What broad-spectrum antibiotic should be started for an adult patient with a urinary tract infection suggested by urinalysis (UA) results showing leukocytes and red blood cell (RBC) casts?

Hold on—This Isn't a Simple UTI

The presence of RBC casts on urinalysis indicates glomerular bleeding and suggests acute glomerulonephritis or other renal parenchymal disease, NOT a urinary tract infection requiring antibiotics. You should not start empiric antibiotics based on these findings alone.

Why RBC Casts Change Everything

RBC casts are virtually pathognomonic for glomerular bleeding and indicate renal parenchymal disease, not infection. 1 The combination of leukocytes and RBC casts points toward:

• Acute glomerulonephritis (post-infectious, IgA nephropathy, lupus nephritis)
• Vasculitis affecting the kidneys
• Acute interstitial nephritis
• Other intrinsic renal diseases

These conditions require completely different evaluation and management than UTI. 1

Critical Diagnostic Steps Required NOW

Before considering any antibiotics, you must:

1. Assess for systemic symptoms of glomerulonephritis:

   • Hypertension, edema, oliguria, or signs of fluid overload 1
   • Fever with rigors suggesting pyelonephritis vs systemic inflammatory process 1
   • Recent pharyngitis or skin infection (post-streptococcal GN) 1

2. Obtain immediate laboratory workup:

   • Serum creatinine and BUN to assess renal function 1
   • 24-hour urine protein or spot protein-to-creatinine ratio (>1,000 mg/24hr suggests glomerular disease) 1
   • Complement levels (C3, C4), ANA, anti-dsDNA, ANCA if vasculitis suspected 1
   • ASO titers if post-streptococcal GN suspected 1

3. Examine RBC morphology under phase-contrast microscopy:

   • 80% dysmorphic RBCs confirms glomerular origin 1

   • Normal "doughnut-shaped" RBCs suggest lower tract bleeding 1

When to Consider Antibiotics (Rarely in This Scenario)

Only start empiric antibiotics if the patient has BOTH:

• Clear systemic signs of pyelonephritis: fever >38.3°C, costovertebral angle tenderness, rigors, or hemodynamic instability 1
• High clinical suspicion that infection is driving the presentation (not just glomerulonephritis with secondary pyuria) 1

If Antibiotics Are Truly Indicated (Suspected Pyelonephritis):

For outpatient oral therapy (mild symptoms, stable patient): 1

• Ciprofloxacin 500-750 mg PO twice daily for 7 days (if local fluoroquinolone resistance <10%) 1
• Levofloxacin 750 mg PO once daily for 5 days (preferred for once-daily dosing) 1
• Ceftibuten 400 mg PO once daily for 10 days OR Cefpodoxime 200 mg PO twice daily for 10 days (give initial IV ceftriaxone 1-2g first) 1

For hospitalized patients requiring IV therapy: 1

• Ceftriaxone 1-2 g IV once daily (most commonly used, excellent tissue penetration) 1
• Ciprofloxacin 400 mg IV twice daily OR Levofloxacin 750 mg IV once daily 1
• Cefepime 1-2 g IV twice daily (broader gram-negative coverage) 1
• Piperacillin-tazobactam 3.375-4.5 g IV every 8 hours (if Pseudomonas risk) 1

Critical Pitfalls to Avoid

Do NOT:

• Assume this is a straightforward UTI just because leukocytes are present—pyuria accompanies many glomerular diseases without infection 1, 2
• Start antibiotics without assessing renal function—nephrotoxic agents could worsen acute kidney injury 1
• Ignore the RBC casts—they are the most important finding and demand nephrology evaluation 1
• Use nitrofurantoin or fosfomycin—these are inappropriate for pyelonephritis and have insufficient data for upper tract infections 1

Immediate Next Steps

Obtain urine culture before any antibiotics (if infection is truly suspected), but simultaneously: 1, 2

1. Check serum creatinine urgently—elevated creatinine with RBC casts = acute glomerulonephritis until proven otherwise 1
2. Quantify proteinuria—>1,000 mg/24hr strongly suggests glomerular disease requiring nephrology referral 1
3. Consult nephrology immediately if creatinine elevated, significant proteinuria present, or RBC casts confirmed 1
4. Perform renal ultrasound to exclude obstruction or structural abnormalities 1

The presence of RBC casts fundamentally changes your diagnostic approach from infectious to nephrologic. 1 Most patients with this presentation will NOT need antibiotics but rather immunosuppressive therapy, supportive care, or treatment of underlying systemic disease. 1