What lab tests should be ordered to assess kidney function in a patient with a history of diabetes or hypertension?

Laboratory Assessment of Kidney Function in Patients with Diabetes or Hypertension

Order serum creatinine with eGFR calculation using the 2009 CKD-EPI equation and a spot urine albumin-to-creatinine ratio (UACR) as your core kidney function assessment—these two tests together provide complete staging of chronic kidney disease and guide treatment decisions. 1, 2

Essential First-Line Tests

Serum Creatinine and eGFR

• Measure serum creatinine and calculate eGFR using the 2009 CKD-EPI equation rather than relying on serum creatinine concentration alone, as recommended by Kidney International. 1
• The eGFR categorizes kidney function into stages: G1 (≥90 mL/min/1.73m²), G2 (60-89), G3a (45-59), G3b (30-44), G4 (15-29), and G5 (<15 mL/min/1.73m²). 1
• Serum creatinine should be measured using a specific assay with calibration traceable to international standard reference materials. 1

Urine Albumin-to-Creatinine Ratio (UACR)

• Order a spot UACR from a first-morning urine sample—this is the preferred screening method over 24-hour collections or urine dipstick. 3, 1
• UACR categorizes albuminuria as: A1 (<30 mg/g, normal), A2 (30-300 mg/g, moderately increased), and A3 (>300 mg/g, severely increased). 3, 1, 2
• Use UACR instead of urine dipstick, which lacks sufficient sensitivity to detect microalbuminuria—the KDOQI commentary explicitly recommends this over the less sensitive dipstick test. 3

Screening Frequency

For Patients with Diabetes

• Screen annually starting at diagnosis for type 2 diabetes and 5 years after diagnosis for type 1 diabetes. 3, 2, 4
• Increase monitoring frequency to every 6 months for patients with eGFR <60 mL/min/1.73m² or UACR >30 mg/g. 2

For Patients with Hypertension

• Screen annually with both serum creatinine/eGFR and UACR as part of routine cardiovascular risk assessment. 3, 1

Confirmatory Testing Requirements

When UACR is Elevated

• Confirm persistent albuminuria by repeating the test—2 of 3 samples collected over 3-6 months must show elevation (>30 mg/g) before diagnosing persistent microalbuminuria. 3
• Patients should refrain from vigorous exercise for 24 hours before sample collection, as exercise can transiently elevate albumin excretion. 3
• Avoid testing during acute febrile illness, urinary tract infections, marked hypertension, or heart failure, as these cause transient elevations. 3

Additional Laboratory Tests at Initial Evaluation

Electrolyte Panel

• Measure sodium, potassium, calcium, chloride, phosphorus, and bicarbonate to identify electrolyte imbalances and complications of kidney disease. 1, 4
• Blood urea nitrogen (BUN) helps calculate the BUN-to-creatinine ratio, differentiating prerenal, intrinsic renal, and postrenal causes. 4

Cardiovascular Risk Assessment

• Order fasting glucose and lipid profile as part of comprehensive CVD risk stratification in hypertensive patients. 3

Monitoring Frequency Based on CKD Stage

Adjust monitoring intervals based on the combined GFR and albuminuria categories:

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

Special Monitoring Situations

Patients on ACE Inhibitors, ARBs, or Diuretics

• Recheck serum creatinine, eGFR, and potassium within 1-2 weeks after initiating or adjusting doses of these medications. 4
• Monitor serum creatinine/eGFR and potassium at least annually thereafter. 2

When to Consider Cystatin C

• Consider cystatin C as a confirmatory test when eGFR based on serum creatinine may be inaccurate due to extremes of muscle mass or dietary protein intake. 1
• The combined creatinine-cystatin C equation provides improved accuracy in certain populations. 1

Critical Action Thresholds

Nephrology Referral Criteria

Refer to nephrology when any of the following occur:

• eGFR <30 mL/min/1.73m² 2, 4
• UACR ≥300 mg/g persistently 2
• Rapidly declining eGFR (>5 mL/min/1.73m² per year or >50% increase in creatinine from baseline) 2, 4
• Serum creatinine >2.5 mg/dL (>250 µmol/L) 4

Treatment Initiation Based on UACR

• Start ACE inhibitor or ARB therapy when UACR ≥30 mg/g, regardless of blood pressure control, to reduce progressive kidney disease risk. 2
• Treatment goal is to reduce UACR by at least 30-50% and ideally achieve <30 mg/g. 2

Important Caveats

Diagnostic Requirements

• A single abnormal test result is insufficient for CKD diagnosis—persistence of abnormalities for >3 months is required. 1
• Both eGFR and UACR are required together to properly stage kidney disease; neither alone is sufficient. 2

Test Limitations

• Certain medications and substances can interfere with creatinine measurements, affecting eGFR accuracy. 1
• Urine albumin measurements are not yet fully standardized across all clinical laboratories, though progress is being made. 3
• At very high proteinuria levels (spot urine protein-to-creatinine ratio 500-1,000 mg/g), measurement of total protein instead of albumin is acceptable. 3

Collection Protocols

• First-morning spot collections are best for children and adolescents to avoid confounding effects of orthostatic proteinuria. 3
• Refrigerate urine samples for assay the same or next day; one freeze is acceptable if necessary, but avoid repeated freeze-thaw cycles. 3