Last updated: November 20, 2023

Summarytoggle arrow icon

Pneumonia is a respiratory infection characterized by inflammation of the alveolar space and/or the interstitial tissue of the lungs. In industrialized nations, it is the leading infectious cause of death. Pneumonia is most commonly transmitted via aspiration of airborne pathogens (primarily bacteria, but also viruses and fungi) but may also result from the aspiration of stomach contents. The most likely causal pathogens can be narrowed down based on patient age, immune status, and where the infection was acquired (community-acquired or hospital-acquired). Pneumonia is classified based on clinical features as either typical and atypical; each type has its own spectrum of commonly associated pathogens. Typical pneumonia manifests with sudden onset of malaise, fever, and a productive cough. On auscultation, crackles and bronchial breath sounds are audible. Atypical pneumonia manifests with gradual onset of unproductive cough, dyspnea, and extrapulmonary manifestations. Auscultation is usually unremarkable. Some patients may present with elements of both types. Diagnostics include blood tests for inflammatory parameters and pathogen detection in blood, urine, or sputum samples. Chest x-ray in cases of typical pneumonia shows opacity restricted to one lobe, while x-ray in atypical pneumonia may show diffuse, often subtle infiltrates. Together with the characteristic clinical features, newly developed pulmonary infiltrate on chest x-ray confirms the diagnosis. Management consists of empiric antibiotic treatment and supportive measures (e.g., oxygen administration, antipyretics).

For specific information on the diagnosis and management of pneumonia in pediatric patients, see “Pneumonia in children.”

Etiologytoggle arrow icon


For atypical pneumonia bacterial causes, remember the mnemonic: Atypically, Legions of Clams Mind their P's and Q's!
Legionella pneumophila
Chlamydophila pneumoniae
Mycoplasma pneumoniae
Psittacosis (Chlamydophila psittaci)
Q fever (Coxiella burnetii)

Pneumonia pathogens according to location
Type of pneumonia Common pathogens
Lobar pneumonia
Interstitial pneumonia
Cryptogenic organizing pneumonia
  • Noninfectious
Pneumonia pathogens according to affected population
Type of pneumonia Common pathogens
Pneumonia in immunocompromised patients
Pneumonia in newborns
Pneumonia in children (4 weeks –18 years)
Pneumonia in young adults (18–40 years)
Pneumonia in adults (40–65 years)
Pneumonia in elderly individuals
Recurrent pneumonia

Track my respiration: chlassic strep formation”: C. trachomatis, Mycoplasma, Respiratory syncytial virus, Chlamydia pneumoniae, and Streptococcus pneumoniae are the most common causative agents of pneumonia in children.

Risk factors [7]

Bear in mind immune status and potential exposures when considering potential pathogens in patients with suspected pneumonia.

Consider aspiration pneumonia in patients with altered mental status or other risk factors for aspiration.

Classificationtoggle arrow icon

Pneumonia can be classified according to etiology, location acquired, clinical features, and the area of the lung affected by the pathology.


Location acquired

  • Community-acquired pneumonia (CAP): pneumonia that is acquired outside of a healthcare establishment
  • Hospital-acquired pneumonia (HAP): nosocomial pneumonia, with onset > 48 hours after admission
  • Healthcare-associated pneumonia (HCAP): pneumonia that is acquired in healthcare facilities (e.g., hospital, nursing homes, hemodialysis centers, and outpatient clinics); this terminology is no longer recommended but is included for historical purposes. [10][11]

Clinical features

Area of lung affected by the pathology

Pathophysiologytoggle arrow icon

Routes of infection


  1. Failure of protective pulmonary mechanisms; (e.g., cough reflex, mucociliary clearance , alveolar macrophages )
  2. Infiltration of the pulmonary parenchyma by the pathogen interstitial and alveolar inflammation
  3. Impaired alveolar ventilation ventilation/perfusion (V/Q) mismatch with intrapulmonary shunting (right to left)
  4. Hypoxia due to increased alveolar-arterial oxygen gradient

