Investigações
Primeiras investigações a serem solicitadas
chest x-ray
Exame
Radiological evidence of new consolidation on chest x-ray is usually required to confirm a diagnosis of CAP in hospitalised patients.[74][77][89]
Perform a chest x-ray in all patients presenting to hospital with suspected CAP within 4 hours of presentation.[78][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].
[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].
[Figure caption and citation for the preceding image starts]: Left-sided pleural effusionFrom the collection of Dr R Light. Used with permission [Citation ends].
[Figure caption and citation for the preceding image starts]: Increased opacification of the right perihilar region and superior segment of the right lower and upper lobes consistent with worsening aspiration pneumoniaFrom the collection of Dr Roy Hammond. Used with permission [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 Other tests to consider).
Practical tip
A good quality x-ray is very important for accurate diagnosis and to avoid inappropriate antibiotic prescribing. One study reported that 29% of hospitalised patients treated for CAP had no radiographic abnormalities.[102]
It may be more difficult to obtain an adequate-quality chest x-ray in people with class III obesity (BMI ≥40 kg/m²).
Do not routinely request a chest x-ray in patients with suspected CAP seen in the community unless:[74][77]
there is diagnostic doubt
clinical progress following treatment is not satisfactory at review
the patient has risk factors for underlying lung pathology such as lung cancer.
Resultado
new shadowing (consolidation)
pulse oximetry
arterial blood gas (ABG)
Exame
Check an ABG in hospitalised patients with CAP who have:[84]
SpO₂ <94% on air or requiring supplemental oxygen to maintain target saturations
high oxygen requirements or increasing FiO₂ needs
risk factors for hypercapnia (e.g., COPD, obesity hypoventilation syndrome, neuromuscular disorders, or chest wall deformity), especially with drowsiness or worsening breathlessness
high-severity disease or critical illness (see Diagnostic criteria for guidance on assessing severity)
clinical deterioration including:
worsening breathlessness
falling oxygen saturation (≥3%)
rising NEWS2 or other early warning score
suspected metabolic disturbance (e.g., diabetic ketoacidosis or renal failure)
any situation where blood gas results are likely to inform management.
Practical tip
Obtaining a blood gas while the patient is not yet receiving supplemental oxygen provides a more accurate reflection of the oxygenation status, but should not delay supplemental oxygen in an unstable patient.
Always record the fraction of inspired oxygen (FiO₂) clearly as this is essential for interpreting blood gas results.
For initial assessment, a venous blood gas (VBG) may be used, particularly when an ABG is not immediately indicated. It may show respiratory alkalosis, metabolic acidosis (e.g. due to hypotension, reduced tissue perfusion, or acute kidney injury), or elevated lactate. However, VBG does not reliably assess oxygenation and may not accurately reflect arterial carbon dioxide levels; if oxygen requirements are high or the VBG suggests possible respiratory acidosis, perform an ABG to assess oxygenation and ventilation.[84]
As an alternative to arterial sampling, arterialised earlobe (capillary) blood gases may be used to obtain an accurate measure of pH and PaCO₂. However, PaO₂ is less accurate (typically underestimated by 0.5-1.0 kPa), so oxygen saturation should be monitored carefully, and an ABG should be taken if there is any concern about accuracy.[84]
Resultado
may show hypoxaemia or hypercapnia
may reveal acidaemia (respiratory or metabolic)
may reveal raised lactate, potentially indicating sepsis
urea and electrolytes
Exame
Request urea and electrolytes in patients being investigated in hospital to support severity assessment and assess renal function.[74][77]
Chronic kidney disease is a significant risk factor for mortality in patients with CAP.[69]
Routine blood tests, including urea and electrolytes, are not required for most patients managed in the community, but should be performed in all hospitalised patients.[74][77]
Result
usually normal; urea may be elevated in patients with severe CAP
urea >7 mmol/L (>19.6 mg/dL) counts for 1 point in the CURB-65 score for assessing mortality risk
acute kidney injury may occur in patients with sepsis complicating CAP
full blood count
Test
Leukocytosis is often seen in people with CAP.
