From the Guidelines
Diagnosing an infected renal cyst requires a combination of clinical assessment, laboratory tests, and imaging studies, with 18FDG PET-CT scan being the preferred imaging modality for confirmation, as recommended by the most recent guideline 1. The diagnosis should be suspected in patients with symptoms such as fever, flank pain, and general malaise, especially in those with known renal cysts or risk factors like diabetes or immunosuppression. Some key points to consider in the diagnosis of infected renal cysts include:
- Blood tests including complete blood count showing elevated white blood cells and C-reactive protein or erythrocyte sedimentation rate indicating inflammation are important initial steps 1.
- Urinalysis and urine culture should be performed, though they may be negative if the infected cyst doesn't communicate with the collecting system 1.
- Blood cultures are recommended as they may identify the causative organism 1.
- Imaging is crucial, with 18FDG PET-CT scan being the preferred modality for confirmation of infected renal cysts, especially in patients with autosomal dominant polycystic kidney disease (ADPKD) 1.
- In some cases, diagnostic aspiration of the cyst under imaging guidance may be necessary to confirm infection and identify the pathogen, especially when empiric antibiotic therapy fails 1. The use of 18FDG PET-CT scan has been shown to have high sensitivity and specificity for diagnosing infected renal cysts, with a meta-analysis showing a sensitivity of 92% and specificity of 76% 1. It is also important to note that the treatment of infected renal cysts often requires long-term use of antibiotics, and precise localization of the infected cyst may be necessary for refractory infection when cyst drainage may be required 1.
From the Research
Diagnostic Criteria for Infected Renal Cyst
To diagnose an infected renal cyst, several criteria can be considered, including:
- Clinical presentation: patients with infected renal cysts often have variable presenting symptoms, but almost universally have flank pain, fever, and elevated serum inflammatory markers 2
- Imaging modalities: computed tomography (CT) scans can show changes concerning for possible early pyelonephritis and an enlarged renal cyst compared to previous imaging 3
- Laboratory evaluation: urine culture, blood culture, and cyst aspirate can help confirm the diagnosis of an infected renal cyst 3, 2
- Microbiological findings: the presence of bacteria such as Escherichia coli in the urine or cyst aspirate can indicate an infected renal cyst 3
Diagnostic Challenges
Diagnosing an infected renal cyst can be challenging due to:
- Variable presenting symptoms: patients may not always have typical symptoms such as flank pain 3
- Imaging modalities can be misleading: CT scans may not always show clear signs of infection 3
- Laboratory evaluation may not always be conclusive: urine and blood cultures may remain negative even in definite cases of infected renal cysts 2
Diagnostic Tools
Several diagnostic tools can be used to diagnose an infected renal cyst, including:
- Cyst aspirate: considered the gold standard in diagnosing cyst infection 2
- (18)fluorodeoxyglucose positron-emission computed tomography ((18)F-FDG PET/CT): has good diagnostic properties for diagnosing cyst infection 2
- CT scans: can show changes concerning for possible early pyelonephritis and an enlarged renal cyst compared to previous imaging 3
- Urine and blood cultures: can help confirm the diagnosis of an infected renal cyst 3, 2