eMedicine Specialties > Pulmonology > Infectious Lung Diseases

Pneumonia, Bacterial: Differential Diagnoses & Workup

Author: Nader Kamangar, MD, FACP, FCCP, FAASM, Associate Professor of Clinical Medicine, Division of Pulmonary, Critical Care and Sleep Medicine, Multi-campus Pulmonary and Critical Care Fellowship Program, University of California, Los Angeles, David Geffen School of Medicine; Medical Director, Hospitalist/Intensivist Program, Olive View-UCLA Medical Center; Associate Program Director, Combined Pulmonary and Critical Care Fellowship Program, Cedars-Sinai/Olive View-UCLA Medical Center/West Los Angeles Veterans Affairs Medical Center
Coauthor(s): Christina Rager, MD, Resident Physician, Internal and Emergency Medicine, Olive View-University of California at Los Angeles Medical Center; Sat Sharma, MD, FRCPC, Professor and Head, Division of Pulmonary Medicine, Department of Internal Medicine, University of Manitoba; Site Director, Respiratory Medicine, St Boniface General Hospital
Contributor Information and Disclosures

Updated: Aug 21, 2009

Differential Diagnoses

Atelectasis
Pneumococcal Infections
Bronchiectasis
Pneumocystis Carinii Pneumonia
Chronic Bronchitis
Pneumonia, Aspiration
Chronic Obstructive Pulmonary Disease
Pneumonia, Community-Acquired
Foreign Body Aspiration
Pneumonia, Fungal
Influenza
Pneumonia, Viral
Klebsiella Infections
Psittacosis
Lung Abscess
Q Fever
Lung Cancer, Non-Small Cell
Respiratory Failure
Lung Cancer, Oat Cell (Small Cell)
Sepsis, Bacterial
Mycobacterium Avium-Intracellulare
Mycobacterium Kansasii

Workup

Laboratory Studies

A note on diagnosing pneumonia
 
Despite the frequency of pneumonia and the large body of research and literature surrounding it, controversy remains regarding certain aspects of evaluation and management. Much emphasis has been placed on the utility of diagnostic testing versus the role empiric treatment. The following 3 aspects of disease are important in the management of pneumonia, in which diagnostic testing can play a pivotal role:

  • Determining the presence of pneumonia
  • Assessing disease severity at the time of presentation
  • Identifying the causative agent
Differentiation between community-acquired pneumonia (CAP), health care–associated pneumonia (HCAP), hospital-acquired pneumonia (HAP), and other pulmonary pathology at presentation is important for several reasons, but primarily because the varying pathogens implicated in each category dictate the empiric therapy likely to be most useful.10

Laboratory studies
Diagnostic testing in patients with suspected pneumonia is driven mostly by the possibility that the results would significantly alter empiric therapy and management decisions and whether the test is likely to have a high yield.10,23

The following initial tests are indicated with suspected pneumonia:
  • Blood culture, prior to antibiotic therapy
  • Sputum Gram stain and culture, prior to antibiotic therapy (if a good-quality, contaminant-sparse specimen containing <10 squamous epithelial cells per low-power field can be obtained)
  • Sputum, serum, and/or urinary antigen test for Streptococcus pneumoniae
  • Sputum and/or urinary antigen test for Legionella pneumophila
  • Endotracheal aspirate for culture in intubated patients
  • Culture and study of pleural fluid if effusion present
The following additional pathogen-specific tests are available, but they may be less useful:
  • Immune serologies for Mycoplasma pneumoniae, Chlamydophila pneumoniae, L pneumophila, and Coxiella burnetii  - Results usually not available until several weeks after infection
  • Nucleic acid detection (eg, polymerase chain reaction) - Still in development, but pose a major weakness in their extreme sensitivity and potential for false-positive results
Diagnostic testing is also useful in classifying the severity of illness and site-of-care decisions (outpatient vs inpatient vs ICU). The most obvious indication for extensive diagnostic testing is in the critically ill patient.10,24
 
Various systems to assess the severity of disease and risk of death exist and are in wide use, including the PSI/PORT (ie, Pneumonia Severity Index/Patient Outcomes Research Team score), the CURB-65 system (ie, confusion, urea of 7 mmol/L, respiratory rate of 30 breaths/min, and low systolic [90 mm Hg] or diastolic [60 mm Hg] blood pressure), and the APACHE (ie, Acute Physiology and Chronic Health Evaluation), among others. A number of laboratory values are commonly used in the calculation of these risk indices. The following laboratory tests may not be useful for diagnostic purposes but are useful for classifying illness severity and site-of-care/admission decisions25,26,27,28 :
  • Serum chemistry panel (sodium, potassium, bicarbonate, BUN, creatinine, glucose)
  • Arterial blood gas determination (serum pH, arterial oxygen saturation, arterial oxygen pressure)
  • Venous blood gas determination (central venous oxygen saturation)
  • Complete blood cell count
  • Serum free cortisol value
  • Serum lactate level

Imaging Studies

Radiology is generally helpful in detecting suspected pneumonia and identifying the presence of complications; only occasionally can imaging suggest specific pathogens.29
 
The discussion of imaging thoracic infection is usually split into subcategories of patients' immune status, as immunocompetent patients, non-AIDS immunocompromised patients, and patients with AIDS.

