What are the initial tests and interventions for assessing and managing impaired renal function?

Initial Assessment of Renal Function

Use serum creatinine with an estimating equation (eGFRcr) as the initial test for assessing renal function, followed by urinalysis for proteinuria and bacteriuria. 1

Primary Laboratory Tests

Serum Creatinine and eGFR

• Serum creatinine measurement with calculated estimated glomerular filtration rate (eGFR) using the Cockcroft-Gault method is the cornerstone initial test 1
• The CKD-EPI equation is recommended by the National Kidney Foundation for general GFR estimation across CKD stages, though Cockcroft-Gault remains preferred for medication dosing decisions 1
• If eGFRcr is expected to be inaccurate (e.g., extremes of muscle mass, certain medications), measure cystatin C and calculate eGFRcr-cys for improved accuracy 1
• Baseline serum creatinine establishes future reference points for monitoring progression 1

Urinalysis

• Perform urinalysis with microscopic examination to detect proteinuria, bacteriuria, hematuria, and cellular casts 1, 2
• Quantify proteinuria using spot urine protein-to-creatinine ratio (PCR) or albumin-to-creatinine ratio (ACR) rather than relying solely on dipstick 3
• Examine urine sediment for red cell casts, dysmorphic red blood cells, white blood cells, and epithelial cells 1, 2

Critical caveat: Urinalysis dipstick for proteinuria has reduced reliability when confounding factors are present, including specific gravity ≥1.020, ≥3+ blood, ≥3+ leukocyte esterase, ketonuria, or significant hematuria—these require confirmatory ACR testing 4

Additional Essential Tests

Blood Work Panel

• Complete blood count (CBC) to evaluate for anemia, infection, or other hematologic abnormalities 2
• Electrolytes including sodium, potassium, calcium, magnesium, chloride, and bicarbonate to assess acid-base balance 2
• Blood urea nitrogen (BUN) provides complementary information about renal function and hydration status 5, 6

Inflammatory Markers (When Infection Suspected)

• C-reactive protein (CRP) may be significantly elevated in kidney infections 2
• Blood cultures should be obtained before antibiotic therapy when kidney infection is suspected 2
• White blood cell count >11 × 10^9/L combined with elevated CRP is highly suggestive of kidney infection with clinical symptoms 2

Imaging Studies

Renal Ultrasound

• Renal ultrasound is recommended as first-line imaging to assess kidney size, symmetry, cortical thickness, and detect obstruction or hydronephrosis 1
• Duplex Doppler ultrasound (DUS) is the preferred modality for suspected renal artery stenosis, with peak systolic velocity ≥200 cm/s indicating >50% stenosis 1
• Ultrasound has minimal impact on diagnosis in patients with chronic kidney disease from diabetes or hypertension alone 1

Advanced Imaging (Selective Use)

• Unenhanced CT abdomen/pelvis is most sensitive for urinary calculi and retroperitoneal pathology when ultrasound is nondiagnostic 1
• DMSA renal imaging can assess cortical scarring and differential function, particularly in grades III-V vesicoureteral reflux 1
• Avoid contrast-enhanced studies (CT, MRA) in acute or severe renal impairment due to nephrotoxicity risk 1

Monitoring Frequency

Stable Chronic Kidney Disease

• Assess GFR and albuminuria at least annually in patients with established CKD 3
• For impaired renal function (CrCl <60 mL/min), evaluate more frequently—divide CrCl by 10 to obtain minimum testing frequency in months (e.g., CrCl 30 mL/min = test every 3 months) 1

High-Risk Situations Requiring Immediate Re-evaluation

• Intercurrent acute illness (infections, acute heart failure) may transiently affect renal function 1
• Rapidly progressive or treatment-resistant hypertension 1
• Rapidly declining renal function or flash pulmonary edema 1

Special Considerations

Acute vs. Chronic Kidney Disease Distinction

• In acute renal failure, serum creatinine may falsely indicate mild or normal function when severe impairment exists—CKD can only be diagnosed in stable situations 1
• Serial creatinine measurements detect the rapid deterioration characteristic of acute kidney injury 1

When to Refer to Nephrology

• Significant proteinuria (>1 g/day or >0.5 g/day if persistent/increasing) 3
• Presence of red cell casts or dysmorphic red blood cells 3
• Bilateral renal cortical abnormalities on imaging 1
• Unexplained renal insufficiency without clear etiology 1

Kidney Biopsy Indications

• Consider when proteinuria predominantly consists of light chains with high serum free light chain levels but unclear etiology 1
• Perform when patients lack complete explanation for renal insufficiency to assess for monoclonal immunoglobulin deposition disease or glomerulonephritis 1