For updates on diagnosis and management of coexisting conditions during the pandemic, see our topic "Management of coexisting conditions in the context of COVID-19".

Patient history and physical exam are important parts of the diagnosis and may elicit symptoms consistent with CAP, immune defects, and/or potential exposure to specific pathogens. However, a definitive diagnosis of pneumonia requires the presence of a new infiltrate on chest x-ray.


The objective of history-taking should be to detect symptoms consistent with CAP, defects of immunity, and possible risk of exposure to specific pathogens.

Risk factors include age >65 years, residence in a healthcare setting, COPD, HIV infection, cigarette smoke exposure, alcohol abuse, poor oral hygiene, contact with children, and use of certain drugs (e.g., acid-reducing drugs, inhaled corticosteroids, antipsychotics, antidiabetic drugs, opioids). Diabetes mellitus and chronic liver or renal disease have also been associated with CAP.

Clinical signs and findings of infection (fever or chills and leukocytosis) and respiratory symptoms (including cough, often with increasing sputum production, expectoration, dyspnea, pleuritic pain, and hemoptysis) are usually present. Nonspecific symptoms such as myalgia and arthralgia may be reported. In patients of advanced age, patients with chronic illness, and immunocompromised patients, the signs and symptoms of pulmonary infection may be less intense and the pneumonia may go unrecognized because of the presence of nonrespiratory symptoms.

Some causes of pneumonia (e.g., legionellosis) may have a specific history. Legionellosis can present with headache, confusion, digestive manifestations such as diarrhea, and clinical manifestations of hyponatremia.

Mycoplasma pneumoniae infection is most common in young patients and patients who have been treated with antibiotics before their current presentation with pneumonia. It may present with extrapulmonary manifestations such as myringitis, encephalitis, uveitis, iritis, and myocarditis.[18][65]

Physical exam

Perform a physical exam. The patient may be febrile, tachycardic, and breathless at rest. Auscultation of the chest may reveal crackles, rales, or bronchial breathing, and there may be presence of dullness on percussion or tactile vocal fremitus.


Order a chest x-ray as soon as possible in all patients admitted to hospital with suspected CAP to confirm or exclude the diagnosis. It is not necessary to perform a chest x-ray in outpatients with suspected CAP unless the diagnosis is in doubt, the patient is not responding to treatment satisfactorily, or the patient is considered to be at risk of an underlying lung pathology.[66] Posteroanterior and latero-lateral projections increase the likelihood of diagnosis of pneumonia and are useful in establishing the severity of the illness.

The primacy of chest x-ray in making the diagnosis of CAP has been challenged by studies using lung ultrasound and computed tomography (CT) scan of the chest. Consider ordering a lung ultrasound if the chest x-ray is negative and the patient is elderly and frail or the clinical suspicion is uncertain. Only consider a chest CT in patients who have an uncertain diagnosis after both a chest x-ray and ultrasound.[67]

Lung ultrasound represents an easy and accessible technique for the diagnosis of CAP. It is radiation-free, and its use is especially valuable when chest x-ray is not available. The diagnosis of CAP via bedside lung ultrasound mainly depends on detecting consolidation. However, consolidation is not always present in CAP, because pneumonia may be interstitial or present as diffuse pulmonary infiltrations.[68] Systematic reviews have found that lung ultrasound can diagnose pneumonia in adults with excellent accuracy, including in the emergency department.[69][70]

Chest CT may improve the diagnosis of CAP, because chest x-ray may lead to misdiagnosis. Chest CT provides detailed information about the lung parenchyma and the mediastinum. However, the principal limitations include exposure to radiation, high cost, and the impossibility of bedside testing. One study has reported that in patients presenting to the emergency department with suspected CAP, early CT scan findings, when CT is used in addition to chest x-ray, markedly affect both diagnosis and clinical management.[71]

These alternative imaging techniques may be the future of care for the diagnosis of CAP as the availability of CT scanners in emergency departments increases, along with their ability to scan as fast as chest x-ray with an equivalent amount of radiation.[28]


The initial antibiotic treatment is empiric in most cases. Determining the microbial etiology reduces inappropriate use of broad-spectrum antibiotics and helps to ensure appropriate antibiotic therapy, which is an important factor in reducing mortality. It also identifies resistant pathogens and pathogens that may have public health implications (e.g., Legionella).

