Community Acquired Pneumonia

Community Acquired Pneumonia

Community-acquired pneumonia (CAP) is an acute infection of the lung parenchyma occurring in a non-hospitalized individual or developing ≤48 hours of hospital admission, not residing in long-term care and without recent healthcare exposure.

Category	Definition	Clinical relevance
CAP	Outside hospital or ≤48 hr admission	Standard community pathogens
HAP	≥48 hr after admission	Higher MDR risk
VAP	≥48 hr after intubation	ICU-specific, highest MDR risk

Term Healthcare-associated pneumonia (HCAP) is obsolete → removed due to poor specificity for MDR pathogens.

Table of Contents

Etiology

A. Typical Bacterial Pathogens

Organism	Notes
Streptococcus pneumoniae	Most common worldwide
Haemophilus influenzae	COPD, smokers
Moraxella catarrhalis	Elderly, COPD
Staphylococcus aureus	Post-influenza, necrotizing
Gram-negative bacilli	Elderly, comorbidities

B. Atypical Pathogens

Organism	Clinical Clues
Mycoplasma pneumoniae	Young adults, dry cough
Chlamydophila pneumoniae	Mild, prolonged
Legionella pneumophila	Hyponatremia, diarrhea, confusion

C. Viral Causes

Influenza A/B
RSV
SARS-CoV-2
Adenovirus
Human metapneumovirus

Viral CAP often predisposes to secondary bacterial pneumonia

D. Risk-Based Pathogens

Risk Factor	Pathogen
Alcoholism	Klebsiella pneumoniae
Post-influenza	Staphylococcus aureus
Aspiration	Anaerobes
Structural lung disease	Pseudomonas aeruginosa
Immunocompromised	Pneumocystis, fungi

Pathogenesis

Microaspiration of oropharyngeal flora (most common)
Inhalation of aerosols
Hematogenous spread (rare)
Impaired host defenses:

Smoking
Alcohol
COPD
Diabetes
Immunosuppression

Lung Response

Alveolar macrophage activation
Cytokine release (IL-1, TNF-α, IL-6)
Neutrophil influx
Consolidation and impaired gas exchange

Clinical Features

Fever
Cough (productive or dry)
Dyspnea
Pleuritic chest pain(Suggests: lobar pneumonia, PE).
Hemoptysis(PE, tuberculosis, bronchiectasis)

History Clue	Think Of
Diarrhea + Hyponatremia + Confusion	Legionella pneumophila
Rust-colored sputum	Streptococcus pneumoniae
Currant jelly sputum + Alcoholic	Klebsiella pneumoniae
Post-influenza severe pneumonia	Staphylococcus aureus
Bird exposure	Chlamydia psittaci
Hotel/cruise travel	Legionella pneumophila
HIV + Dry cough + Severe hypoxemia	Pneumocystis jirovecii Pneumonia
Aspiration risk + Foul sputum	Anaerobic pneumonia
COPD/Bronchiectasis	Pseudomonas aeruginosa
TB contact + Weight loss

Mycobacterium tuberculosis

Differential Diagnosis

CAP Mimic	Key Clue
Acute heart failure	Orthopnea, elevated BNP,Usually symmetric (but asymmetric pulmonary edema may occur in patients with mitral disease).
Pulmonary embolism	Sudden onset dyspnea, pleuritic pain,Wedge-shaped pulmonary infarcts may masquerade as PNA.
Tuberculosis	Chronic symptoms (>2–3 weeks)
Lung cancer	Non-resolving pneumonia
PJP(Pneumocystis jiroveci pneumonia)	HIV/immunosuppression + severe hypoxemia
Diffuse alveolar hemorrhage	Hemoptysis + falling hemoglobin
Organizing pneumonia	Failure to respond to antibiotics
Aspiration pneumonitis	Aspiration event precedes symptoms
ARDS	Bilateral infiltrates with another trigger
Acute eosinophilic pneumonia	Blood eosinophils over ~300/uL (unusual for severe pneumonia)., new smoking exposure
COPD exacerbation	COPD exacerbation may clinically mimic pneumonia, including hypoxemia, fever, and sputum production. This can be nearly impossible to sort out based on history and physical examination.
atelectasis
septic pulmonary emboli	Diffuse infiltrates that tend to cavitate.