Routine blood tests, including FBC, are not required for most patients managed in the community, but should be performed in all hospitalised patients.[74][77]
Result
leukocytosis is common
WBC count >15 × 109/L is suggestive of bacterial (particularly pneumococcal) infection, although lower counts do not exclude a bacterial cause
C-reactive protein (CRP)
Test
Order CRP in patients being investigated in hospital to support diagnosis and provide a baseline measurement.[74]
Raised CRP levels do not reliably distinguish bacterial from non-bacterial CAP; however, low CRP levels make a bacterial aetiology less likely.[118]
The UK National Institute for Health and Care Excellence (NICE) also recommends considering measurement of CRP (or procalcitonin) 3-4 days after starting treatment if there is clinical concern about treatment failure.[78]
High CRP levels, or levels that do not significantly improve with treatment, are associated with treatment failure and may indicate the need for senior clinical review.[78]
A failure of CRP to fall by ≥50% by day 4 is associated with higher 30-day mortality, an increased need for mechanical ventilation, and a greater likelihood of other complications.[119]
Routine blood tests, including CRP, are not required for most patients managed in the community, but should be performed in all hospitalised patients.[74][77]
Result
raised
CRP >100 mg/L supports a diagnosis of pneumonia
CRP <20 mg/L (in a patient with symptoms for >24 hours) makes pneumonia unlikely
liver function tests
Test
Obtain baseline liver function tests (LFTs) in all hospitalised patients.[74][77]
Results are usually normal but may be abnormal in those with underlying liver disease or Legionella pneumophila infection.[74]
Chronic liver disease is a risk factor for pulmonary complications in patients hospitalised due to pneumococcal pneumonia.[71]
Routine blood tests, including LFTs, are not required for most patients managed in the community, but should be performed in all hospitalised patients.[74][77]
Result
usually normal; may be abnormal in patients with underlying liver disease or Legionella pneumophila infection
Investigations to consider
blood culture
Test
In the hospital setting, consider blood cultures in patients with moderate- or high-severity CAP (as determined by the CURB-65 score and clinical judgement - see Diagnostic criteria) if there are additional clinical indications, such as suspected sepsis.[78]
Blood cultures should ideally be obtained before starting antibiotics, as prior antibiotic therapy can reduce culture yield and affect results.[78]
Isolation of bacteria can be highly specific in determining the microbial aetiology in people with moderate- or high-severity CAP.[74][77]
Bacteraemia is also a marker of illness severity. However, many patients with CAP do not have associated bacteraemia.[74]
Microbial causes of CAP that can be associated with bacteraemia include:[74][120]
Streptococcus pneumoniae (identified in approximately 60% of positive blood cultures)
Haemophilus influenzae (identified in 2% to 13% of positive cultures)
Staphylococcus aureus and Klebsiella pneumoniae
Do not order microbiological tests, including blood cultures, routinely in patients with low-severity CAP or in patients presenting with CAP who are managed in the community.[78][74][77]
Result
growth of causative bacterial pathogen
sputum culture
Test
In hospital
Consider sputum culture in:
All patients with moderate- or high-severity CAP (as determined by the CURB-65 score and clinical judgement - see Diagnostic criteria), taking into account the person's history of antibiotic treatment, their clinical trajectory, the presence of any comorbidities, any recent hospitalisation and the likelihood of getting a good-quality sputum sample.[74][77][78]
Patients who do not improve with initial therapy, regardless of disease severity.[74][77]
Do not order microbiological tests, including sputum culture, routinely in patients with low-severity CAP.[78]
In the community
Do not order microbiological tests routinely, including sputum culture, in patients presenting with CAP who are managed in the community.[74][77]
Only consider ordering microbiological tests in the community if:[74]
The patient’s symptoms do not improve with empirical antibiotic therapy
The patient has a persistent productive cough, especially if they also have malaise, weight loss, or night sweats, or risk factors for tuberculosis (e.g., ethnic origin, social deprivation, older patients, previous history of tuberculosis, contact history of tuberculosis): consider sputum examination for Mycobacterium tuberculosis.
