
• Do not use albumin-only dipsticks without creatinine measurement, as results are susceptible to false results due to hydration variations 4

Additional Evaluation

After confirming CKD, evaluate:

• Cause of CKD through clinical context, personal and family history, medications, physical examination, and imaging 1
• Blood pressure measurement 1, 6
• Blood glucose or hemoglobin A1c to assess for diabetes 6
• Electrolytes, particularly potassium, when initiating treatment 5