Sputum and blood cultures:

  • Obtain pretreatment Gram stain and culture of lower respiratory secretions and blood cultures in the following patients in the hospital setting:[17]

    • Patients with severe CAP as defined by American Thoracic Society (ATS)/Infectious Diseases Society of America (IDSA) criteria for defining severe CAP (see Diagnostic criteria section), especially if they are intubated

    • Patients being empirically treated for methicillin-resistant Staphylococcus aureus (MRSA) or Pseudomonas aeruginosa

    • Patients who have been previously infected with MRSA or P aeruginosa, especially those with a prior respiratory tract infection

    • Patients who have been hospitalized and received parenteral antibiotics in the past 90 days.

  • These tests are not routinely recommended in other inpatients, and are not recommended in the outpatient setting. Take into account local antimicrobial stewardship protocols, local etiologic factors, and the clinical presentation when deciding whether to obtain these tests.[17]

  • Sputum Gram stain is sensitive and highly specific for identifying the causative pathogens in patients with CAP. A meta-analysis found that this test is highly specific for identifying Streptococcus pneumoniae, Haemophilus influenzae, S aureus, and gram-negative bacilli. However, the proportion of false-negative results ranged from 22% (for H influenzae) to 44% (for S pneumoniae), indicating that a negative result does not conclusively confirm the absence of causative pathogens, and antibiotic therapy should not necessarily be stopped based on a negative sputum Gram stain.[72]

Pneumococcal and Legionella urinary antigen testing:

  • Test urine for pneumococcal antigen in patients with severe CAP. Test urine for Legionella antigen in patients with epidemiologic factors (e.g., association with legionella outbreak or recent travel) or patients with severe CAP. Collect lower respiratory tract secretions for Legionella culture or nucleic acid amplification in patients with severe CAP at the same time. Urinary antigen testing has been associated with a reduction in mortality in large observational studies, and is important to consider given the increase in Legionella infections, especially among severely ill patients.[17]

Influenza virus testing:

  • Test for influenza virus using a rapid influenza molecular assay (rather than antigen-based detection tests) when influenza viruses are circulating in the community. Testing may also be considered during periods of low influenza activity.[17]

Laboratory investigations

Order a complete blood count, blood glucose, serum electrolytes, blood urea nitrogen, and liver function tests in hospitalized patients. An elevated white cell count is suggestive of infection. Chronic renal disease and chronic liver disease are risk factors for mortality and complications in patients hospitalized with CAP.

Measure arterial blood gases in severely ill or hospitalized patients. Oximetry is noninvasive and can be used continually.

Consider ordering biomarkers such as C-reactive protein (CRP) and procalcitonin (PCT). These biomarkers have been found to be useful for predicting inadequate host response. High levels of CRP or PCT at initial presentation represent a risk factor for inadequate host response,[73] whereas low levels are protective. In patients with suspected pneumonia, a CRP level >10 mg/dL makes pneumonia likely.[74] Increased values of PCT are correlated with bacterial pneumonia whereas lower values are correlated with viral and atypical pneumonia. PCT is especially elevated in cases of pneumococcal pneumonia.[75][76] Initial empiric antibiotic therapy should be started in patients with clinically suspected and radiographically confirmed CAP regardless of the initial serum PCT level.[17]

Consider pleural fluid aspiration and culture in all patients with a pleural effusion. Parapneumonic effusions are exudates; a positive Gram stain of pleural fluid indicates an empyema.


Consider bronchoscopy in immunosuppressed patients, in patients with severe CAP, and in cases of treatment failure. The most common sampling techniques are bronchoalveolar lavage (BAL) and protected specimen brushing (PSB). A threshold of 10⁴ colony-forming units (CFU)/mL in BAL samples indicates infection. For PSB, a threshold of 10³ CFU/mL has been recommended to distinguish colonization from infection.[77]

Molecular techniques

Routine bacterial cultures are too slow to be immediately therapeutically useful. Nucleic acid amplification tests such as polymerase chain reaction have improved diagnostic accuracy in CAP. Molecular techniques provide high sensitivity and specificity in the diagnosis of single or polymicrobial infections, and they can help to determine antimicrobial resistance (as may occur with Staphylococcus aureus, nonfermenting gram-negative bacilli, and Enterobacteriaceae) associated with severe CAP.[78]

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