Diagnosis of mild-to-moderate community-acquired pneumonia is typically based on medical history, physical exam, chest X-ray, and a complete blood count (Aoun 2016). Pulse oximetry, a noninvasive test that measures the oxygen level in the blood, can help assess pneumonia severity and whether hospitalization is needed (Torres 2013; Ortega 2011; Kaysin 2016). White blood cell numbers are generally elevated in people with pneumonia, but can also be low in some cases of overwhelming bacterial infection. CT scan may also be helpful in certain ambiguous cases (UMMC 2012; Musher 2012; Sethi 2014; Aoun 2016). Sputum gram stain and culture can help identify the presence and species of bacteria (UMMC 2012; Musher 2012).
While identifying the infectious organism causing pneumonia can be helpful in choosing effective antibiotic treatment, physicians generally employ “empiric therapy” beginning immediately upon diagnosis of community-acquired pneumonia not requiring hospitalization (Cilloniz 2016). In empiric therapy, clinical judgment and knowledge of the patient and his or her condition guide treatment. This method is considered to have a high rate of success, and some authorities have recommended that attempts to identify the causative organism be reserved for more severe or problematic cases (Sethi 2014).
In hospitalized patients, blood cultures can reveal widespread bacterial infection, usually from Streptococcus pneumoniae (Sethi 2014). Specific blood tests for atypical organisms are available (Cilloniz 2016). Urine antigen tests can detect the presence of proteins from Streptococcus pneumoniae and Legionella pneumophila more rapidly than cultures (Sinclair 2013; Molinos 2015; Aoun 2016).Invasive diagnostic procedures such as bronchoscopy or biopsy may be necessary in cases that do not respond to treatment as expected or in which unusual infectious organisms are suspected (Torres 2013).