Pattern of involvement

  • Lobar pneumonia
    • Classic (typical) pneumonia of an entire lobe; primarily caused by pneumococci
    • Characterized by inflammatory intra-alveolar exudate, resulting in consolidation
    • Can involve the entire lobe or the whole lung
Stages of lobar pneumonia
Stages Macroscopic findings Microscopic findings
Congestion (day 1–2)
  • Alveolar lumens with serous exudate, bacteria, and rare inflammatory cells
Red hepatization (day 3–4)
  • Parenchymal consolidation
  • Red-brown
  • Dry and firm
  • Liver–like consistency
  • Reversible
Gray hepatization (day 5–7)
  • Uniformly gray
  • Liver–like consistency
Resolution (day 8 to week 4)
  • Gradual aeration of the affected segment

Clinical featurestoggle arrow icon

Typical pneumonia

Typical pneumonia is characterized by a sudden onset of symptoms caused by lobar infiltration.

Suspect bacterial pneumonia in immunocompromised patients with acute high fever and pleural effusion.

Atypical pneumonia

Atypical pneumonia typically has an indolent course (slow onset) and commonly manifests with extrapulmonary symptoms.

This classification does not have a major impact on patient management because it is not always possible to clearly distinguish between typical and atypical pneumonia.

Diagnosticstoggle arrow icon

Pneumonia is a clinical diagnosis based on history, physical examination, laboratory findings, and CXR findings. Consider microbiological studies and advanced diagnostics based on patient history, comorbidities, severity, and entity of pneumonia. [13]

In ambulatory settings, the combination of normal vital signs and an unremarkable lung examination indicates a very low likelihood of CAP. [14][15]

Laboratory studies


Do not rely on laboratory markers like CRP or procalcitonin to determine the need for antibiotic therapy. [11][18]

Microbiological studies

Indication Microbiological studies to consider [10][11][13]
Any admitted patient
  • MRSA nares swab (PCR and/or culture)

Any patient being treated empirically for MRSA or P. aeruginosa

Severe CAP



Influenza season

All patients in pandemic or epidemic settings

In pandemic or epidemic settings, rule out the prevalent respiratory pathogen (e.g., COVID-19) in all patients with suspected pneumonia. [19]

Avoid routine blood cultures and sputum Gram stain in patients with CAP, except if severe or there is concern for MRSA or Pseudomonas infection. [11]


Chest x-ray (posteroanterior and lateral)

A new pulmonary infiltrate on chest x-ray in a patient with classic symptoms of pneumonia confirms the diagnosis.

Typical pneumonia usually appears as lobar pneumonia on x-ray, while atypical pneumonia tends to appear as interstitial pneumonia. However, the underlying pathogen cannot be conclusively identified based on imaging results alone.

Consider chest CT or empiric treatment if clinical suspicion for CAP remains high despite a negative CXR, as the initial CXR may be falsely negative. [20][21]

Chest CT (usually without contrast)

CT is more sensitive and specific than CXR for the diagnosis of pneumonia.

Lung ultrasound in pneumonia

Point-of-care ultrasound (POCUS) has high sensitivity and specificity for the diagnosis of pneumonia. [23][24]

In the emergency department, consider POCUS to quickly confirm pneumonia and assess for other causes of dyspnea.

Advanced diagnostics for pneumonia


  • Indications
    • Suspected mass (e.g., recurrent pneumonia)
    • Need for pathohistological diagnosis (e.g., biopsy of a central mass discovered on CT)
    • Inconclusive results on CT
    • Poor response to treatment

Diagnostic thoracentesis

Treatmenttoggle arrow icon


Initial stabilization [29][30]

Criteria for hospitalization [11][18]

Every patient should be assessed individually and clinical judgment is the most important factor. The pneumonia severity index (PSI) and the CURB-65 score are tools that can help to determine whether to admit a patient.