Plain chest radiography

Considered the criterion standard for diagnosing the presence of pneumonia, a chest radiograph demonstrating the presence of an infiltrate is required for diagnosis of pneumonia. Note that the accuracy of plain chest radiography for detecting pneumonia decreases depending on the setting of infection: CAP → HAP → ventilator-associated pneumonia (VAP) → patients with acute respiratory distress syndrome (ARDS).10,24
 
The radiographic classification of pneumonia was discussed earlier (see "Anatomic/radiographic patterns of pneumonia" in Background.

Pathogen-specific findings2 are as followed:

  • Lobar pneumonias
    • S pneumoniae: Infection is characterized by homogenous parenchymal lobar opacities with air bronchograms; it can occasionally manifest as a round opacity stimulating a pulmonary mass, called round pneumonia.
    • K pneumoniae: Radiographs may have evidence of lobar expansion with bulging of interlobular fissures due to voluminous inflammatory exudate, as well as cavitations.
    • L pneumophila: Radiologic resolution tends to lag far behind clinical improvement (8 wk to clear).
  • Bronchopneumonias
    • S aureus: Lobar enlargement with bulging of interlobular fissures can be seen in severe cases; abscesses, cavitations (with air-fluid levels), and pneumatoceles are commonly seen; 30-50% of patients develop pleural effusions, half of which are empyemas.
    • P aeruginosa: Radiographic findings tend to be nonspecific and difficult to differentiate from underlying lung disease; usually all lobes are involved, with a predilection for the lower lobes; necrosis and cavitation occur frequently; pulmonary vasculitis can produce areas of pulmonary infarction that radiographically resembles invasive aspergillosis.
    • H influenzae: Pleural effusion is present in approximately half of infected individuals.
  • Aspiration pneumonias: This is seen in the gravity-dependent portions of the lungs (affected by patient positioning); the right lung is affected twice as often as the left lung.

Computed tomography

The role of CT scanning in diagnosis of pneumonia is not yet well defined. For inpatients, CT scanning may identify pulmonary infections earlier than plain radiography.24 In most cases, it can be helpful in the analysis of more complex lung findings and evaluation of other intrathoracic structures.

CT patterns of disease30 may be broken down into abnormalities that cause either increased or decreased lung opacity.

Abnormalities that cause increased lung opacity include the following:
  • Nodular, based on the anatomy of the secondary pulmonary lobule
    • Centrilobular - Further characterized by the presence or absence of tree-in-bud morphology, the presence of which is almost always seen in infection; its absence is likely to expand the differential beyond infectious processes
    • Perilymphatic nodules
    • Random nodules
  • Linear
    • Interlobular septal thickening (smooth, nodular, irregular)
    • Parenchymal bands
    • Subpleural lines
    • Irregular linear opacities
  • Reticular
  • Ground-glass opacity
  • Consolidation
Abnormalities that cause decreased lung opacity include the following:
  • Bronchiectasis
  • Emphysematous change (centilobular, panlobular, paraseptal, irregular)
  • Honeycomb lung and cystic disease
  • Mosaic perfusion and inhomogeneous lung opacity
Ultrasonography
Ultrasonography is useful in evaluating suspected parapneumonic effusions, especially if septations are present within the fluid collection that may not be visible on CT scans. Ultrasonography also has great utility for direct operation by physicians at the patient bedside.29

Procedures

  • Bronchoscopy with or without bronchoalveolar lavage (BAL): Lung tissue can be visually evaluated and bronchial washing specimens can be obtained with the aid of a fiberoptic bronchoscope. Protected brushings and BAL can be performed for fluid analysis and cultures.
  • Nonbronchoscopic bronchoalveolar lavage, mini-BAL: BAL can be performed without the use of a bronchoscope.
  • Transtracheal aspiration for culture: This procedure is mentioned primarily for historical significance. This method of obtaining lower respiratory secretions has been replaced by fiberoptic bronchoscopy.
  • Thoracentesis: This is an essential procedure in patients with a parapneumonic pleural effusion. Obtaining fluid from the pleural space for laboratory analysis allows for the differentiation between simple and complicated effusions. This determination may help guide further therapeutic intervention. The fluid can also be sent for Gram stain and culture.

