Investigations

1st investigations to order

Test
Result
Test

A definitive diagnosis of CAP requires evidence of consolidation on chest x-ray.[1][64][75] Perform a chest x-ray in all patients presenting in hospital as soon as possible and within 4 hours of admission.Posterior-anterior chest radiograph showing right upper lobe consolidation in a patient with community-acquired pneumonia[Figure caption and citation for the preceding image starts]: Posterior-anterior chest radiograph showing right upper lobe consolidation in a patient with community-acquired pneumoniaDurrington HJ, et al. Recent changes in the management of community acquired pneumonia in adults. BMJ 2008;336:1429. [Citation ends].

  • If the chest x-ray shows atypical changes or complicated pneumonia (e.g., cavitation, pleural effusion, multifocal consolidation), consider other imaging investigations (see our section on Other tests to consider).

Practical tip

A high-quality chest radiograph is very important to ensure accurate diagnosis and to avoid inappropriate antibiotic prescribing. One study reported that 29% of hospitalised patients treated for CAP did not have radiographic abnormalities.[83]

Bear in mind that it is more difficult to obtain a high-resolution image from a person with class III obesity (BMI ≥40).

Do not perform a chest x-ray in patients with suspected CAP seen in the community unless:[1]

  • There is diagnostic doubt

  • Progress following treatment is not satisfactory at review

  • The patient is at risk of underlying lung pathology such as lung cancer.

Result

new shadowing (consolidation)

Test
Result
Test

Use pulse oximetry (preferably while breathing air) to assess oxygen saturation in hospital to inform supportive treatment.

  • Oxygen saturation <94% in a patient with CAP is an adverse prognostic factor and may be an indication for oxygen therapy and urgent referral to hospital.[76]

General practitioners should consider assessing oxygenation via pulse oximetry if available (e.g., if working in an out-of-hours setting).[1]

Result

  • may reveal low arterial oxygen saturation

  • oxygen saturation <94% in a patient with CAP is an adverse prognostic factor and may be an indication for oxygen therapy and/or urgent referral to hospital[76]

Test
Result
Test

Measure ABG in patients with CAP receiving oxygen therapy with an SpO2 <94%, those with a risk of hypercapnic ventilatory failure (CO2 retention), and all patients with high-severity CAP.

  • Patients may be hypoxaemic and require supplemental oxygen.

  • Oxygen saturation <94% in a patient with CAP is an adverse prognostic factor and may be an indication for oxygen therapy and urgent referral to hospital.[76]

Practical tip

Always record the inspired oxygen concentration clearly as this is essential for interpreting blood gas results.

Result

may reveal low arterial oxygen saturation

Test
Result
Test

Request urea and electrolytes to inform disease severity and renal function in patients being investigated in hospital.[1]

  • Chronic renal failure is a significant risk factor for mortality in patients with CAP.[57]

General investigations are not necessary for the majority of patients with CAP who are managed in the community.[1][64]

Result

  • usually normal; elevated in patients with severe CAP

  • urea >7 mmol/L (>19.6 mg/dL) counts for 1 point in the 6-point CURB-65 score to assess severity[1][64]

Test
Result
Test

Leukocytosis is often seen in people with CAP.

General investigations are not necessary for the majority of patients with CAP who are managed in the community.[1][64]

Result

  • leukocytosis

  • WBC count > 15 x109/L indicates a bacterial aetiology (particularly pneumococcal,) although lower counts do not exclude a bacterial cause

Test
Result
Test

Order CRP as a baseline measurement and to help rule out other acute respiratory illnesses in patients being investigated in hospital.

  • A failure of CRP to fall by 50% or more at day 4 is associated with an increased risk for 30-day mortality, need for mechanical ventilation and/or inotropic support, and complications.[91]

General investigations are not necessary for the majority of patients with CAP who are managed in the community.[1][64]

Result

  • elevated

  • a level >100 mg/L makes pneumonia likely[90]

  • a level <20 mg/L with symptoms for more than 24 hours makes the presence of pneumonia highly unlikely[90]

Test
Result
Test

Take blood for a baseline measurement. Provides information about liver function.

