pulmonary disorders
Community Acquired Pneumonia
Diagnosis of Community Acquired Pneumonia
Chest x-ray Consideration of alternative diagnoses (eg, heart failure, pulmonary embolism, inflammatory lung conditions) Sometimes identification of pathogen Evaluation of severity and risk stratification Diagnosis of pneumonia is suspected on the basis of clinical presentation and infiltrate seen on chest x-ray. When there is high clinical suspicion of pneumonia and the chest x-ray does not reveal an infiltrate, doing computed tomography (CT) or repeating the chest x-ray in 24 to 48 hours is recommended. Severity of the pneumonia is estimated using a variety of clinical and laboratory factors (see Risk stratification ) which are sometimes organized using quantitative scoring systems. Typically, testing includes oxygen saturation, complete blood count, and a basic or complete metabolic profile. Differential diagnosis in patients presenting with pneumonia-like symptoms includes Acute Bronchitis and Quantitative cultures of bronchoscopic or suctioned specimens, if they are obtained before antibiotic administration, can help distinguish between bacterial colonization (ie, presence of microorganisms at levels that provoke neither symptoms nor an inflammatory response) and infection. However, bronchoscopy is usually done only in patients receiving mechanical ventilation or for those with other risk factors for unusual microorganisms or complicated pneumonia (eg, immunocompromise, failure of empiric therapy). Distinguishing between bacterial and viral pneumonias is challenging. Many studies have investigated the utility of clinical, imaging, and routine blood tests, but no test is reliable enough to make this differentiation. Even identification of a virus does not preclude concomitant infection with a bacteria; therefore, antibiotics are indicated in almost all patients with a community-acquired pneumonia. In outpatients with mild pneumonia, no further diagnostic testing is needed (see table Risk Stratification for Community-Acquired Pneumonia (the Pneumonia Severity Index) ). In patients with moderate or severe pneumonia, a white blood cell count and measurement of electrolytes, blood Diagnosis of etiology can be difficult. A thorough history of exposures, travel, pets, hobbies, and other exposures is essential to raise suspicion of less common organisms. For example, exposure to farm animals may suggest Q fever, and a recent hotel or cruise ship stay may suggestLegionellainfection. Identification of the pathogen can be useful to direct therapy and verify bacterial susceptibilities to antibiotics. However, because of the limitations of current diagnostic tests and the success of empiric antibiotic treatment, experts recommend limiting attempts at microbiologic identification (eg, cultures, specific antigen testing) unless patients are at high risk or have complications (eg, severe pneumonia, immunocompromise, asplenia, failure to respond to empiric therapy). In general, the milder the pneumonia, the less such diagnostic testing is required. Critically ill patients require the most intensive testing, as do patients in whom a antibiotic-resistant or unusual organism is suspected (eg,Mycobacterium tuberculosis,P. jirovecii) and patients whose condition is deteriorating or who are not responding to treatment within 72 hours. Chest x-rayfindings generally cannot distinguish one type of infection from another, although the following findings are suggestive: Multilobar infiltrates suggestS. pneumoniaeorLegionella pneumophilainfection. Interstitial pneumonia (on chest x-ray, appearing as increased interstitial markings and subpleural reticular opacities that increase from the apex to the bases of the lungs) suggests viral or mycoplasmal etiology. Cavitating pneumonia suggestsS. aureusor a fungal or mycobacterial etiology. This chest x-ray shows an infiltrate that appears to blend with the right heart border (silhouette sign). The silhouette sign indicates contiguous positioning of the 2 structures that have similar radiodensity; the part of the lung contiguous with the right heart border is the right middle lobe, so that is the part with the infiltrate and pneumonia. SCIENCE PHOTO LIBRARY This chest x-ray shows an infiltrate that does not obscure the right heart border (ie, there is no silhouette sign). Because the silhouette sign develops when 