How to differentiate between kidney stone and Urinary Tract Infection (UTI) in an asymptomatic patient with leukocyturia (leucocytes), hematuria (erythrocytes), and moderate mixed bacterial growth on urine culture?

From the Guidelines

To determine if an asymptomatic patient with elevated urinary leukocytes (117) and erythrocytes (1392) and moderate mixed growth on culture has a kidney stone or UTI, a non-contrast CT scan of the abdomen and pelvis is recommended as the initial diagnostic test, given its high sensitivity for detecting kidney stones 1.

Diagnostic Approach

The patient's presentation with elevated urinary leukocytes and erythrocytes, along with moderate mixed growth on culture, suggests the need for further evaluation to differentiate between a kidney stone and a urinary tract infection (UTI).

• The high number of erythrocytes (1392) is concerning for a kidney stone or hemorrhagic cystitis, as these conditions can cause significant bleeding into the urinary tract 1.
• The elevated leukocytes (117) indicate inflammation, which could be due to either a kidney stone or a UTI, as both conditions can cause an inflammatory response in the urinary tract 2.

Considerations for UTI

• The presence of moderate mixed growth on culture may represent contamination, and a repeat clean-catch midstream urine culture should be collected to confirm the presence of a significant infection 2.
• Empiric antibiotic therapy with nitrofurantoin 100mg twice daily for 5 days or trimethoprim-sulfamethoxazole 160/800mg twice daily for 3 days could be considered if UTI is strongly suspected, based on the patient's clinical presentation and laboratory results 2.

Additional Diagnostic Clues

• Urine pH and crystal analysis from the urinalysis may provide additional clues to differentiate between a kidney stone and a UTI, with acidic urine and calcium oxalate crystals suggesting stones, and alkaline urine indicating infection with urease-producing organisms 1.
• The absence of symptoms is unusual for both conditions but more common with small kidney stones or mild UTIs, highlighting the importance of further diagnostic testing to determine the underlying cause of the patient's laboratory abnormalities 1.

From the Research

Determining Kidney Stone or UTI in Asymptomatic Patients

To determine if a patient has a kidney stone or a urinary tract infection (UTI) when they are asymptomatic, several factors must be considered:

• The presence of leucocytes (117) and erythrocytes (1392) in the urinalysis, which may indicate an infection or inflammation in the urinary tract 3.
• The moderate mixed growth on culture, which suggests the presence of bacteria in the urine, but may not necessarily indicate a UTI in asymptomatic patients 3, 4.
• The patient's symptoms, or lack thereof, as asymptomatic bacteriuria is common, particularly in older women, and should not be treated with antibiotics 3, 4.

Diagnostic Considerations

When diagnosing a UTI, the following should be considered:

• Urine culture is the gold standard for detection of UTI, but asymptomatic bacteriuria is common and should not be treated with antibiotics 3, 4.
• Dipstick urinalysis is popular for its availability and usefulness, but results must be interpreted in context of the patient's pretest probability based on symptoms and characteristics 3.
• Microscopic urinalysis is likely comparable to dipstick urinalysis as a screening test, but bacteriuria is more specific and sensitive than pyuria for detecting UTI 3.

Treatment Considerations

When treating a UTI, the following should be considered:

• First-line treatments for UTI include nitrofurantoin, fosfomycin, and trimethoprim-sulfamethoxazole (when resistance levels are <20%) 3, 4, 5.
• Antibiotic prophylaxis is also effective in preventing recurrence, but has a risk of adverse effects and antimicrobial resistance 4, 5.
• Increased fluids, intake of cranberry products, and methenamine hippurate can prevent recurrent infections 5.

Kidney Stone Considerations

To determine if a patient has a kidney stone, the following should be considered:

• The presence of erythrocytes (1392) in the urinalysis, which may indicate a kidney stone or other urinary tract abnormality.
• Imaging studies, such as a CT scan or ultrasound, may be necessary to confirm the presence of a kidney stone.
• The patient's symptoms, or lack thereof, as some kidney stones may be asymptomatic.