There is a clinical or epidemiological reason, such as an outbreak (e.g., Legionnaires’ disease) or during mycoplasma epidemics: consider urinary antigen testing, PCR of upper (e.g., nose and throat swabs) or lower (e.g., sputum) respiratory tract samples or serological investigations; if available, PCR is preferred over serological investigations.
Result
growth of causative bacterial pathogen
urinary antigen testing for Streptococcus pneumoniae
Test
In hospital
Consider pneumococcal urinary antigen testing (detects S pneumoniae) in people with moderate- or high-severity CAP.[78]
Urinary antigen testing is useful for diagnosing pneumococcal pneumonia in adults and is less affected by prior antibiotic therapy than blood/sputum cultures.[74][77]
Do not routinely order microbiological tests, including urinary antigen tests, in patients with low-severity CAP who present to hospital.[78]
In the community
Do not order microbiological tests routinely, including urinary antigen tests, in patients presenting with CAP who are managed in the community.[74][77]
Evidence: Urinary antigen testing for pneumococcal pneumonia
Studies have shown that the pneumococcal urinary antigen test has significantly greater sensitivity than routine blood or sputum cultures.[121]
Results remain positive in 80% to 90% of patients for up to 7 days after starting antimicrobial treatment.[121]
Result
may be positive
urinary antigen testing for Legionella pneumophila
Test
In hospital
The National Institute for Health and Care Excellence (NICE) recommends considering legionella urinary antigen testing in patients presenting to hospital with moderate- or high-severity CAP who have specific risk factors for legionella infection.[78]
Epidemiological risk factors include:[45][131]
Recent travel, particularly with overnight stays away from home (including hotels or cruise ships)
Exposure to potentially contaminated water systems (e.g., hot tubs, spa pools, or complex plumbing systems)
Recent care at a healthcare facility
Use of respiratory therapy equipment
Association with a known or suspected legionella outbreak
In addition, the following patient groups are at increased risk of legionella infection:[131][132]
Age >50 years
Male sex
Smoking (current or previous)
Chronic lung disease (such as COPD or emphysema)
Underlying comorbidities, including heart disease, diabetes, renal failure, or hepatic failure
Systemic malignancy
Immunocompromise, due to disease or immunosuppressive drugs
Prompt diagnosis of L pneumonia is important because it is associated with significant mortality and has public health implications.[74][77]
The urinary antigen enzyme immunoassay provides rapid, highly specific (>95%), and moderately sensitive (80%) detection of L pneumophila serogroup 1, the predominant cause of legionella-associated CAP in the UK.[74][124][77]
For all patients who test positive for legionella urinary antigen, a lower respiratory tract specimen (e.g., sputum, bronchoalveolar lavage, or postmortem lung tissue) should be obtained as soon as possible and tested for Legionella species by both PCR and culture.[74][77][132]
NICE recommends against routinely ordering microbiological tests, including urinary antigen testing, in patients with low-severity CAP.[78] However, the British Thoracic Society (BTS) advises that urinary antigen testing for L pneumophila may be considered in patients with low-severity CAP in the following situations:[74]
During outbreaks
When there is a specific clinical or epidemiological reason
In the community
Do not order microbiological tests routinely, including urinary antigen tests, in patients presenting with CAP who are managed in the community.[74][77] According to the BTS, legionella urinary antigen testing may be considered in selected circumstances where there is a specific clinical or epidemiological indication, such as during outbreaks (e.g., Legionnaires’ disease) or when relevant epidemiological exposure raises suspicion of legionella infection.[74][77]
Result
may be positive
polymerase chain reaction (PCR) and/or serological testing
Test
PCR can help with rapid identification of the causative pathogen. The British Thoracic Society (BTS) does not recommend routine PCR testing for all patients with CAP. Instead, PCR is positioned as a targeted diagnostic tool to be used in selected clinical circumstances, particularly where identification of viral or atypical pathogens may influence management.[74][77]
In patients with high-severity CAP, the BTS advises that PCR testing of respiratory tract samples (e.g. sputum or upper respiratory tract swabs) may be considered to allow rapid identification of respiratory viruses and atypical pathogens, particularly when:[74][77]
there is poor response to initial antibiotic therapy, and/or
there is a strong clinical suspicion of a viral or atypical cause of pneumonia.