  • CURB-65 score [31]
    • Confusion (disorientation, impaired consciousness)
    • Serum Urea > 7 mmol/L (42 mg/dL)
    • Respiratory rate ≥ 30/min
    • Blood pressure: systolic BP ≤ 90 mm Hg or diastolic BP ≤ 60 mm Hg
    • Age 65 years
    • Interpretation
    • CRB-65 score (if serum urea is not known or unavailable)
      • CRB-65 score of 0: The patient may be treated as an outpatient.
      • CRB-65 score of ≥ 1: Hospitalization is recommended.
  • Pneumonia severity index (PSI/PORT score) [32]
    • Patients are assigned to one of five risk classes based on a more complex point system than in CURB-65.
    • Points are distributed based on patient age, comorbidities, and lab results.

The CURB-65 score and PSI are tools for evaluating the risk of mortality. They have not been validated for determining the necessity for ICU admission.

Criteria for ICU admission [11][18]

IDSA/ATS criteria for severe CAP [11]
Major criteria
Minor criteria


  • Severe CAP: one major criterion or ≥ 3 minor criteria

Empiric antibiotic therapy for community-acquired pneumonia [11][33]


Empiric antibiotic therapy for community-acquired pneumonia in an outpatient setting
Patient profile Recommended empiric antibiotic regimen [11]

Previously healthy patients without comorbidities or risk factors for resistant pathogens

Patients with comorbidities or risk factors for resistant pathogens

  • Duration of treatment
    • 5 days of therapy is usually sufficient for CAP that is treated in the outpatient setting.
    • Any patient being treated in a primary care setting should be re-examined after 48–72 hours to evaluate the efficacy of the prescribed antibiotic.
  • Additional considerations: Knowing local resistance patterns of S. pneumoniae to macrolides is critical when deciding on an empiric antibiotic regimen.


Empiric antibiotic therapy for community-acquired pneumonia in an inpatient setting
Patient profile Recommended empiric antibiotic regimen [11]

Nonsevere CAP/non-ICU treatment

Severe CAP/ICU treatment

Risk factors for Pseudomonas aeruginosa

Risk factors for MRSA

If aztreonam is used as an alternative to other β-lactam antibiotics, additional coverage for MSSA must be included (e.g., a fluoroquinolone).

Empiric antibiotic therapy for hospital-acquired pneumonia [10]

Empiric antibiotic therapy for hospital-acquired pneumonia
Patient profile Recommended empiric antibiotic regimen [10]

Patients not at high risk for mortality and without risk factors for MRSA infection

Patients not at high risk for mortality but with risk factors for MRSA infection

Patients at high risk for mortality

Patients with structural lung disease (e.g., cystic fibrosis, bronchiectasis)

  • Duration of treatment
    • Empiric antibiotic therapy should be narrowed and/or de-escalated as soon as feasible.
    • Seven days of therapy are usually sufficient. [10]
  • Additional considerations: Resistance patterns can vary widely; local antibiograms should be considered when starting empiric treatment.

Patients with structural lung disease and/or at high risk for mortality should receive double antipseudomonal coverage!

Empiric antibiotic therapy for ventilator-associated pneumonia [10]

Supportive therapy for pneumonia

Acute management checklisttoggle arrow icon

Pathogen-specific pneumoniatoggle arrow icon

Mycoplasma pneumonia [35]

Other types of pathogen-specific pneumonia

Aspiration pneumoniatoggle arrow icon


Patients may develop aspiration pneumonitis without pneumonia, aspiration pneumonia without pneumonitis, or aspiration pneumonitis complicated by pneumonia. [36]


Aspiration pneumonitis and pneumonia are unusual following aspiration of tube feeds or blood, which are typically high-pH and sterile. [36]

Clinical features [36][37]

Diagnostics of pulmonary aspiration

Clinical diagnosis supported by characteristic laboratory and imaging findings

The initial CXR may be negative in early aspiration pneumonia. [36]

Treatment of pulmonary aspiration [36]

Aspiration pneumonia requires antibiotic therapy while aspiration pneumonitis typically self-resolves within 24–48 hours with supportive care alone. [11][41]

Avoid routine anaerobic coverage for aspiration pneumonia without lung abscess, empyema, or severe periodontal disease. [40]


Prevention [36]

Complicationstoggle arrow icon

We list the most important complications. The selection is not exhaustive.