  • Chronic liver disease is a risk factor for pulmonary complications in patients hospitalised due to pneumococcal pneumonia.[59]

General investigations are not necessary for the majority of patients with CAP who are managed in the community.[1][64]

Result

usually normal; abnormal in patients with underlying liver disease or legionella infection[1]

Investigations to consider

Test
Result
Test

Order blood cultures, ideally before antibiotics are given, in all patients with moderate- or high-severity CAP (as determined by the CURB-65 score) presenting in hospital.[1][64]

  • Blood cultures can be highly specific in determining the microbial aetiology.[1][64]

  • Bacteraemia is also a marker of illness severity. However, many patients with CAP do not have associated bacteraemia.[1] Microbial causes of CAP that can be associated with bacteraemia include:[1]

    • Streptococcus pneumoniae ( identified in approximately 60% of positive blood cultures)[92]

    • Haemophilus influenzae (identified in 2% to 13% of positive cultures)[92]

    • Staphylococcus aureus and Klebsiella pneumoniae.

Do not order blood cultures in patients with confirmed CAP who have low-severity disease and no comorbid conditions.[1] Empirical antibiotic therapy is associated with a good prognosis in these patients.[1]

There is debate around the practicality of ordering routine blood cultures in patients hospitalised with CAP. This is mainly due to low sensitivity, cost, and the fact that results hardly ever influence antimicrobial management.

  • In a study of 355 patients admitted to hospital for CAP, the proportion of false-positive blood cultures was 10%, and the proportion of true positives was 9% (95% CI, 3.3% to 5.5%).[93] 

  • Antibiotic therapy was changed on the basis of blood culture results in only 5% of patients (95% CI, 3% to 8%).[93] 

  • However, despite these limitations, most experts still recommend blood cultures in patients with high-severity CAP.[1]

Result

growth of causative bacterial pathogen

Test
Result
Test

Take sputum samples for culture (± Gram stain) in all patients with moderate- and high-severity CAP (as determined by the CURB-65 score) presenting in hospital before starting antibiotics, and in patients who do not improve regardless of disease severity.[1][64] 

Order Gram stain of sputum cultures in patients with high-severity CAP or complications if available in your local laboratory. Gram stain is an immediate indicator of the likely pathogen and can help with interpreting culture results.[1]

Do not order microbiological tests routinely in patients presenting with CAP in the community.[1][64]

A prospective study of 1390 patients with bacteraemic CAP found a sensitivity for sputum Gram stain of:[94]

  • 82% for pneumococcal pneumonia

  • 79% for Haemophilus influenzaepneumonia

  • 76% for staphylococcal pneumonia.

Specificities ranged from 93% to 96%.

Practical tip

Carrying out routine sputum Gram stains for all patients is unnecessary.[1] The test has a low sensitivity and specificity, and often does not contribute to initial management. Problems include:[1]

  • Patients may not be able to produce good specimens

  • Prior exposure to antibiotics

  • Delays in transport and processing of samples, which reduces the yield of bacterial pathogens

  • Difficulty interpreting the results due to contamination of the sample by upper respiratory tract flora, which may include potential pathogens such as Streptococcus pneumoniae and 'coliforms' (especially in patients already given antibiotics).

Result

growth of causative bacterial pathogen

Test
Result
Test

Consider pneumococcal and legionella urine antigen tests in people with moderate- or high-severity CAP.[64] Order legionella urine antigen testing in all patients with specific risk factors and during an outbreak (e.g, Legionnaires’ disease) or during epidemic mycoplasma years.[1]

  • Pneumococcal urinary antigen testing is useful for diagnosing pneumococcal pneumonia in adults and is less affected than blood/sputum cultures by prior antibiotic therapy.[1]

  • Legionella urinary antigen testing allows for rapid results early in the illness.[1]

    • For all patients who are positive for legionella urine antigen, order sputum cultures from respiratory samples (e.g., obtained from bronchoscopy) for Legionella species. This is to aid outbreak and source investigation with the aim of preventing further cases.[1]

    • Legionella antigen testing by enzyme immunoassay is highly specific (>95%) and sensitive (80%) for detecting Legionella pneumophila serogroup 1, which is the most common cause of sporadic CAP and CAP due to foreign travel in the UK.[97]

Do not order microbiological tests routinely in patients presenting with CAP in the community.[1][64]

Studies have shown significantly greater sensitivity rates for the pneumococcal urine antigen test than for routine blood or sputum cultures.[95]

  • Results remain positive in 80% to 90% of patients for up to 7 days after starting antimicrobial treatment.[95]

Result

positive for Legionella or pneumococcal antigens

Test
Result
Test

Allows for rapid identification of the pathogen. Order PCR of sputum or other respiratory samples (e.g., nose and throat swabs) in patients with high-severity CAP:[1]

  • If unresponsive to beta-lactam antibiotics

  • If there is a strong suspicion of a respiratory virus (i.e., influenza A and B, parainfluenza 1-3, adenovirus, respiratory syncytial virus) or an 'atypical' pathogen: 

    • Mycoplasma pneumoniae

    • Chlamydophila pneumoniae

    • Chlamydophila psittaci

    • Coxiella burnetii

    • Pneumocystis jirovecii (if at risk).

