What is the best initial test for a male in his mid-30s presenting with flank pain and low-grade fever?

Best Initial Test for Male in Mid-30s with Flank Pain and Low-Grade Fever

Non-contrast CT of the abdomen and pelvis is the best initial test, as it serves as the gold standard with 98-100% sensitivity and specificity for detecting urinary stones while also identifying alternative diagnoses including infectious complications like pyelonephritis or perinephric abscess that are suggested by the presence of fever. 1, 2

Why Non-Contrast CT is the Optimal Choice

The combination of flank pain with low-grade fever creates a diagnostic challenge that requires imaging capable of detecting both obstructive urolithiasis and infectious/inflammatory complications:

• Non-contrast CT abdomen/pelvis achieves 98-100% sensitivity and specificity for urinary stones of any size, location, or composition 1, 2, 3
• This imaging modality identifies alternative diagnoses in approximately one-third of patients presenting with flank pain 2
• The presence of fever shifts the differential diagnosis toward infectious etiologies such as pyelonephritis or perinephric abscess, which can be visualized on non-contrast CT 3, 4

Critical Clinical Context: The Fever Changes Everything

The low-grade fever is a crucial distinguishing feature that elevates diagnostic urgency:

• Fever is nearly universal in acute pyelonephritis, and flank pain is present in almost all cases 4
• Pain that worsens significantly with external flank pressure suggests infection (pyelonephritis or perinephric abscess) rather than uncomplicated stone disease 1, 3
• The combination of fever, flank pain, and elevated white blood cell count significantly increases the odds of urinary tract infection, even in the presence of stones 5

Important Nuance: When to Consider Contrast-Enhanced CT

While non-contrast CT is the initial test of choice, there are specific scenarios where contrast may be needed:

• If infectious etiology is strongly suspected based on clinical presentation, contrast-enhanced CT may be more appropriate to fully evaluate for pyelonephritis or abscess 3
• However, contrast-enhanced CT may obscure stones within the renal collecting system if stone disease is the primary concern 6, 3
• In patients with negative history of renal stones and negative urinalysis, contrast-enhanced CT is indicated as 15% will have a non-renal stone diagnosis 6

Algorithmic Approach to This Clinical Scenario

Step 1: Obtain urinalysis and complete blood count immediately

• Pyuria (>10 WBCs per HPF), fever >37.9°C, and leukocytosis >11,300 significantly increase the risk of positive urine culture 5
• However, absence of hematuria cannot exclude ureterolithiasis—11% of patients with confirmed stones have no RBCs on urinalysis 7

Step 2: Order non-contrast CT abdomen/pelvis as the initial imaging study

• This provides comprehensive evaluation for stones, hydronephrosis, and signs of infection 1, 2
• Low-dose CT protocols are non-inferior to standard-dose for detecting ureteral stones with 92-99% sensitivity 6

Step 3: Obtain urine culture before initiating antibiotics

• Urine culture should be obtained in all patients to guide antibiotic therapy if the patient does not respond to initial empiric regimens 4

Step 4: Consider contrast-enhanced imaging if:

• No improvement in symptoms after initial management 4
• Strong clinical suspicion for complicated infection (high fever, severe systemic symptoms, hemodynamic instability) 3
• Initial non-contrast CT is inconclusive 6

Common Pitfalls to Avoid

• Do not rely on hematuria to rule in or rule out ureterolithiasis—33% of patients with confirmed stones have ≤5 RBCs per HPF, and 11% have none 7
• Do not assume all flank pain with fever is pyelonephritis—other serious conditions include renal infarction (which presents with flank pain, fever, elevated LDH, and elevated WBC count) 8
• Do not delay imaging in the presence of fever—the average time to diagnosis of partial renal infarction was 65.2 hours, resulting in significant morbidity 8
• Do not miss alternative diagnoses such as right colonic diverticulitis, inflammatory bowel disease, or Page kidney (subcapsular hematoma causing secondary hypertension) 2, 9

Red Flags Requiring Urgent Evaluation

• Hemodynamic instability or shock requires immediate intervention 2
• Inability to urinate or decreased urine output mandates urgent evaluation 2
• Failure of analgesia after 1 hour requires immediate hospital admission 2
• Signs of systemic infection (high fever, chills, rigors) suggest complicated pyelonephritis or sepsis 2