Pathogens of interest include respiratory viruses (e.g., influenza A and B, parainfluenza 1-3, adenovirus, respiratory syncytial virus) and atypical bacteria (e.g., Mycoplasma pneumoniae, Chlamydia pneumoniae, Chlamydia psittaci, Coxiella burnetii, and Pneumocystis jirovecii).[74][77]
In patients with low- or moderate-severity CAP, the BTS advises that PCR testing should not be performed routinely. PCR may be considered only in selected circumstances, including:[74]
Outbreaks
Epidemic years for specific pathogens (e.g., M pneumoniae)
When there is a specific clinical or epidemiological indication raising suspicion of an atypical or viral pathogen
In the BTS CAP guideline, serological tests refer to antibody-based blood tests used to detect infection with atypical respiratory pathogens (e.g., M pneumoniae, Chlamydia species, C burnetii). The BTS does not recommend serology for routine diagnosis of CAP because of its limited value in acute management and states that PCR is preferred where available because it provides earlier and more clinically useful results.[74] However, it notes that serological testing may be considered in selected circumstances, including:[74][77]
when PCR testing is unavailable, particularly for suspected atypical pathogens,
in patients with high-severity CAP where no microbiological diagnosis has been made using other methods and the patient fails to improve,
during outbreaks or epidemic periods, where confirmation of the causative organism is important for public health surveillance, and
for retrospective diagnosis, epidemiological investigation, or audit, rather than to guide initial therapy.
In these settings, paired serology (acute and convalescent samples) is preferred to single titres, as this improves diagnostic reliability.[74]
Result
detection of viral/atypical pathogen antigens or antibodies
SARS-CoV-2 testing (PCR or antigen)
Test
Consider SARS-CoV-2 testing if COVID-19 is clinically suspected and the result would influence management. See Coronavirus disease 2019 (COVID-19).[90]
Result
positive in patients with COVID-19 infection
CT scan of chest
Test
Consider a CT scan of the chest when there is diagnostic doubt (e.g., if the chest x-ray is of poor quality or shows ill-defined consolidation) or there are complicated or atypical findings on the chest x-ray, such as:[74][111]
Cavitation: CT can help identify alternative diagnoses such as tuberculosis, lung cancer, pulmonary infarction, septic pulmonary emboli, infected bulla, or lung abscess.
[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].
Multifocal consolidation: CT can help identify alternative diagnoses such as staphylococcal infection, tuberculosis, aspiration pneumonia, allergic bronchopulmonary aspergillosis, cryptogenic organising pneumonia, or drug-induced pneumonitis.
Pleural effusion: CT (in conjunction with chest ultrasound and guided aspiration) can help identify parapneumonic effusions, empyema, tuberculosis, lung cancer, or reactive/inflammatory effusions.
Approximately 20% to 57% of patients hospitalised with pneumonia develop a parapneumonic effusion.[112]
[Figure caption and citation for the preceding image starts]: Left-sided pleural effusionFrom the collection of Dr R Light. Used with permission [Citation ends].
Result
may show cavitation, pleural effusion, multifocal consolidation, or an underlying neoplasm
lung ultrasound
Test
Depending on availability, lung ultrasound may represent a practical and accessible technique for diagnosing CAP. It is radiation-free and a useful alternative imaging modality when the clinician has appropriate training and equipment. Lung ultrasound is particularly valuable when chest x-ray is not available.
