What lab tests should be ordered to assess kidney function in a general adult population with no specific medical history?

Laboratory Tests to Assess Kidney Function

Order serum creatinine with calculated eGFR using the CKD-EPI equation and a spot urine albumin-to-creatinine ratio (ACR) as the core tests to assess kidney function in adults. 1, 2, 3

Essential Initial Laboratory Tests

Serum Creatinine and eGFR

• Serum creatinine alone is insufficient and should never be used in isolation to assess kidney function because it is heavily influenced by muscle mass, age, sex, and race. 1, 3, 4
• Laboratories must calculate eGFR automatically using a validated equation (preferably CKD-EPI) whenever creatinine is measured, reporting the result alongside the creatinine value. 1, 3
• The CKD-EPI equation is superior to the older MDRD equation, particularly at GFR ≥60 mL/min/1.73 m², with less bias and greater accuracy. 3, 5
• Normal eGFR is ≥90 mL/min/1.73 m²; values <60 mL/min/1.73 m² for ≥3 months confirm chronic kidney disease regardless of cause. 3

Urine Albumin Assessment

• Obtain a spot (untimed) urine sample for albumin-to-creatinine ratio (ACR) rather than 24-hour urine collection, as it corrects for hydration variations and is far more convenient with equivalent accuracy. 1, 2, 3
• Use a first morning void specimen when possible, though a random sample is acceptable for initial screening. 1, 2
• Normal ACR is <30 mg/g; values ≥30 mg/g indicate kidney damage and require confirmation with repeat testing. 2, 3
• If initial dipstick shows ≥1+ protein (≥30 mg/dL), confirm with quantitative spot urine protein-to-creatinine ratio within 3 months. 2
• Persistent proteinuria requires two or more positive quantitative tests over a 3-month period to establish chronicity. 2, 3

Additional Tests in Specific Circumstances

When eGFR is Borderline or Confirmation Needed

• Measure serum cystatin C in adults with eGFR 45-59 mL/min/1.73 m² who lack other markers of kidney damage if confirmation of CKD is required for critical decisions. 1, 3
• Calculate eGFR using both creatinine and cystatin C (eGFRcr-cys) when greater accuracy is needed, as combined equations outperform either marker alone. 1, 5
• Consider measured GFR using exogenous filtration markers (such as iohexol clearance) only when even greater precision is essential for major clinical decisions like kidney donation or chemotherapy dosing. 1

Urinalysis and Urine Sediment

• Perform comprehensive urinalysis with microscopic examination of urine sediment to detect red blood cells, white blood cells, casts, and crystals that indicate specific kidney pathology. 1
• The presence of dysmorphic red blood cells or red cell casts suggests glomerular disease and warrants nephrology evaluation. 1
• Persistent hematuria with proteinuria significantly narrows the differential diagnosis toward glomerular pathology. 1

Blood Urea Nitrogen (BUN)

• BUN is less reliable than creatinine for assessing kidney function because it is heavily influenced by extrarenal factors including dietary protein intake, hydration status, gastrointestinal bleeding, and catabolic states. 6
• The BUN-to-creatinine ratio can help distinguish prerenal azotemia (ratio >20:1) from intrinsic kidney disease (ratio typically 10-15:1), but this is a secondary consideration. 6

Classification and Staging

Once CKD is confirmed through persistent abnormalities for ≥3 months, 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). 3
• Albuminuria category: A1 (<30 mg/g), A2 (30-299 mg/g), A3 (≥300 mg/g). 3

Critical Pitfalls to Avoid

• Never diagnose CKD based on a single abnormal test—chronicity (≥3 months duration) must be established through repeat testing. 3
• Do not order 24-hour urine collections routinely; spot ACR provides adequate assessment and monitoring in nearly all clinical situations. 1, 2, 7
• Avoid testing during transient conditions that elevate proteinuria (menstruation, vigorous exercise, urinary tract infection, fever) as up to 98% of false-positives occur with confounding factors present. 2
• Do not use albumin-only dipsticks without simultaneous creatinine measurement, as results are unreliable due to hydration variations. 2
• Laboratories must use enzymatic assays rather than the Jaffe method for creatinine measurement to avoid interference from drugs and other substances. 3

When to Consider Additional Workup

If kidney disease is confirmed and the cause is unclear, additional tests may include imaging (renal ultrasound), serologic tests (ANCA, anti-GBM antibodies, complement levels, serum protein electrophoresis), or kidney biopsy depending on clinical presentation. 1 However, the USPSTF found insufficient evidence to recommend routine screening for CKD in asymptomatic adults without risk factors such as diabetes or hypertension. 1