Diagnosis

1. Clinical Diagnosis

There is no single universally accepted diagnostic criterion for CAP. Diagnosis is based on a combination of:

Compatible clinical features
Radiological evidence of a new pulmonary infiltrate
Exclusion of alternative diagnoses

Current recommendations from the Infectious Diseases Society of America and American Thoracic Society require demonstration of an infiltrate on chest imaging along with clinical features suggestive of pneumonia.CAP is substantially misdiagnosed. ~10-30% of patients diagnosed with CAP don’t truly have it.

2. Imaging

Chest X-ray (Mandatory)—>No infiltrate = No pneumonia (except very early disease)

Chest X-ray Finding	Likely Etiology / Clinical Significance
Lobar consolidation (Lobar pneumonia)	Common causes: Streptococcus pneumoniae, Haemophilus influenzae, Klebsiella pneumoniae, Legionella pneumophila
Interstitial infiltrates	Suggests atypical pneumonia due to Mycoplasma pneumoniae, Chlamydia pneumoniae, viral pneumonia (e.g., influenza), or Pneumocystis jirovecii pneumonia (PJP). Noninfectious mimics include heart failure, lymphangitic carcinomatosis, and drug-induced pneumonitis.
Patchy multifocal opacities (Bronchopneumonia/Lobular pneumonia)	Seen with bronchogenic spread of infection; commonly caused by Staphylococcus aureus, H. influenzae, gram-negative bacilli, aspiration, and mixed bacterial infections.
Pleural effusion	May indicate parapneumonic effusion, empyema, tuberculosis, malignancy, or heart failure. Requires further evaluation if moderate to large.
Cavitation	Suggests necrotizing infection. Common causes include Staphylococcus aureus, Streptococcus spp., Klebsiella pneumoniae, Pseudomonas aeruginosa, tuberculosis, and fungal infections.
Unilateral cavity larger than surrounding infiltrate	Strongly suggests an anaerobic lung abscess, especially in patients with aspiration risk factors.
Bulging fissure sign	Classically associated with Klebsiella pneumoniae; can occasionally occur with severe Streptococcus pneumoniae infection due to marked lobar expansion.
Chest wall invasion or extension beyond lung parenchyma	Suggests fungal infection (e.g., aspergillosis, mucormycosis) or tuberculosis. CT chest with IV contrast is recommended to assess extent of invasion.
Normal or inconclusive chest X-ray despite high clinical suspicion of pneumonia	Obtain CT chest, which is significantly more sensitive for detecting infiltrates, abscesses, cavitation, pleural disease, and alternative diagnoses.

CT Chest

Reserved for:

Complications
Non-resolving pneumonia
Immunocompromised
Suspected malignancy
Unusual chest imaging (e.g., radiograph suggests nodular/cavitating pneumonia).

3. Laboratory Investigations

Routine Tests

CBC (↑ WBC / leukopenia = severe)
Neutrophilia with left shift
CRP, Procalcitonin(bacterial > viral; helps in antibiotic stewardship)
Renal function
LFTs
ABG (if hypoxemia)

4. Microbiological Work-up

Outpatients-Not routinely required

Hospitalized / Severe CAP

Sputum Gram stain & culture

Finding	Interpretation
>25 neutrophils (leukocytes) and <10 squamous epithelial cells per low-power field (LPF)	Indicates a good-quality sputum sample that is likely representative of lower respiratory tract secretions.
>10 squamous epithelial cells per LPF	Suggests significant contamination with oropharyngeal secretions; the specimen is generally considered inadequate and should be rejected.
Gram stain shows one predominant bacterial morphology in a good-quality sample	Strongly suggests that the identified organism is the likely cause of pneumonia.
Gram stain shows multiple bacterial morphologies in similar numbers	Usually reflects normal oral flora contamination and has limited diagnostic value for identifying the causative pathogen.

