Can kidney function be assessed through lab tests?

Can Kidney Function Be Assessed Through Lab Tests?

Yes, kidney function can be reliably assessed through laboratory tests, with estimated glomerular filtration rate (eGFR) calculated from serum creatinine being the primary method, supplemented by markers of kidney damage such as proteinuria. 1

Primary Laboratory Assessment Methods

GFR Estimation as the Gold Standard

• GFR is the best measure of overall kidney function in both health and disease. 1
• Clinical laboratories should routinely calculate eGFR from serum creatinine using prediction equations rather than reporting creatinine values alone. 1
• The MDRD equation (using serum creatinine, age, sex, and race) is recommended for adults 18 years or older, while the Schwartz or Counahan-Barratt equation should be used for those younger than 18 years. 1
• Normal GFR in young adults is approximately 120-130 mL/min per 1.73 m², declining with age. 1
• A GFR level less than 60 mL/min per 1.73 m² represents loss of half or more of normal kidney function and marks clinically significant chronic kidney disease. 1

Markers of Kidney Damage

• Persistent proteinuria is the principal marker of kidney damage. 1
• An albumin-creatinine ratio greater than 30 mg/g in untimed (spot) urine samples is considered abnormal. 1
• Sex-specific cutpoints are greater than 17 mg/g in men and greater than 25 mg/g in women. 1
• Other markers include abnormalities in urine sediment, blood and urine chemistry measurements, and imaging findings. 1

Advanced Laboratory Methods for Confirmatory Testing

Cystatin C-Based Assessment

• Cystatin C-based equations (CKD-EPI-CystC or CKD-EPI-Cr-CystC) should be used as the primary alternative to creatinine for confirmatory testing. 2
• Cystatin C is less influenced by muscle mass, gender, age, and nutritional status compared to creatinine. 2
• The combination of creatinine and cystatin C provides the most precise estimate of measured GFR and improves risk stratification. 2
• KDIGO guidelines recommend using cystatin C for confirmatory testing when eGFR based on creatinine is less accurate, particularly in patients with eGFR 45-59 mL/min/1.73 m² without albuminuria. 2

Direct GFR Measurement

• Direct GFR measurement using exogenous filtration markers like iohexol represents the gold standard when accurate ascertainment will impact treatment decisions. 2
• Iohexol is the preferred exogenous marker, filtered without secretion or reabsorption by renal tubules. 2
• The 2024 KDIGO guidelines recommend harmonizing and standardizing measured GFR protocols to improve accuracy and comparability. 2

Critical Laboratory Requirements

Serum Creatinine Standardization

• Clinical laboratories must measure serum creatinine accurately and calibrate assays to national and international reference standards. 1
• Serum creatinine values less than 1 mg/dL should be reported to the hundredths (2 decimal points). 1
• Laboratory reports should include confidence intervals, reference values for age and sex, and explanations for body surface area adjustment. 1

Additional Laboratory Tests for Comprehensive Assessment

• Creatinine-based eGFR remains the primary kidney function test. 1
• Urinalysis should include diptest, albumin-creatinine ratio, and protein-creatinine ratio. 1
• Metabolic testing should assess serum bicarbonate, chloride, phosphate, and uric acid levels. 1
• Serum and urine glucose levels should be measured for Fanconi syndrome assessment. 1

Common Pitfalls and Caveats

Critical Errors to Avoid

• Never use serum creatinine concentration alone to assess kidney function. 1, 2
• GFR must decline to approximately half the normal level before serum creatinine rises above the upper limit of normal. 2
• 24-hour urine creatinine clearance does not provide more accurate estimates than prediction equations and is fraught with collection errors. 2
• Standard eGFR equations (MDRD, CKD-EPI) cannot be used in ICU settings because they require serum creatinine to be in steady-state. 2

Special Populations

• In patients with low muscle mass due to severe neurological complications, standard creatinine-based equations can overestimate eGFR, and cystatin C-based equations might be more accurate. 1
• Decreased GFR in the elderly is an independent predictor of adverse outcomes such as death and cardiovascular disease, requiring the same diagnostic criteria regardless of age. 1
• Approximately 17% of persons older than 60 years have an estimated GFR less than 60 mL/min per 1.73 m². 1

Clinical Implications

Early Detection Benefits

• Earlier stages of chronic kidney disease can be detected through routine laboratory measurements, allowing prevention or delay of adverse outcomes. 1
• Decreased GFR is associated with complications including hypertension, anemia, malnutrition, bone disease, neuropathy, and decreased quality of life, which can be prevented or ameliorated by treatment at earlier stages. 1
• Treatment can slow progression to kidney failure. 1

Cardiovascular Risk Stratification

• Patients with chronic kidney disease should be considered in the highest risk group for subsequent cardiovascular disease events. 1
• Cardiovascular disease events are more common than kidney failure in patients with chronic kidney disease. 1