What are the steps to diagnose renal failure?

How to Diagnose Renal Failure

Begin by measuring serum creatinine with calculated eGFR using the 2009 CKD-EPI equation, obtain urinalysis with microscopy, and perform renal ultrasound to categorize the failure as prerenal, intrinsic renal, or postrenal. 1, 2

Initial Laboratory Assessment

Essential Blood Tests

• Serum creatinine is the primary marker and must be measured using an assay with calibration traceable to international standard reference materials 2
• Calculate eGFR using the 2009 CKD-EPI equation rather than relying on creatinine alone—this provides standardized assessment of kidney function 2, 3
• Complete blood count to evaluate for anemia (common in chronic kidney disease) and infection 2, 3
• Blood urea nitrogen (BUN) with calculation of BUN-to-creatinine ratio: >20:1 suggests prerenal cause, <10:1 suggests intrinsic renal disease 3
• Complete electrolyte panel including sodium, potassium, calcium, chloride, phosphorus, and magnesium to identify life-threatening imbalances 2, 3

Critical Urine Tests

• Urinalysis with microscopy is essential to detect cells, casts, and crystals—this differentiates between various causes of renal failure 1, 2, 3
• Urine albumin-to-creatinine ratio (ACR) from an untimed spot urine sample is the preferred method for assessing proteinuria 2, 3
• Fractional excretion of sodium (FENa) distinguishes prerenal (<1%) from intrinsic renal (>1%) causes 2, 3

Imaging Evaluation

Renal ultrasound is the first-line imaging modality to evaluate kidney size, echogenicity, and rule out obstruction 1, 2, 3. This should be performed in most patients, particularly older men where obstruction is more common 1.

If ultrasound detects hydronephrosis, unenhanced CT of the abdomen and pelvis should be obtained to determine the level and cause of obstruction 1, 2.

Diagnostic Classification Framework

Distinguishing Acute from Chronic Kidney Disease

Acute Kidney Injury (AKI) is defined as:

• Increase in creatinine by ≥0.3 mg/dL within 48 hours, OR 1
• Increase in serum creatinine to ≥1.5 times baseline (within prior 7 days), OR 1
• Urine volume <0.5 mL/kg/hr for 6 hours 1

Chronic Kidney Disease (CKD) requires:

• Abnormality of kidney structure or function present for >3 months 1, 2, 3
• A single abnormal test result is insufficient—persistence of abnormalities for >3 months is mandatory for CKD diagnosis 2, 3

Categorizing by Etiology

Prerenal causes (impaired blood flow):

• FENa <1% 2, 3
• BUN/creatinine ratio >20:1 3
• Causes include hypotension, hypovolemia, decreased cardiac output, or renal artery occlusion 1

Intrinsic renal causes (parenchymal damage):

• FENa >1% 2, 3
• Urinalysis shows casts, cells, or proteinuria 1, 2
• Causes include vasculitis, acute tubular necrosis, glomerulonephritis, interstitial nephritis, infection, drugs, and toxins 1

Postrenal causes (obstruction):

• Hydronephrosis on ultrasound 1, 2
• Results from ureteral, bladder, or urethral obstruction 1
• Important caveat: Obstruction accounts for <3% of AKI cases 1

Severity Staging

CKD Stages Based on eGFR:

• Stage 1 (G1): ≥90 mL/min/1.73m² (normal or high) 2
• Stage 2 (G2): 60-89 mL/min/1.73m² (mildly decreased) 2
• Stage 3a (G3a): 45-59 mL/min/1.73m² (mildly to moderately decreased) 2
• Stage 3b (G3b): 30-44 mL/min/1.73m² (moderately to severely decreased) 2
• Stage 4 (G4): 15-29 mL/min/1.73m² (severely decreased) 2
• Stage 5 (G5): <15 mL/min/1.73m² (kidney failure) 1, 2

Albuminuria Categories:

• A1: <30 mg/g (normal to mildly increased) 2
• A2: 30-300 mg/g (moderately increased) 2
• A3: >300 mg/g (severely increased) 2

When to Perform Additional Testing

Cystatin C should be measured when eGFR based on creatinine may be inaccurate (extremes of muscle mass, malnutrition, or amputation) 2, 3. The combined creatinine-cystatin C equation provides improved accuracy in these populations 2.

Renal biopsy is indicated when glomerular disease is suspected, particularly with significant proteinuria, red cell casts, or unexplained acute kidney injury requiring differentiation of nephritic and nephrotic syndromes 1, 3.

Critical Pitfalls to Avoid

• Do not use iodinated contrast in acute kidney injury unless there is an overriding clinical question that cannot be answered with alternative imaging 1, 2, 3
• Normal kidney size on imaging does not exclude chronic kidney disease—renal size is initially preserved in diabetic nephropathy and infiltrative disorders 2, 3
• Certain medications interfere with creatinine measurements, affecting eGFR accuracy—review medication list carefully 2
• In patients with AKI superimposed on CKD, interpreting results requires comparison to baseline values 2
• Urine output changes may be physiologic—serum creatinine measurement is more important than urine volume for AKI diagnosis 1

Monitoring Strategy

Frequency of monitoring should be guided by GFR category, albuminuria category, and rate of progression 2. More frequent monitoring is warranted for patients with rapidly declining kidney function or those at higher risk for progression 2.

Serial monitoring of serum electrolytes and renal function is essential, particularly when initiating diuretics or renin-angiotensin-aldosterone system inhibitors 3.