How to treat an elderly patient with left lower quadrant pain, impaired renal function (eGFR 57 mL/min/1.73), and potential electrolyte imbalance (hyperkalemia), with lab results indicating elevated BUN (Blood Urea Nitrogen) and creatinine levels?

Management of Left Lower Quadrant Pain in an Elderly Patient with Mild Renal Impairment

Immediate Diagnostic Priority

Your first step is to obtain a CT scan with IV contrast to definitively diagnose the cause of left lower quadrant pain, as this imaging is essential to distinguish diverticulitis from other serious pathologies and guide appropriate treatment. 1

• In elderly patients presenting to the ED with acute abdominal pain, CT imaging changes the clinical diagnosis in 43% of cases and influences treatment plans in 65% overall 1
• The patient's eGFR of 57 mL/min/1.73m² represents Stage 3a chronic kidney disease, but this should not discourage CT with IV contrast 1
• Recent meta-analyses show that IV contrast for CT does not significantly increase risk of contrast-induced acute kidney injury (CI-AKI) even in patients with chronic kidney disease 1
• The benefit of prompt diagnosis in this frail population justifies the minimal risk of CI-AKI, especially when combined with appropriate hydration and avoidance of nephrotoxic medications 1

Critical Lab Interpretation

Your patient's labs reveal several important findings that guide management:

• BUN 30 mg/dL (elevated) with BUN/creatinine ratio of 24 suggests possible volume depletion or prerenal azotemia, though the ratio is at the upper limit of normal 1
• eGFR 57 mL/min/1.73m² indicates Stage 3a CKD, requiring medication dose adjustments 1
• Normal potassium (4.5 mmol/L) is reassuring and does not contraindicate ACE inhibitors or other renally-adjusted medications 1
• CRP <1 mg/L suggests absence of significant systemic inflammation, making severe complicated diverticulitis or abscess less likely 1
• All other labs are within normal limits, including no leukocytosis

Most Likely Diagnosis and Treatment Algorithm

If CT Confirms Uncomplicated Diverticulitis:

Initiate oral antibiotics covering gram-negative and anaerobic organisms, with doses adjusted for renal function:

• First-line option: Ciprofloxacin 500 mg PO every 12 hours PLUS metronidazole 500 mg PO every 8 hours for 7-10 days 1, 2

  • Ciprofloxacin requires no dose adjustment at eGFR >30 mL/min/1.73m² 2
  • However, use caution in elderly patients due to increased risk of tendon rupture, CNS effects, and QT prolongation 2

• Alternative option: Amoxicillin-clavulanate 875/125 mg PO every 12 hours for 7-10 days 1

  • Requires dose adjustment: reduce frequency to every 12-24 hours if eGFR 10-30 mL/min/1.73m² 3
  • At eGFR 57, standard dosing is appropriate

If CT Shows Complicated Diverticulitis (abscess, perforation, obstruction):

Admit for IV antibiotics and surgical consultation:

• Ertapenem 1 g IV daily is an excellent choice for elderly patients with renal impairment 3
• No dose adjustment needed at eGFR >30 mL/min/1.73m² 3
• If eGFR were <30 mL/min/1.73m², reduce to 500 mg IV daily 3

Critical Medication Adjustments for Renal Function

Calculate creatinine clearance using Cockcroft-Gault equation rather than relying on eGFR alone for medication dosing decisions 1, 4:

For this patient (assuming age ~70-80 years, weight ~70 kg):

• CrCl ≈ 50-60 mL/min (similar to eGFR)

Key Medication Cautions:

• Avoid NSAIDs - they can worsen renal function and cause diuretic resistance 1, 5
• Avoid nitrofurantoin if considering UTI as differential - contraindicated at CrCl <60 mL/min due to inadequate urinary concentrations and toxicity risk 6
• Avoid aminoglycosides - high nephrotoxicity risk in elderly with baseline renal impairment 1

Hydration and Supportive Care

Optimize hydration status immediately:

• The elevated BUN with borderline BUN/creatinine ratio suggests possible volume depletion 1
• Administer IV normal saline 500-1000 mL bolus, then reassess volume status 1, 6
• This hydration also minimizes any theoretical CI-AKI risk from contrast 1

Monitoring Plan

Recheck renal function and electrolytes in 48-72 hours after initiating treatment 4, 6:

• Monitor for worsening renal function (creatinine, BUN, eGFR)
• Monitor potassium if any ACE inhibitors or potassium-sparing agents are added 1
• Daily weights to assess fluid status if admitted 1

Alternative Diagnoses to Consider

While diverticulitis is most likely in an elderly patient with LLQ pain 7, the CT will also evaluate for:

• Renal pathology (infarct, subcapsular hematoma, nephrolithiasis) - can present with LLQ pain and elevated BUN 8, 9
• Colonic malignancy with perforation 7
• Epiploic appendicitis - benign, self-limited 7
• Left-sided appendicitis (rare, associated with situs inversus or malrotation) 10

Key Pitfalls to Avoid

• Do not rely on serum creatinine alone - it underestimates renal dysfunction in elderly patients with low muscle mass 1, 11
• Do not withhold contrast CT due to mild-moderate renal impairment - the diagnostic benefit far outweighs minimal risk 1
• Do not use fluoroquinolones as first-line in elderly if other options exist, due to increased adverse effects 6, 2
• Do not treat asymptomatic bacteriuria if urine culture is obtained - common in elderly but does not require treatment 6