Acute bacterial nephritis. A descriptive study of an underdiagnosed entity

Background: Acute bacterial nephritis is an infectious process diagnosed through imaging studies. The clinical course of the disease has been shown to be more aggressive than acute pyelonephritis. It continues to be underdiagnosed, thus there are few studies on the entity in the literature. Objective: To describe the clinical characteristics and imaging features of acute bacterial nephritis, as well as its clinical course. Design: A descriptive, retrospective case series was conducted. Materials and methods: Thirty-two cases of acute bacterial nephritis in patients admitted to the hospital within the time frame of 2009 to 2016 were reviewed. The patients’ clinical characteristics upon admission were registered, as well as inpatient clinical progression, culture results, and antibiotic therapy response. The imaging studies were re-evaluated and the diagnostic consistency with either the focal or multifocal disease presentation was confirmed. Results: Cases predominated in women (n=29, 90.62%) and the most frequently associated comorbidities were diabetes (n=16, 50%) and obesity (n=9, 28.25%). The most important clinical findings upon admission were fever (n=15, 46.87%) and leukocytosis (n=27, 84.38%). Escherichia coli was the most commonly isolated bacterium (63.63%). Both acute focal bacterial nephritis and acute multifocal bacterial nephritis were observed in 46.87% (n=15) and 53.13% (n=17) of the patients, respectively. Imaging studies were required for all diagnoses. Conclusion: Fever and leukocytosis are the main findings in acute bacterial nephritis. Imaging studies are necessary for making the diagnosis, given that acute pyelonephritis and acute bacterial nephritis cannot be clinically differentiated.


Introduction
Acute bacterial nephritis (ABN), formerly known as acute lobar nephronia and first described by Rosenfield, (1) is a bacterial infectious process of low prevalence that affects the renal parenchyma. It is caused by different infectious agents that reach the kidney through ascending invasion or hematogenous spread. (2) Escheri-chia coli (E. coli) is the most commonly isolated etiologic agent. (1,3,4) Hematogenous spread appears to be an important infection mechanism for ABN, and wedge-shaped lesions on the renal parenchyma suggest bacterial emboli dissemination. (5) ABN shares the same clinical features as acute pyelonephritis (APN) and both may present with fever, flank pain, pyuria, bacteriuria, and leukocytosis. (3,4,6,7) Therefore, imaging studies are needed to differentiate between ABN and APN. The main sonographic findings in ABN are hypoechoic lesions with irregular, poorly defined margins, which may be associated with nephromegaly or perinephric fluid.
Hyperechoic lesions may also be present. (8,9) Ultrasound (US) sensitivity and specificity have been reported at 74% and 56.7%, respectively. (10) Contrast computed tomography (CT) is the imaging study of choice, with a sensitivity of 90% and specificity of 86%, (11) but may be reserved for cases in which US is inconclusive. (1,12) The majority of case series reported to date focus on pediatric patients, demonstrating a strong association between ABN and vesicoureteral reflux in 40% of cases. (2,7,13) Acute inflammation, edema, and diffuse leukocyte infiltration are histopathologic findings in ABN, as well as in APN. (4,6,14,15) It is important to differentiate ABN from other infectious renal processes because a more aggressive and prolonged treatment is required and there is also a high risk of renal abscess progression. (15) Few case series on ABN in adults are available, but at present, greater access to imaging studies has resulted in increased awareness and diagnosis of the disease.

Study design and definitions
A descriptive, retrospective case series was con- Clinical features, such as obesity, were defined, according to the CDC criteria (BMI ≥ 30), (17) diabetes was defined, according to the 2018 ADA criteria (fasting plasma glucose ≥ 126 mg/dl or random plasma glucose ≥ 200 mg/dl plus classic symptoms of hyperglycemia and/ or hyperglycemic crisis), (18) and hypertension, in accordance with the JNC 8 report (blood pressure ≥ 140/90 mmHg). (19) Renal function was assessed utilizing the Cockcroft-Gault formula and chronic kidney disease (CKD) was considered when the patient presented with a GFR < 60 ml/min.

Exclusion criteria
Patients whose imaging studies did not meet the definition or inclusion criteria for ABN described above, or whose studies were inconclusive, were excluded, as were patients that presented with pyelonephritis or lower urinary tract infections.

Imaging
The imaging studies of all 32 patients were reassessed and checked for diagnostic consistency with ABN, in either the focal or multifocal presentation, ensuring that the patients were accurately diagnosed through the imaging study.
The diagnosis was first checked in relation to the imaging report made by the radiologist, and we carried out further investigation to make sure that either focal or multifocal nephritis was present. Images were stored in Carestream

Results
The clinical records and imaging studies of 32 patients with ABN were evaluated. The majority of patients were women (n=29, 90.62%) and mean patient age was 36.34 years (SD ±13.23).
Fever (n=15, 46.87%) and leukocytosis (n=27, 84.38%) were the main clinical findings upon admission and the mean leukocyte count was 16.77 k/µl ± 7.33 (Table 1).    Table   2). Antibiotic therapy had to be escalated to carbapenems in 25% (n=8) of the patients, and a single case required nephrectomy (3.12%). Hospital stay was 7.84 ± 6.91 days and there were 2 deaths. Progression to renal abscess was observed in 9.37% (n=3) of the patients.
The data is shown in Table 2.

Discussion
In the present case series report, we describe APN and renal abscess. (24) Our patients were given IV antibiotics for 7.84 ± 6.91 days, concurring with the EAU guidelines, which recommend IV antibiotic treatment for 7 to 14 days. (25) Patients were discharged from the hospital after 48 hours of defervescence ( Figure 2). No differences between the focal and multifocal presentations were observed.

Conclusions
The limitations of our study include its retrospective design, the small sample size, and the fact that not every patient had a standardized imaging study workup. Only 3 patients developed renal abscess, which hindered the progression analysis.
Our study showed that early and aggressive IV antibiotic therapy was effective in ABN management. There should be high clinical suspicion of ABN in all patients with presumptive APN, given that it presents with greater morbidity. Imaging assessment is crucial.
We reviewed 32 cases of ABN at our institution in the present analysis, and our results were similar to findings in other studies. (1,6,8,20,26,27) We believe more studies showing that ABN is a different entity from APN are needed. The comparison of the two presentations has not been analyzed in depth, and their distinction can change patient prognosis and outcome.

Financial disclosure
No sponsorship of any kind was received in relation to this article.