Consider PCR in patients with low- or moderate-severity CAP during outbreaks (e.g., Legionnaires’ disease) or during epidemic mycoplasma, or when there is a particular clinical or epidemiological reason.[1]

Do not order microbiological tests routinely in patients presenting with CAP in the community.[1][64]

Result

detection of viral/atypical pathogen antigens or antibodies

Test
Result
Test

Consider a CT scan of the chest when there is diagnostic doubt: for example, if the chest x-ray is of poor quality or there is an ill-defined consolidation.[1]

  • Findings on chest x-ray that should prompt you to perform a CT scan include:

    • Cavitation – CT helps identify alternative diagnoses such as tuberculosis, lung cancer, pulmonary infarct, septic pulmonary emboli, infected bulla, lung abscessChest radiograph showing left upper lobe cavitating pneumonia[Figure caption and citation for the preceding image starts]: Chest radiograph showing left upper lobe cavitating pneumoniaFrom the collection of Dr Jonathan Bennett. Used with permission [Citation ends].

    • Consolidation pattern (multifocal) – CT helps identify alternative diagnoses such as staphylococcal infection, tuberculosis, aspiration pneumonia, allergic bronchopulmonary aspergillosis, cryptogenic organising pneumonia, or drug hypersensitivity reaction

    • Pleural effusion – CT (in conjunction with chest ultrasound and guided aspiration) helps identify parapneumonic effusion, empyema, tuberculosis, lung cancer 

      • Approximately 40% of patients who are hospitalised for pneumonia develop a parapneumonic effusion.[85]

      Left-sided pleural effusion[Figure caption and citation for the preceding image starts]: Left-sided pleural effusionFrom the collection of Dr R Light. Used with permission [Citation ends].

Consider a chest and upper abdomen CT scan in patients with persistent signs and symptoms or changes on chest x-ray prior to bronchoscopy to rule out alternative diagnoses. 

Result

may show cavitations, pleural effusion, multifocal consolidation, neoplasm

Test
Result
Test

Consider chest ultrasound following chest x-ray when there is diagnostic doubt.[1]

  • Pleural effusion seen on chest x-ray may prompt you to perform a chest ultrasound (with or without guided aspiration and chest x-ray) to help make an alternative diagnosis such as tuberculosis, lung cancer, or pulmonary embolism.Left-sided pleural effusion[Figure caption and citation for the preceding image starts]: Left-sided pleural effusionFrom the collection of Dr R Light. Used with permission [Citation ends].

Although a chest radiograph showing new shadowing that cannot be attributed to any other cause is the ‘gold-standard’ for the diagnosis of pneumonia, it may not always be feasible in a community setting and it involves exposure to radiation.

  • Emerging evidence has shown that lung ultrasound is a possible accurate diagnostic test for people with CAP. However, the benefits of its use in practice over chest radiography are still unclear.

  • A meta-analysis of 12 studies looking at the diagnostic accuracy of lung ultrasound in people with CAP found a sensitivity and specificity of 0.88 and 0.86, respectively.[88] However, there were limitations, such as the large variability in the findings and the lack of heterogeneity of the studies reviewed. 

  • Further evidence is required before recommendations can be made.

Result

consolidation may be seen; parapneumonic effusion may be seen

Test
Result
Test

Consider thoracocentesis in all patients with pleural effusion as this can reveal an infected pleural space consistent with a parapneumonic effusion or empyema.[1]

  • A positive Gram stain of pleural fluid indicates an empyema. In these patients, drain pleural fluid in those with an empyema or clear pleural fluid with pH <7.2.[1]

Result

exudate; growth of causative bacterial species in case of empyema

Test
Result
Test

Consider CTPA to rule out pulmonary embolism if symptoms came on quickly (within minutes) or pain and breathlessness preceded infective symptoms.[87]

  • CTPA has the best diagnostic accuracy of all advanced non-invasive imaging methods in the detection of pulmonary embolism.[100]

Result

may reveal a thrombus in a pulmonary artery; appears as a partial or complete intraluminal filling defect

Test
Result
Test

Consider bronchoscopy during recovery in patients with persisting signs and symptoms of CAP and radiological abnormalities at around 6 weeks after completing treatment.[1]

  • The most common techniques are bronchoalveolar lavage (BAL) and protected specimen brushing (PSB).

Result

  • BAL: 104 colony-forming units (CFU)/mL indicates infection

  • PSB: 103 CFU/mL has been recommended to distinguish colonisation from infection 

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