What tests are used to check for kidney damage?

Tests to Check for Kidney Damage

The two essential tests to check for kidney damage are: (1) serum creatinine to calculate estimated glomerular filtration rate (eGFR), and (2) urine albumin-to-creatinine ratio (UACR) to detect proteinuria. 1

Core Screening Tests

Blood Tests

• Serum creatinine with eGFR calculation is the primary test to assess kidney function, with eGFR <60 mL/min/1.73 m² indicating chronic kidney disease regardless of other findings 1
• Use the CKD-EPI equation (preferred) or MDRD equation to estimate GFR from serum creatinine, age, sex, and race—never rely on serum creatinine alone as it grossly overestimates kidney function 1
• Cystatin C can be used as an alternative marker when creatinine-based estimates are unreliable (extremes of muscle mass, dietary variations) 2

Urine Tests

• Spot urine albumin-to-creatinine ratio (UACR) is the preferred method to detect kidney damage, with values ≥30 mg/g indicating abnormal albumin excretion 1
• Urinalysis screens for proteinuria (≥1+ on dipstick roughly correlates to ≥30 mg/dL), hematuria, and active sediment that may indicate glomerular disease 1
• First morning void is preferred for UACR to minimize variability, though random samples are acceptable in adults 3

Screening Frequency and Target Populations

Who Should Be Screened

• All patients with diabetes or hypertension should have annual eGFR and UACR testing 1
• High-risk populations requiring annual screening include: African Americans, patients with hepatitis C, those with HIV RNA >4,000 copies/mL or CD4+ <200 cells/mL, patients with cardiovascular disease, obesity, or family history of kidney disease 1
• Patients over age 50 should be considered for screening given age-related decline in kidney function 1, 4

Testing Schedule

• Perform annual screening for at-risk populations with normal baseline results 1
• Increase to every 6 months when eGFR is 45-60 mL/min/1.73 m² 1
• Test every 3 months when eGFR is 30-44 mL/min/1.73 m² 1

Staging Kidney Disease

Chronic kidney disease is defined as either kidney damage or eGFR <60 mL/min/1.73 m² persisting for ≥3 months 1:

• Stage 1: eGFR ≥90 with evidence of kidney damage (proteinuria, imaging abnormalities) 1
• Stage 2: eGFR 60-89 with evidence of kidney damage 1
• Stage 3: eGFR 30-59 (kidney disease present regardless of other markers) 1
• Stage 4: eGFR 15-29 (severe kidney dysfunction) 1
• Stage 5: eGFR <15 or on dialysis (kidney failure) 1

Additional Diagnostic Tests When Kidney Disease Is Confirmed

Laboratory Evaluation

• Electrolytes and potassium should be checked at least yearly in all CKD patients 1
• Complete metabolic panel including bicarbonate, calcium, phosphorus when eGFR <60 1
• Hemoglobin to screen for anemia of CKD 1
• Parathyroid hormone when eGFR <60 to detect mineral bone disorder 1
• Hepatitis B and C serology, complement levels, antinuclear antibody, cryoglobulin, immunoglobulin levels, serum and urine protein electrophoresis to identify underlying causes of CKD 1

Imaging Studies

• Renal ultrasound provides information on kidney size, stones, masses, and structural abnormalities 1
• Small kidneys (<9 cm length) suggest advanced irreversible disease, while large echogenic kidneys may indicate certain diagnoses but are not diagnostic alone 1

Critical Pitfalls to Avoid

• Never use serum creatinine alone without calculating eGFR—a "normal" creatinine of 1.3 mg/dL can mask significant kidney dysfunction 1
• Do not rely on 24-hour creatinine clearance for routine screening—it is less accurate than eGFR equations and prone to collection errors 1
• Confirm proteinuria with quantitative testing (UACR or protein-to-creatinine ratio) if dipstick is positive, as transient proteinuria from exercise, fever, or UTI is common 1, 3
• Avoid testing during menstruation or within 24 hours of vigorous exercise, as both cause false-positive proteinuria 3

When to Refer to Nephrology

Referral is indicated when 1:

• eGFR <30 mL/min/1.73 m² (Stage 4 CKD)
• Rapid decline in eGFR (>25% decrease)
• Severe proteinuria (UACR >300 mg/g or protein-to-creatinine ratio >1,000 mg/g)
• Active urinary sediment with dysmorphic red blood cells or red cell casts
• Uncertain etiology of kidney disease
• Resistant hypertension or difficult-to-manage complications (anemia, hyperparathyroidism, electrolyte disturbances)