Evidence: Ultrasound in the diagnosis of CAP
Evidence shows that lung ultrasound can diagnose pneumonia in adults with excellent accuracy, including in the emergency department.[103][104][105] A meta-analysis using CT as the reference standard found that ultrasonography was more accurate than chest radiography for diagnosing CAP.[106]
In the UK, the National Institute for Health and Care Excellence (NICE) advises that lung ultrasound can be a useful adjunct in the diagnosis of CAP in selected situations, including:[78]
Rapid point-of-care assessment of a sick or deteriorating patient
Evaluation of possible alternative diagnoses, such as heart failure
Investigation of complications, including pleural effusion or other pleural disease
[Figure caption and citation for the preceding image starts]: Left-sided pleural effusionFrom the collection of Dr R Light. Used with permission [Citation ends].
Similar recommendations are reflected in US guidance:
The American Thoracic Society states that, for adults with suspected CAP, lung ultrasound is an acceptable diagnostic alternative to chest x-ray in medical centres where appropriate clinical expertise is available.[107]
The American College of Physicians recommends point-of-care ultrasound (POCUS) in cases of diagnostic uncertainty in patients presenting with acute dyspnoea.[108]
The American College of Radiology notes that POCUS has particular utility when access to CT is limited, such as when rapid assessment is required or when a patient’s condition limits mobility or transport to the radiology suite.[109]
Practical tip
The diagnosis of CAP using bedside lung ultrasound mainly depends on detecting consolidation. However, consolidation is not always present, as pneumonia may be interstitial or manifest as diffuse pulmonary infiltrates. In such cases, correlate with clinical findings and consider alternative imaging if diagnostic uncertainty remains.
Result
consolidation may be seen; parapneumonic effusion may be seen
thoracocentesis and pleural fluid culture
Test
Perform early diagnostic thoracocentesis in all patients with pleural effusion to assess for an infected pleural space due to a complicated parapneumonic effusion (a parapneumonic effusion that requires drainage, even if not frankly purulent) or empyema.[74][77]
The presence of pus, cloudy fluid, or organisms on Gram stain or culture is diagnostic of empyema.
Non-purulent pleural fluid with pH <7.2 in the context of CAP is consistent with complicated parapneumonic effusion, suggesting bacterial invasion of the pleural space.
Drain pleural fluid in patients with confirmed empyema or non-purulent pleural fluid with pH <7.2.[74]
Result
empyema: pus; cloudy pleural fluid; organisms on Gram stain or culture
complicated parapneumonic effusion: pleural fluid pH <7.2 in the context of CAP
computer tomographic pulmonary angiography (CTPA)
Test
Consider CTPA to evaluate for pulmonary embolism if symptoms came on quickly (within minutes) or if chest pain and breathlessness preceded infective symptoms.[113]
CTPA has the highest diagnostic accuracy among non-invasive imaging modalities for pulmonary embolism.[114]
Result
may show a partial or complete intraluminal filling defect within a pulmonary artery, consistent with thromboembolism.
bronchoscopy
Test
Guidelines from the British Thoracic Society recommend considering bronchoscopy in patients with persisting signs, symptoms, and radiological abnormalities approximately 6 weeks after completing treatment.[74][77]
Result
mucopurulent secretions; endobronchial inflammation; mucus plugging; alternative pathology including malignancy, atypical or resistant pathogens (including tuberculosis), bronchiectasis
serum procalcitonin
Test
The UK National Institute for Health and Care Excellence (NICE) recommends considering measurement of procalcitonin (or C-reactive protein) 3-4 days after starting treatment if there is clinical concern about treatment failure.[78]
Procalcitonin is a peptide precursor of calcitonin, which is responsible for calcium homeostasis.
Raised procalcitonin concentrations are associated with bacterial pneumonia, whereas lower concentrations are more commonly seen in viral and atypical pneumonia. Procalcitonin is particularly raised in cases of pneumococcal pneumonia.[127][128]
Result
may be raised in bacterial pneumonia, particularly pneumococcal pneumonia
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