Prognosistoggle arrow icon

  • Mortality increases with age.
  • The mortality risk can be evaluated with the CURB-65 score. [42]
    • Score 0: ∼ 1%
    • Score 1–2: ∼ 10%
    • Score 3: ∼ 14%
    • Score 4: ∼ 40%
  • HAP is associated with a mortality rate of > 20%.

Preventiontoggle arrow icon

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Referencestoggle arrow icon

  1. Olubamwo OO, Onyeka IN, Aregbesola A, et al. Determinants of hospitalizations for pneumonia among Finnish drug users. SAGE Open Medicine. 2018; 6: p.205031211878431.doi: 10.1177/2050312118784311 . | Open in Read by QxMD
  2. Mishra K, Bhardwaj P, Mishra A, Kaushik A. Acute Chlamydia trachomatis respiratory infection in Infants. Journal of Global Infectious Diseases. 2011; 3 (3): p.216.doi: 10.4103/0974-777x.83525 . | Open in Read by QxMD
  3. Hammerschlag MR. Chlamydia trachomatis and Chlamydia pneumoniae Infections in Children and Adolescents. Pediatrics in Review. 2004; 25 (2): p.43-51.doi: 10.1542/pir.25-2-43 . | Open in Read by QxMD
  4. Chlamydia pneumoniae Infection. Updated: January 10, 2019. Accessed: December 10, 2020.
  5. Simonetti AF, Viasus D, Garcia-Vidal C, Carratalà J. Management of community-acquired pneumonia in older adults. Therapeutic Advances in Infectious Disease. 2014; 2 (1): p.3-16.doi: 10.1177/2049936113518041 . | Open in Read by QxMD
  6. Stupka JE, Mortensen EM, Anzueto A, Restrepo MI. Community-acquired pneumonia in elderly patients. Aging Health. 2009; 5 (6): p.763-774.doi: 10.2217/ahe.09.74 . | Open in Read by QxMD
  7. Jameson JL, Fauci AS, Kasper DL, Hauser SL, Longo DL, Loscalzo J. Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2). McGraw-Hill Education / Medical ; 2018
  8. Cordier J-F. Cryptogenic organising pneumonia. European Respiratory Journal. 2006; 28 (2): p.422-446.doi: 10.1183/09031936.06.00013505 . | Open in Read by QxMD
  9. Abers MS, Sandvall BP, Sampath R et al. Postobstructive Pneumonia: An Underdescribed Syndrome. Clin Infect Dis. 2016; 62 (8): p.957-961.doi: 10.1093/cid/civ1212 . | Open in Read by QxMD
  10. Kalil AC, Metersky ML, Klompas M, et al. Management of Adults With Hospital-acquired and Ventilator-associated Pneumonia: 2016 Clinical Practice Guidelines by the Infectious Diseases Society of America and the American Thoracic Society. Clin Infect Dis. 2016; 63 (5): p.e61-e111.doi: 10.1093/cid/ciw353 . | Open in Read by QxMD
  11. Metlay JP, Waterer GW, Long AC, et al. Diagnosis and Treatment of Adults with Community-acquired Pneumonia. An Official Clinical Practice Guideline of the American Thoracic Society and Infectious Diseases Society of America. Am J Respir Crit Care Med. 2019; 200 (7): p.e45-e67.doi: 10.1164/rccm.201908-1581st . | Open in Read by QxMD
  12. Löffler B, Niemann S, Ehrhardt C et al. Pathogenesis of Staphylococcus aureus Necrotizing Pneumonia. Expert Rev Anti Infect Ther. 2014; 11 (10): p.1041-1051.doi: 10.1586/14787210.2013.827891 . | Open in Read by QxMD
  13. Mandell LA, Niederman MS. Aspiration Pneumonia. N Engl J Med. 2019; 380 (7): p.651-663.doi: 10.1056/nejmra1714562 . | Open in Read by QxMD
