What lab test is used to check kidney function in patients with suspected kidney disease or those at risk of developing kidney disease, including the elderly and those with pre-existing medical conditions such as diabetes, hypertension, or a family history of kidney disease?

Laboratory Tests for Kidney Function Assessment

The primary lab test to check kidney function is serum creatinine with automatic calculation of estimated glomerular filtration rate (eGFR), supplemented by urine albumin-to-creatinine ratio (ACR) to detect kidney damage. 1, 2

Core Testing Panel

Serum Creatinine and eGFR

• Serum creatinine alone should never be used as the sole measure of kidney function because it fails to detect significant renal impairment until GFR declines to approximately half of normal, and it is affected by age, sex, race, and muscle mass. 3, 1
• Clinical laboratories must automatically report eGFR whenever serum creatinine is measured, using the MDRD equation (or CKD-EPI equation) that incorporates serum creatinine, age, sex, and race without requiring height or weight. 3, 1, 2
• The MDRD equation is more accurate than the Cockcroft-Gault equation for patients with GFR less than 90 mL/min per 1.73 m². 3
• eGFR values greater than 60 mL/min/1.73 m² should be reported as ">60 mL/min/1.73 m²" rather than a precise number due to reduced accuracy at higher GFR levels. 1

Urine Albumin Assessment

• Use spot (untimed) urine samples for albumin-to-creatinine ratio (ACR) as the preferred screening method, replacing 24-hour urine collections for routine assessment. 1, 2
• First morning void specimens are preferred, but random samples are acceptable. 1, 2
• Two or more positive tests over 3 months are required to establish persistent proteinuria and diagnose chronic kidney disease—a single abnormal result is insufficient. 1, 2
• ACR categorizes albuminuria as: A1 (<30 mg/g, normal), A2 (30-300 mg/g, moderately increased), and A3 (>300 mg/g, severely increased). 2

Additional Laboratory Tests

Supplementary Markers

• Blood urea nitrogen (BUN) helps calculate the BUN-to-creatinine ratio to differentiate prerenal, intrinsic renal, and postrenal causes of kidney dysfunction. 4, 2
• Serum electrolytes (sodium, potassium, chloride, calcium, phosphorus, bicarbonate) identify complications of kidney disease. 4, 2

Screening Frequency by Risk Category

High-Risk Populations Requiring Annual Screening

• Patients with diabetes mellitus, hypertension, age >60 years, African American race, family history of chronic kidney disease, HIV infection, or hepatitis C coinfection. 1
• For diabetes specifically: screen annually starting at diagnosis for type 2 diabetes and 5 years after diagnosis for type 1 diabetes. 2

Monitoring Based on CKD Stage

• Stage 1-2 CKD (eGFR ≥60): Annual monitoring 2
• Stage 3 CKD (eGFR 30-59): Every 6-12 months 4, 2
• Stage 4 CKD (eGFR 15-29): Every 3-5 months 4, 2
• Stage 5 CKD (eGFR <15): Every 1-3 months 4, 2

Critical Caveats and Pitfalls

Laboratory Standardization

• Clinical laboratories must calibrate serum creatinine assays to international standards—differences in calibration can cause errors in eGFR estimates as high as 20%, particularly in patients with near-normal creatinine levels. 3, 1

When eGFR Equations Are Unreliable

• Direct GFR measurement using exogenous filtration markers (iohexol, iothalamate, or inulin clearance) is required in: extremes of age (<18 or >70 years), extremes of body size, severe malnutrition or obesity, skeletal muscle diseases, paraplegia or quadriplegia, vegetarian diet, and rapidly changing kidney function. 3, 1

Common Misinterpretation

• A "normal" serum creatinine does not rule out significant kidney disease—studies show that 11.6% of patients with impaired kidney function (eGFR <60) have serum creatinine in the normal range, with females disproportionately affected (15% vs 9% in males). 5, 6
• In elderly patients, serum creatinine does not reflect age-related decline in GFR due to concomitant decline in muscle mass. 3

Proteinuria Testing Limitations

• Intra-individual variability of urinary albumin is high (30-50% coefficient of variance), and body position, exercise, and fever can affect results. 3
• Patients should refrain from vigorous exercise for 24 hours before sample collection. 2
• 37% of persons with microalbuminuria on initial testing do not have persistent albuminuria on repeat testing 2 months later. 3

Nephrology Referral Thresholds

• Refer to nephrology when: eGFR <30 mL/min/1.73 m², ACR ≥300 mg/g persistently, rapidly declining eGFR (>5 mL/min/1.73 m² per year or >50% increase in creatinine from baseline), or serum creatinine >2.5 mg/dL. 4, 2