2 contiguous structures have a similar radiodensity, the part of the lung affected by this infiltrate is the part not contiguous with the right heart border; that part is the right lower lobe. LIVING ART ENTERPRISES, LLC/SCIENCE PHOTO LIBRARY Alveolar infiltrate of the left lower lobe in a man with bacterial pneumonia. By permission of the publisher. From Roberts R. InAtlas of Infectious Diseases: Pleuropulmonary and Bronchial Infections. Edited by GL Mandell (series editor) and MS Simberkoff. Philadelphia, Current Medicine, 1996. Consolidation of the right upper, middle, and lower lobes in a 64-year-old man with pneumococcal pneumonia. By permission of the publisher. From Roberts R. InAtlas of Infectious Diseases: Pleuropulmonary and Bronchial Infections. Edited by GL Mandell (series editor) and MS Simberkoff. Philadelphia, Current Medicine, 1996. Bilateral interstitial opacities in an elderly man with respiratory syncytial virus pneumonia. By permission of the publisher. From Betts R, Falsey A, Hall C, et al. InAtlas of Infectious Diseases: Pleuropulmonary and Bronchial Infections. Edited by GL Mandell (series editor) and MS Simberkoff. Philadelphia, Current Medicine, 1996. This intubated patient has multiple bilateral infiltrates, most prominently in the right upper lobe. Arrow indicates the right horizontal fissure. Photo courtesy of Thomas M. File, Jr., MD MSc MACP FIDSA FCCP. Blood cultures,which are often obtained in patients hospitalized for pneumonia, can identify causative bacterial pathogens if bacteremia is present. About 12% of all patients hospitalized with pneumonia have bacteremia;S. pneumoniaeaccounts for two thirds of these cases. Sputum testingcan include Gram stain and culture for identification of the pathogen, but the value of these tests is uncertain because specimens often are contaminated with oral flora and overall diagnostic yield is low. Regardless, identification of a bacterial pathogen in sputum cultures allows for susceptibility testing. Obtaining sputum samples also allows for testing for viral pathogens via direct fluorescence antibody testing or polymerase chain reaction (PCR), but caution needs to be exercised in interpretation because 15% of healthy adults carry a respiratory virus or potential bacterial pathogen. In patients whose condition is deteriorating and in those unresponsive to broad-spectrum antibiotics, sputum should be tested with mycobacterial and fungal stains and cultures. Sputum samplescan be obtained noninvasively by simple expectoration or after hypertonic saline nebulization (induced sputum) for patients unable to produce sputum. Alternatively, patients can undergo bronchoscopy or endotracheal suctioning, either of which can be easily done through an endotracheal tube in mechanically ventilated patients. Otherwise, bronchoscopic sampling is usually done only for patients with other risk factors (eg, immunocompromise, failure of empiric therapy). Urine testingforLegionellaantigen and pneumococcal antigen is now widely available. These tests are simple and rapid and have higher sensitivity and specificity than sputum Gram stain and culture for these pathogens. Patients at risk ofLegionellapneumonia (eg, severe illness, failure of outpatient antibiotic treatment, presence of pleural effusion, active alcohol abuse, recent travel) should undergo testing for urinaryLegionellaantigen, which remains present long after treatment is initiated, but the test detects onlyL. pneumophilaserogroup 1 (70% of cases). Thepneumococcal antigen testis recommended for patients who are severely ill; have had unsuccessful outpatient antibiotic treatment; or who have pleural effusion, active alcohol abuse, severe liver disease, or asplenia. This test is especially useful if adequate sputum samples or blood cultures were not obtained before initiation of antibiotic therapy. A positive test can be used to tailor antibiotic therapy, though it does not provide antimicrobial susceptibility. Diagnosis None Serum procalcitonin can help distinguish bacterial infection from other causes of infection or inflammation. However, use of the serum procalcitonin level as a criterion for whether to initiate antibiotic therapy in community-acquired pneumonia is not recommended. It can be used, along with clinical judgement, to guide early discontinuation of antibiotics in lower respiratory tract infections. 