  14. Neill S, Dean N. Aspiration pneumonia and pneumonitis. Curr Opin Infect Dis. 2019; 32 (2): p.152-157.doi: 10.1097/qco.0000000000000524 . | Open in Read by QxMD
  15. Son YG, Shin J, Ryu HG. Pneumonitis and pneumonia after aspiration.. Journal of dental anesthesia and pain medicine. 2017; 17 (1): p.1-12.doi: 10.17245/jdapm.2017.17.1.1 . | Open in Read by QxMD
  16. Sun JC, V. Joffe H. The Most Common Inpatient Problems in Internal Medicine. Saunders ; 2007
  17. Vedamurthy A, Rajendran I, Manian F. Things We Do for No Reason™: Routine Coverage of Anaerobes in Aspiration Pneumonia. J Hosp Med. 2020; 15 (12): p.754-756.doi: 10.12788/jhm.3506 . | Open in Read by QxMD
  18. Dragan V, Wei Y, Elligsen M, Kiss A, Walker SAN, Leis JA. Prophylactic Antimicrobial Therapy for Acute Aspiration Pneumonitis. Clin Infect Dis. 2018; 67 (4): p.513-518.doi: 10.1093/cid/ciy120 . | Open in Read by QxMD
  19. Lim WS, Baudouin SV, George RC, et al. BTS guidelines for the management of community acquired pneumonia in adults: update 2009. Thorax. 2009; 64 (Suppl 3): p.iii1-iii55.doi: 10.1136/thx.2009.121434 . | Open in Read by QxMD
  20. Ebell MH, Chupp H, Cai X, Bentivegna M, Kearney M. Accuracy of Signs and Symptoms for the Diagnosis of Community‐acquired Pneumonia: A Meta‐analysis. Acad Emerg Med.. 2020; 27 (7): p.541-553.doi: 10.1111/acem.13965 . | Open in Read by QxMD
  21. Marchello CS, Ebell MH, Dale AP, Harvill ET, Shen Y, Whalen CC. Signs and Symptoms That Rule out Community-Acquired Pneumonia in Outpatient Adults: A Systematic Review and Meta-Analysis. J Am Board Fam Med.. 2019; 32 (2): p.234-247.doi: 10.3122/jabfm.2019.02.180219 . | Open in Read by QxMD
  22. Kamat IS, Ramachandran V, Eswaran H, Abers MS, Musher DM. Low procalcitonin, community acquired pneumonia, and antibiotic therapy. Lancet Infect Dis. 2018; 18 (5): p.496-497.doi: 10.1016/s1473-3099(18)30215-9 . | Open in Read by QxMD
  23. Rhee C. Using Procalcitonin to Guide Antibiotic Therapy. Open Forum Infectious Diseases. 2016: p.ofw249.doi: 10.1093/ofid/ofw249 . | Open in Read by QxMD
  24. Smith MD, Fee C, Mace SE, et al. Clinical Policy: Critical Issues in the Management of Adult Patients Presenting to the Emergency Department With Community-Acquired Pneumonia. Ann Emerg Med. 2021; 77 (1): p.e1-e57.doi: 10.1016/j.annemergmed.2020.10.024 . | Open in Read by QxMD
  25. COVID-19 Treatment Guidelines Panel. Coronavirus Disease 2019 (COVID-19) Treatment Guidelines. National Institutes of Health. 2022. Updated: May 31, 2022. Accessed: June 11, 2022.
  26. Loubet P, Tubiana S, Claessens YE, et al. Community-acquired pneumonia in the emergency department: an algorithm to facilitate diagnosis and guide chest CT scan indication. Clin Microbiol Infect.. 2020; 26 (3): p.382.e1-382.e7.doi: 10.1016/j.cmi.2019.06.026 . | Open in Read by QxMD
  27. Upchurch CP, Grijalva CG, Wunderink RG, et al. Community-Acquired Pneumonia Visualized on CT Scans but Not Chest Radiographs. Chest. 2018; 153 (3): p.601-610.doi: 10.1016/j.chest.2017.07.035 . | Open in Read by QxMD
