What initial labs are recommended to assess kidney function in a patient with no known kidney disease?

Initial Laboratory Assessment for Kidney Function

For patients with no known kidney disease, obtain serum creatinine with calculated eGFR using the CKD-EPI equation and a spot urine albumin-to-creatinine ratio as the essential initial tests to assess kidney function. 1

Core Laboratory Tests

Primary Kidney Function Assessment

• Serum creatinine with eGFR calculation using the 2009 CKD-EPI equation is the primary assessment of kidney function 1
• The eGFR provides a more accurate estimate of kidney function than serum creatinine alone, which reflects not only renal excretion but also generation, intake, and metabolism of creatinine 2, 3
• A single abnormal result is insufficient for chronic kidney disease diagnosis; abnormalities must persist >3 months to establish CKD 1

Urine Testing

• Spot urine albumin-to-creatinine ratio (ACR) is the preferred method for detecting proteinuria, replacing 24-hour urine collections 4, 1
• First morning void is preferred, though random specimens are acceptable 4
• An ACR >30 mg/g indicates abnormal proteinuria and warrants confirmation with repeat testing within 3 months 4
• Urinalysis with microscopy should be performed to detect cells, casts, and crystals 4, 1

Supporting Laboratory Tests

Blood Tests

• Complete blood count to evaluate for anemia and infection 1
• Blood urea nitrogen (BUN) with calculation of BUN-to-creatinine ratio (>20:1 suggests prerenal cause, <10:1 suggests intrinsic renal disease) 1
• Serum electrolytes including sodium, potassium, calcium, magnesium, chloride, and phosphorus to identify imbalances 4, 1
• Glucose and fasting lipid profile 1

Additional Metabolic Testing

• Serum bicarbonate, phosphate, and uric acid levels 4
• Liver function tests and thyroid-stimulating hormone 1

Clinical Context Considerations

For High-Risk Populations

• Patients with diabetes, hypertension, or family history of kidney disease require albumin-specific testing even if standard dipstick is negative 4
• Adults at increased risk should begin with albumin-specific dipstick or direct measurement of albumin-to-creatinine ratio 4

When to Use Alternative Markers

• Cystatin C should be measured when eGFR based on creatinine may be inaccurate due to extremes of muscle mass or malnutrition 1
• Certain medications can interfere with creatinine measurements, affecting eGFR accuracy 5

Imaging Considerations

• Renal ultrasound is the initial imaging study of choice if structural abnormalities are suspected, to assess kidney size, echogenicity, and rule out obstruction 1, 5
• However, imaging is not routinely required for initial kidney function assessment in asymptomatic patients without known kidney disease 4

Important Caveats

• Normal kidney size on imaging does not exclude chronic kidney disease, particularly in diabetic nephropathy or infiltrative disorders 4
• Serum creatinine alone does not provide adequate estimation of GFR and should always be used to calculate eGFR 2, 3
• Two or more positive results on quantitative tests temporally spaced over 3 months are required to confirm persistent proteinuria 4
• Early-stage CKD is asymptomatic, making laboratory screening essential for detection 6