What is the initial workup for a patient suspected of having pyelonephritis?

Workup for Pyelonephritis

In patients with suspected pyelonephritis, obtain urinalysis with microscopy and urine culture with susceptibility testing before initiating antibiotics—imaging is not routinely indicated for uncomplicated cases. 1

Clinical Diagnosis

The diagnosis of pyelonephritis is primarily clinical, supported by laboratory confirmation rather than imaging in most cases. 1

Key Clinical Features to Assess

• Flank pain or costovertebral angle tenderness is nearly universal; its absence should raise suspicion of an alternative diagnosis 2
• Fever is present in most patients, though it may be absent early in the illness 2
• Voiding symptoms (dysuria, urgency, frequency) are common but absent in up to 20% of patients 1
• Systemic symptoms including chills and fatigue 1

Risk Factors Requiring Documentation

• Prior history of pyelonephritis, diabetes mellitus, anatomic urinary tract abnormalities, vesicoureteral reflux, pregnancy, renal calculi, immunosuppression, or nosocomial acquisition 1, 3
• These factors identify high-risk patients who may require earlier imaging 3

Essential Laboratory Workup

Urinalysis (Mandatory)

• Pyuria and/or bacteriuria on urinalysis with microscopy showing >5 WBC/μL has 90-96% sensitivity 4
• Leukocyte esterase has 72-97% sensitivity for UTI 5
• Nitrite has high specificity (92-100%) but lower sensitivity (19-48%) 5
• Obtain urinalysis before antibiotic administration 4

Urine Culture (Mandatory)

• Always obtain urine culture with antimicrobial susceptibility testing before initiating antibiotics 1, 4
• Cultures yielding >10,000 colony-forming units per milliliter of a uropathogen is the fundamental confirmatory diagnostic test 1
• Urine cultures are positive in 90% of patients with acute pyelonephritis 6

Blood Cultures (Selective)

• Reserve for patients with uncertain diagnosis, immunocompromised status, suspected hematogenous infection, or sepsis 6
• May assist with diagnosis when positive 1

Imaging: When and What to Order

Uncomplicated Pyelonephritis

Imaging is NOT routinely indicated for initial evaluation of uncomplicated pyelonephritis, as approximately 95% of patients become afebrile within 48 hours and nearly 100% within 72 hours of appropriate antibiotic therapy. 3, 4

Indications for Imaging

Obtain imaging in the following scenarios:

Treatment Failure

• Persistent fever or lack of clinical improvement after 72 hours of appropriate antibiotic therapy 3, 4
• Recurrence of symptoms after initial improvement 3

High-Risk Patient Populations (Consider Earlier Imaging)

• Diabetes mellitus (50% may not present with typical flank tenderness) 3, 5
• Anatomic urinary tract abnormalities 3
• Vesicoureteral reflux 3
• Pregnancy 3
• History of urolithiasis or renal obstruction 3, 4
• Nosocomial infections 3
• Treatment-resistant pathogens 3
• Transplant recipients 3
• Prior renal surgery 1

Suspected Complications

• Renal or perinephric abscess 3
• Emphysematous pyelonephritis (especially in diabetic patients) 3
• Pyonephrosis 3
• Obstruction 7

Choice of Imaging Modality

Contrast-Enhanced CT (Preferred)

Contrast-enhanced CT is the imaging study of choice when imaging is indicated, with 90-92% accuracy for detecting parenchymal changes. 3, 5

• Superior to ultrasound for detecting parenchymal abnormalities (84.4% vs 40% detection rate) 3, 5
• Better demonstrates complications such as renal or perinephric abscesses 3
• Superior for detecting calculi and gas in emphysematous pyelonephritis 3
• Recommended protocol includes nephrographic phase (90-100 seconds post-contrast) 3

Ultrasound (Limited Role)

• Limited ability to detect parenchymal abnormalities in acute pyelonephritis 3
• Preferred in pregnancy to avoid radiation exposure 3
• Useful to rule out urinary tract obstruction or renal stones in patients with history of urolithiasis 3

MRI (Alternative When Contrast Contraindicated)

• Useful when iodinated contrast cannot be administered 3
• Dynamic post-contrast sequences and diffusion-weighted imaging are helpful 3
• Poor accuracy for detecting small ureteral calculi and reduced ability to detect gas 3

Common Pitfalls to Avoid

• Do not delay obtaining urine culture before starting antibiotics—this is mandatory for guiding therapy if initial empiric treatment fails 1
• Do not routinely image uncomplicated cases—this wastes resources and exposes patients to unnecessary radiation 3
• Do not rely solely on ultrasound when imaging is indicated—it misses parenchymal abnormalities that CT detects 3
• Do not wait beyond 72 hours to image patients who fail to respond to appropriate therapy 3, 4
• Do not overlook high-risk features (diabetes, anatomic abnormalities, immunosuppression) that warrant lower threshold for imaging 3, 5