Scenario	Interpretation
Positive culture for S. aureus or gram-negative bacilli, but organism not seen on Gram stain of a high-quality sample	May represent airway colonization or contamination rather than true pneumonia; clinical correlation is required.
Negative culture for S. pneumoniae or H. influenzae	Does not exclude infection, because these organisms can be difficult to recover in culture, especially after prior antibiotic exposure.

Blood cultures (before antibiotics)
Urinary antigen:

Streptococcus pneumoniae(False-positive may occur due to pneumonia within the past several weeks, or recent pneumococcal vaccination.)
Legionella(A negative result doesn’t exclude legionella.)

Viral PCR (influenza, SARS-CoV-2)
Nares PCR for MRSA.
Fungal culture and smear should be ordered if imaging or epidemiological data suggests the possibility of a fungal pneumonia
Bronchoalveolar lavage (BAL) -Bronchoscopy is generally most valuable when pneumonia is atypical, imaging findings are unusual, or the patient is severely immunocompromised and at risk for opportunistic infections. Isolation of an organism from BAL alone should not automatically be considered proof of invasive pneumoniais because of contamination with oropharyngeal flora(This may be reduced by the use of protected brush cultures.)

Severity Assessment

CURB-65 Score

Parameter	Point
Confusion	1
Urea >7 mmol/L	1
RR ≥30/min	1
BP <90 systolic or ≤60 diastolic	1
Age ≥65	1

Interpretation

0–1: Outpatient
2: Hospital admission
≥3: Severe CAP → ICU(ICU admission based on ATS/IDSA criteria, not CURB-65 alone)

PSI (Pneumonia Severity Index)

More accurate but complex
Preferred for mortality prediction

ATS/IDSA Severe CAP Criteria (2019)

Major Criteria (Any 1 = ICU)

Septic shock requiring vasopressors
Respiratory failure requiring mechanical ventilation

Minor Criteria (≥3 = ICU)

RR ≥30
PaO₂/FiO₂ ≤250
Multilobar infiltrates
Confusion
Uremia( BUN >20 mg/dL (>7 mM))
Leukopenia (WBC <4,000/mm3.)
Thrombocytopenia (Platelets <100,000/mm3.)
Hypothermia <36C.
Hypotension requiring fluids

Management

Empiric Antibiotic Therapy (ATS/IDSA 2019)

1. Outpatient – No Comorbidities

Drug	Dose	Duration	Comments
Amoxicillin	1 g every 8 hr	Minimum 5 days	Preferred first-line
Doxycycline	100 mg BID	5 days	Good atypical coverage
Azithromycin*	500 mg Day 1 then 250 mg daily	Total 5 days	Only if local pneumococcal resistance <25%
Azithromycin (Alternative)	500 mg daily	3 days	Acceptable regimen

*Macrolide monotherapy no longer preferred in many regions due to resistance.

2. Outpatient – With Comorbidities

(Chronic heart/lung/liver/kidney disease, diabetes, alcoholism)

Regimen Option 1

(β-Lactam + Macrolide/Doxycycline)

Drug	Dose
Amoxicillin-Clavulanate	875/125 mg BID
OR Amoxicillin-Clavulanate ER	2 g/125 mg BID
PLUS Azithromycin	500 mg Day 1 → 250 mg daily
OR Doxycycline	200 mg PO loading dose f/b 100 mg BID

Doxycycline is generally preferred over azithromycin

Animal exposure (covers zoonotic pneumonias).
Low-key MRSA coverage: Patients are at moderate risk for community-acquired MRSA pneumonia, but not enough risk to justify linezolid/vancomycin.
Azithromycin is preferred for patients with suspected Legionella pneumonia.