  28. Nambu A. Imaging of community-acquired pneumonia: Roles of imaging examinations, imaging diagnosis of specific pathogens and discrimination from noninfectious diseases. World Journal of Radiology. 2014; 6 (10): p.779.doi: 10.4329/wjr.v6.i10.779 . | Open in Read by QxMD
  29. Llamas-Álvarez AM, Tenza-Lozano EM, Latour-Pérez J. Accuracy of Lung Ultrasonography in the Diagnosis of Pneumonia in Adults. Chest. 2017; 151 (2): p.374-382.doi: 10.1016/j.chest.2016.10.039 . | Open in Read by QxMD
  30. Orso D, Guglielmo N, Copetti R. Lung ultrasound in diagnosing pneumonia in the emergency department. Eur J Emerg Med.. 2018; 25 (5): p.312-321.doi: 10.1097/mej.0000000000000517 . | Open in Read by QxMD
  31. Qaseem A, Etxeandia-Ikobaltzeta I, Mustafa RA, et al. Appropriate Use of Point-of-Care Ultrasonography in Patients With Acute Dyspnea in Emergency Department or Inpatient Settings: A Clinical Guideline From the American College of Physicians. Ann Intern Med. 2021; 174 (7): p.985-993.doi: 10.7326/m20-7844 . | Open in Read by QxMD
  32. Volpicelli G, Elbarbary M, et al. International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Med. 2012; 38 (4): p.577-591.doi: 10.1007/s00134-012-2513-4 . | Open in Read by QxMD
  33. Blaivas M. Lung Ultrasound in Evaluation of Pneumonia. J Ultrasound Med.. 2012; 31 (6): p.823-826.doi: 10.7863/jum.2012.31.6.823 . | Open in Read by QxMD
  34. Light RW. Parapneumonic Effusions and Empyema. Proc Am Thorac Soc. 2006; 3 (1): p.75-80.doi: 10.1513/pats.200510-113jh . | Open in Read by QxMD
  35. Phua J, Dean NC, Guo Q, Kuan WS, Lim HF, Lim TK. Severe community-acquired pneumonia: timely management measures in the first 24 hours. Crit Care. 2016; 20 (1).doi: 10.1186/s13054-016-1414-2 . | Open in Read by QxMD
  36. Walls R, Hockberger R, Gausche-Hill M. Rosen's Emergency Medicine. Elsevier Health Sciences ; 2018
  37. Lim WS. Defining community acquired pneumonia severity on presentation to hospital: an international derivation and validation study. Thorax. 2003; 58 (5): p.377-382.doi: 10.1136/thorax.58.5.377 . | Open in Read by QxMD
  38. Fine MJ, Auble TE, Yealy DM, et al. A Prediction Rule to Identify Low-Risk Patients with Community-Acquired Pneumonia. N Engl J Med. 1997; 336 (4): p.243-250.doi: 10.1056/nejm199701233360402 . | Open in Read by QxMD
  39. Mandell LA, Wunderink RG, Anzueto A, et al. Infectious Diseases Society of America/American Thoracic Society Consensus Guidelines on the Management of Community-Acquired Pneumonia in Adults. Clinical Infectious Diseases. 2007; 44 (Supplement_2): p.S27-S72.doi: 10.1086/511159 . | Open in Read by QxMD
  40. Siempos II, Vardakas KZ, Kopterides P, Falagas ME. Adjunctive therapies for community-acquired pneumonia: a systematic review. J Antimicrob Chemother. 2008; 62 (4): p.661-668.doi: 10.1093/jac/dkn283 . | Open in Read by QxMD
  41. File TM Jr. Treatment of Hospital-acquired and Ventilator-associated Pneumonia in Adults. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. Last updated: June 26, 2017. Accessed: January 15, 2018.
  42. Mycoplasma pneumoniae Infections. Updated: June 5, 2020. Accessed: December 10, 2020.

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