Diagnostic Approach to Left Lower Lobe Infiltrates

Clinical Criteria and Imaging

The American College of Radiology recommends that a left lower lobe infiltrate on chest radiograph does not automatically represent pneumonia and requires clinical correlation with at least two signs or symptoms of infection to establish the diagnosis 1, 2
The diagnosis of pneumonia requires both radiographic findings and clinical evidence that the infiltrate is of infectious origin, including new or progressive radiographic infiltrate plus at least two clinical features such as fever, leukocytosis, new cough, dyspnea, or purulent secretions 3
Using only one clinical criterion increases sensitivity but significantly decreases specificity, leading to overtreatment with antibiotics, while requiring all three criteria is too insensitive and will miss true pneumonia cases 3
The Infectious Diseases Society of America (IDSA) and the American Thoracic Society (ATS) consider CT a reasonable alternative to empiric antibiotic therapy with follow-up chest radiographs when there is high clinical suspicion 1, 2, 5

CT detects pneumonia in 27-33% of patients with negative chest radiographs when clinical suspicion is high 2, 4, 5
CT excluded pneumonia in 29.8% of patients who had opacities on chest radiograph, preventing unnecessary antibiotic use 4, 5
Ultrasound demonstrates higher sensitivity (81.4%) than chest radiograph (64.3%) for detecting pneumonia and may add value when initial radiographs are negative or equivocal 1, 2, 4

Pulmonary edema should be considered in patients with cardiomegaly or heart failure as a non-infectious cause of lower lobe infiltrates 6
Aspiration without infection should be considered as not all aspiration events lead to bacterial pneumonia requiring antibiotics 7