How to differentiate between a urinary tract infection (UTI) and renal calculi in a patient, considering demographics, medical history, and symptoms such as dysuria, frequency, or flank pain?

Differentiating UTI from Renal Calculi

The differentiation between UTI and renal calculi relies primarily on clinical presentation: UTI typically presents with dysuria, frequency, urgency, and suprapubic pain (with fever and flank pain if pyelonephritis), while renal calculi present with acute, severe colicky flank pain radiating to the groin, often with hematuria but without fever unless complicated by infection. 1, 2

Key Clinical Distinguishing Features

UTI Presentation

• Lower tract symptoms: Dysuria (painful urination) is central to diagnosis with >90% accuracy when not accompanied by vaginal symptoms, along with increased frequency and urgency 2
• Upper tract symptoms (pyelonephritis): High fever, malaise, vomiting, flank pain, and costovertebral angle tenderness are almost universal and distinguish pyelonephritis from lower UTI 3
• Systemic signs: Fever is typically present with pyelonephritis, whereas it is absent in uncomplicated cystitis 2, 3
• Pain character: Suprapubic pain or discomfort in lower abdomen, not the acute colicky pain of stones 2

Renal Calculi Presentation

• Pain pattern: Acute, severe colicky flank pain that radiates to the groin is the hallmark presentation 4
• Hematuria: Commonly present with stones, though can also occur with UTI 2
• Absence of fever: Unless the stone is complicated by infection or obstruction 4
• Hydronephrosis: Moderate to severe hydronephrosis on imaging suggests stone with obstruction 4

Critical Diagnostic Considerations

Laboratory Assessment

• Urinalysis findings: Pyuria alone does not differentiate infection from other causes and has low positive predictive value 3, 5
• Nitrites: More sensitive and specific than other dipstick components for UTI, particularly in elderly patients 5
• Bacteriuria: More specific and sensitive than pyuria for detecting UTI 5
• Urine culture: Mandatory for complicated UTIs, recurrent UTIs, and when diagnosis is uncertain 2

Important Clinical Pitfalls

• Coexistence of both conditions: Renal calculi can predispose to UTI, particularly infection stones composed of magnesium ammonium phosphate that develop following urease-producing bacterial infections 4, 6
• Diabetic patients: Up to 50% will not have typical flank tenderness, making differentiation more difficult 4
• Asymptomatic bacteriuria: Positive urine culture without symptoms should not be treated and does not indicate active infection 1, 3
• Pyuria in stone patients: Can occur without infection, particularly in older adults with lower urinary tract symptoms 5

Imaging Strategy for Unclear Cases

When to Image

• Persistent symptoms beyond 72 hours: Consider imaging if UTI symptoms do not respond to appropriate antibiotics within 48-72 hours, as this suggests possible stone or complication 4, 1
• Suspected stone: When clinical presentation suggests renal colic with moderate to severe hydronephrosis 4
• Complicated UTI risk factors: Anatomical abnormalities, obstruction, diabetes, immunosuppression, pregnancy, or indwelling catheters warrant imaging 4, 1

Imaging Modality Selection

• CT urography (CTU): Primary test for comprehensive evaluation when differentiation is needed, as it evaluates for both stones and complications of infection 4
• Unenhanced CT abdomen/pelvis: Excellent for detecting stones and hydronephrosis, but limited for evaluating infection complications 4
• Ultrasound: Useful initial test for detecting hydronephrosis and can identify moderate to severe cases that suggest stone, though less sensitive than CT 4
• Contrast-enhanced CT: Better for evaluating pyelonephritis, renal abscesses, and infection complications 4

Algorithmic Approach

Step 1: Assess Pain Character

• Colicky, radiating flank-to-groin pain → Suspect stone, proceed to unenhanced CT if diagnosis needed for management 4
• Suprapubic pain with dysuria/frequency → Suspect lower UTI, treat empirically if uncomplicated 2, 3
• Fever with flank pain/CVA tenderness → Suspect pyelonephritis, obtain urine culture and treat 3

Step 2: Evaluate for Fever

• Fever present with flank pain → Pyelonephritis until proven otherwise; if no response in 48-72 hours, image to exclude stone with obstruction or abscess 4, 1
• No fever with severe flank pain → Stone more likely; consider imaging if management will change 4

Step 3: Risk Stratify

• Complicated UTI risk factors present (diabetes, obstruction, catheter, immunosuppression) → Lower threshold for imaging with CTU 4, 1
• Uncomplicated presentation → Treat empirically and image only if no response to therapy 4, 3

Step 4: Consider Coexistence

• Stone with infection: Presence of renal calculi increases risk of renal involvement with UTI through obstruction and urinary stasis 4, 6
• Infection stones: Recurrent UTIs with urease-producing organisms (Proteus) can cause staghorn calculi; alkaline urine and branched/staghorn appearance on imaging are typical 6

Special Populations

Elderly Patients

• May present with atypical symptoms including mental status changes, functional decline, or behavioral changes rather than classic UTI symptoms 2
• Baseline incontinence and cognitive changes should not be attributed to UTI without other supporting evidence 1

Diabetic Patients

• Higher risk for complications including renal abscesses and emphysematous pyelonephritis 4
• May lack typical flank tenderness in up to 50% of cases 4
• Lower threshold for imaging if symptoms persist 4