More on Pneumonia, Bacterial

Overview: Pneumonia, Bacterial
Differential Diagnoses & Workup: Pneumonia, Bacterial
Treatment & Medication: Pneumonia, Bacterial
Follow-up: Pneumonia, Bacterial
References
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References

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Further Reading

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Keywords

bacterial pneumonia, pneumonia, pneumococcus, Streptococcus pneumoniae, S pneumoniae, Haemophilus influenzae, H influenzae, Staphylococcus aureus, S aureus, Legionella, Legionella pneumophila, Mycoplasma, Mycoplasma pneumoniae,  Chlamydophila psittaci, Coxiella burnetii, C burnetii, Pseudomonas, Klebsiella, Klebsiella pneumoniae, K pneumoniae, Moraxella catarrhalis, M catarrhalis, Nocardia, Escherichia coli, Enterobacter, Serratia species, Bacteroides, Peptostreptococcus, Fusobacterium species, hospital-acquired pneumonia, community-acquired pneumonia, CAP, nosocomial pneumonia, viral pneumonia, typical pneumonia, atypical pneumonia, lobular pneumonia, lobar pneumonia, bronchial pneumonia

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Contributor Information and Disclosures

Author

Nader Kamangar, MD, FACP, FCCP, FAASM, Associate Professor of Clinical Medicine, Division of Pulmonary, Critical Care and Sleep Medicine, Multi-campus Pulmonary and Critical Care Fellowship Program, University of California, Los Angeles, David Geffen School of Medicine; Medical Director, Hospitalist/Intensivist Program, Olive View-UCLA Medical Center; Associate Program Director, Combined Pulmonary and Critical Care Fellowship Program, Cedars-Sinai/Olive View-UCLA Medical Center/West Los Angeles Veterans Affairs Medical Center
Nader Kamangar, MD, FACP, FCCP, FAASM is a member of the following medical societies: American Academy of Sleep Medicine, American Association of Bronchology, American College of Chest Physicians, American College of Physicians, American Lung Association, American Medical Association, American Thoracic Society, California Thoracic Society, and Society of Critical Care Medicine
Disclosure: Nothing to disclose.

Coauthor(s)

Christina Rager, MD, Resident Physician, Internal and Emergency Medicine, Olive View-University of California at Los Angeles Medical Center
Christina Rager, MD is a member of the following medical societies: American College of Physicians, American Medical Student Association/Foundation, and Phi Beta Kappa
Disclosure: Nothing to disclose.

Sat Sharma, MD, FRCPC, Professor and Head, Division of Pulmonary Medicine, Department of Internal Medicine, University of Manitoba; Site Director, Respiratory Medicine, St Boniface General Hospital
Sat Sharma, MD, FRCPC is a member of the following medical societies: American Academy of Sleep Medicine, American College of Chest Physicians, American College of Physicians-American Society of Internal Medicine, American Thoracic Society, Canadian Medical Association, Royal College of Physicians and Surgeons of Canada, Royal Society of Medicine, Society of Critical Care Medicine, and World Medical Association
Disclosure: Nothing to disclose.

Medical Editor

Ryland P Byrd Jr, MD, Professor, Department of Internal Medicine, Division of Pulmonary Medicine and Critical Care Medicine, Program Director of Pulmonary Diseases and Critical Care Medicine Fellowship, James H Quillen College of Medicine, East Tennessee State University; Medical Director of Respiratory Therapy, James H Quillen Veterans Affairs Medical Center
Ryland P Byrd Jr, MD is a member of the following medical societies: American College of Chest Physicians and American Thoracic Society
Disclosure: Nothing to disclose.

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: eMedicine Salary Employment

CME Editor

Timothy D Rice, MD, Associate Professor, Departments of Internal Medicine and Pediatrics and Adolescent Medicine, Saint Louis University School of Medicine
Timothy D Rice, MD is a member of the following medical societies: American Academy of Pediatrics and American College of Physicians
Disclosure: Nothing to disclose.

Chief Editor

Zab Mosenifar, MD, Director, Division of Pulmonary and Critical Care Medicine, Director, Women's Guild Pulmonary Disease Institute, Executive Vice Chair, Department of Medicine, Cedars Sinai Medical Center; Professor of Medicine, David Geffen School of Medicine at UCLA
Zab Mosenifar, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, American Federation for Medical Research, and American Thoracic Society
Disclosure: Nothing to disclose.

 
 
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