What are the next steps in managing a patient with impaired renal function, indicated by elevated creatinine (2.3) and blood urea nitrogen (BUN) (37), and potential underlying conditions such as pre-existing kidney disease, diabetes, hypertension, or heart failure?

Management of Elevated Creatinine and BUN

Your immediate priority is to calculate the estimated GFR using the MDRD equation (accounting for age, sex, and race) rather than relying on serum creatinine alone, and determine the BUN/creatinine ratio to distinguish prerenal azotemia from intrinsic kidney disease. 1

Initial Assessment and Risk Stratification

Calculate Key Parameters

• Estimated GFR: With creatinine 2.3 mg/dL, this patient likely has an eGFR <30 mL/min/1.73 m², indicating chronic kidney disease stage 4, which requires immediate nephrology referral 2
• BUN/Creatinine Ratio: 37/2.3 = 16.1, which is below the traditional threshold of 20:1, suggesting this may represent intrinsic kidney disease rather than simple prerenal azotemia 1, 3
• Serum creatinine >2.0 mg/dL is an independent risk factor for cardiac complications and increased long-term morbidity and mortality 2

Determine Acuity: Acute vs Chronic Kidney Disease

Obtain prior creatinine values immediately to determine if this represents acute kidney injury or chronic kidney disease, as management differs substantially 4. Look specifically for:

• Acute kidney injury indicators: Rapid rise in creatinine (>1.5 times baseline or >50% drop in eGFR), especially with rising NT-proBNP or elevated lactate levels 2
• Chronic kidney disease indicators: Small kidney size on ultrasound (<9 cm), anemia, hyperphosphatemia, hypocalcemia, or elevated parathyroid hormone 2

Diagnostic Workup

Essential Immediate Tests

Order the following tests within 24 hours 1:

• Urinalysis with microscopy: Look for proteinuria, hematuria, or abnormal sediment (casts, dysmorphic RBCs) that would indicate intrinsic kidney disease requiring renal biopsy 2, 4
• Spot urine albumin-to-creatinine ratio: Values >30 mg/g indicate significant kidney damage 2
• Serum electrolytes: Check potassium (risk increases when creatinine >1.6 mg/dL), sodium, chloride, bicarbonate 2, 5
• Renal ultrasound: Rule out obstruction and assess kidney size to distinguish acute from chronic disease 4

Assess for Prerenal Causes

Evaluate hydration status and cardiac function immediately 1:

• Volume depletion signs: Orthostatic hypotension, decreased skin turgor, dry mucous membranes, reduced urine output 1
• Heart failure assessment: Elevated jugular venous pressure, peripheral edema, pulmonary congestion, reduced cardiac output 2
• Maintain transkidney perfusion pressure (mean arterial pressure minus central venous pressure) >60 mm Hg 2

Medication Review

Identify and address nephrotoxic medications 1:

• ACE inhibitors/ARBs: Creatinine increases up to 30% or <3.0 mg/dL are acceptable and do not require discontinuation, as these represent hemodynamic changes rather than true kidney injury 2, 5
• Diuretics: Assess for excessive diuresis causing prerenal azotemia 3
• NSAIDs: Discontinue immediately if present 5
• Contrast agents: Avoid unless absolutely necessary 2

Management Based on Underlying Etiology

If Prerenal Azotemia (Volume Depletion)

Provide adequate fluid resuscitation and recheck BUN and creatinine after 2 days 3:

• Expected response: Creatinine should decrease toward baseline with adequate hydration 1
• If elevation persists after 2 days of adequate rehydration, consider intrinsic kidney disease and proceed with further workup 3

If Heart Failure-Related

Note that elevated BUN/creatinine ratio in heart failure identifies a particularly high-risk phenotype 6, 7:

• In patients with elevated BUN/Cr ratio, renal dysfunction (eGFR <45) is strongly associated with mortality (hazard ratio 2.2) 7
• Initiate or optimize guideline-directed medical therapy including ACE inhibitors/ARBs, beta-blockers, and mineralocorticoid receptor antagonists, as these improve survival despite potential transient creatinine increases 2
• Bortezomib-containing regimens can be administered without dose adjustment in patients with severe renal impairment if multiple myeloma is present 2

If Intrinsic Kidney Disease Suspected

Pursue further workup if 3:

• Elevation persists after 2 days of adequate rehydration
• Proteinuria, hematuria, or abnormal urinary sediment present
• eGFR <30 mL/min/1.73 m²
• Rapidly progressive kidney disease (creatinine rising >0.5 mg/dL per week)

Consider renal biopsy if proteinuria predominantly consists of light chains with high serum free light chain levels, or if there is no clear explanation for renal insufficiency 2

Medication Management in Renal Impairment

ACE Inhibitors/ARBs

Do not discontinue ACE inhibitors/ARBs unless 2, 5:

• Serum creatinine rises >30% over baseline during the first 2 months after initiation
• Serum creatinine exceeds 3.0 mg/dL
• Hyperkalemia develops (serum potassium ≥5.6 mmol/L)

In patients with preexisting chronic renal insufficiency, an early rise in serum creatinine of approximately 25% above baseline (to ~1.7 mg/dL) is expected and associated with long-term renoprotection 5

Diuretics

Continue diuretics in congested patients despite rising creatinine, as de-escalating diuretic therapy to preserve eGFR can lead to worsening congestion and adverse consequences 2

Other Medications Requiring Adjustment

• Lenalidomide: Requires dose adjustment based on creatinine clearance 2
• Denosumab: Does not require dose adjustment for any level of kidney function, including severe renal impairment or dialysis, making it preferred over bisphosphonates 8

Monitoring and Follow-Up

Short-Term Monitoring

Monitor serum creatinine and electrolytes 1:

• Grade 1 elevation: Weekly monitoring
• Grade 2 elevation: Every 2-3 days
• Check potassium closely if on ACE inhibitors/ARBs, as hyperkalemia risk increases progressively when creatinine exceeds 1.6 mg/dL 5

Long-Term Monitoring

For suspected chronic kidney disease, monitor every 2-3 months initially 1:

• Serum creatinine and estimated GFR
• Electrolytes (sodium, potassium, chloride, bicarbonate)
• Urinalysis for proteinuria progression

Nephrology Referral Criteria

Immediate nephrology referral is indicated if 1, 3:

• eGFR <30 mL/min/1.73 m² (present in this patient with creatinine 2.3)
• Proteinuria, hematuria, or abnormal urinary sediment
• Rapidly progressive kidney disease
• Uncertain etiology of renal impairment after initial workup
• Persistent or recurrent grade 2-3 renal impairment despite treatment 2

Blood Pressure Management

Target blood pressure <140/90 mm Hg (ideally <130/85 mm Hg in patients with renal disease) to slow progression of kidney disease 1

Common Pitfalls to Avoid

• Do not rely on serum creatinine alone without calculating eGFR, especially in elderly patients with low muscle mass who may have advanced renal insufficiency at creatinine levels as low as 2.0 mg/dL 2, 1
• Do not automatically discontinue ACE inhibitors/ARBs for modest creatinine increases, as this removes long-term renoprotection 2, 5
• Do not assume BUN/Cr ratio >20:1 always indicates "simple" prerenal azotemia, as this interpretation is fundamentally flawed in critically ill patients and those with heart failure 3
• Do not withhold diuretics in congested patients to preserve eGFR, as this leads to worsening congestion and adverse outcomes 2