Regimen Option 2

(Respiratory Fluoroquinolone Monotherapy)

Drug	Dose	Duration
Levofloxacin	750 mg daily PO	5 days
Moxifloxacin	400 mg daily PO	5–7 days

3. Inpatient – Non-Severe CAP

β-lactam + macrolide or Respiratory fluoroquinolone alone

Drug	Dose
Ceftriaxone	1–2 g IV daily
Cefotaxime	1–2 g IV q8h
Ampicillin-Sulbactam(poor coverage against Gram-negative bacteria.)	1.5–3 g IV q6h
PLUS Azithromycin	500 mg IV/PO daily

4. Severe CAP (ICU)

β-lactam + macrolide(Preferred Regimen)
OR
β-lactam + fluoroquinolone

β-Lactam Options

Drug	Dose
Ceftriaxone(Good choice for severe CAP in patients without risk factors for drug-resistant organisms.)	2 g IV OD
Cefotaxime	2 g IV q8h
Ampicillin-Sulbactam	3 g IV q6h
Ceftaroline	600 mg IV q12h

Macrolide Options

Drug	Dose
Azithromycin(preferred.)	500 mg IV/PO OD
Clarithromycin	500 mg PO BID

5. MRSA Risk(uncommon cause of community-acquired pneumonia)

Prior MRSA infection/colonization
Influenza-associated pneumonia
Recent hospitalization + IV antibiotics (last 90 days)
iv drug abuse
Skin pustule
Shorr score(emcrit)

Drug	Dose
Vancomycin	15–20 mg/kg IV q8–12 hr
OR Linezolid(Preferred-superior lung penetration)	600 mg IV/PO q12 hr

Add to standard CAP regimen.

MRSA PCR has an excellent negative predictive value, so a negative PCR can generally allow for discontinuation of
MRSA coverage should be stopped within <48 hours unless there is some objective data that the patient has MRSA (e.g., positive PCR or positive culture data).

6. Pseudomonas Risk

Previous Pseudomonas isolation
Structural lung disease (bronchiectasis)
Recent hospitalization with IV antibiotics
Severe COPD with frequent exacerbations

The DRIP score may be the best-validated strategy to determine patients at elevated risk of a drug-resistant pneumonia:

Antipseudomonal β-Lactam

Drug	Dose
Piperacillin-Tazobactam	4.5 g IV q6h
Cefepime	2 g IV q8h
Ceftazidime	2 g IV q8h
Meropenem	1 g IV q8h
Imipenem	500 mg IV q6h

PLUS

Second Drug	Dose
Levofloxacin	750 mg IV daily

Influenza-Associated Severe CAP

Drug	Dose
Oseltamivir	75 mg PO/NG BID

Duration: 5 days (longer in ICU patients may be considered)

Aspiration Severe CAP

Routine anaerobic coverage is NOT recommended unless:

Lung abscess
Empyema
Necrotizing pneumonia
Severe periodontal disease

Duration of Therapy

Latest ATS 2025 Update

CAP Type	ATS 2025 Recommendation
Outpatient CAP reaching clinical stability	<5 days may be sufficient (minimum 3 days)
Non-severe inpatient CAP reaching clinical stability	<5 days may be sufficient (minimum 3 days)
Severe CAP	≥5 days recommended

ATS/IDSA 2019 Guideline

Situation	Recommended Duration
Most adults with CAP (outpatient or inpatient)	Minimum 5 days
Stop antibiotics only if clinical stability achieved	≥5 days + clinically stable
Severe CAP, slow response, complications, MDR pathogens	Longer duration (7–14 days or individualized)

When to Extend Therapy Beyond 5 Days

Condition	Typical Duration
Severe CAP with slow clinical improvement	7–10 days
Bacteremic pneumococcal pneumonia	Usually 7 days
MRSA pneumonia	7–21 days (commonly 7–14 days)
Pseudomonas pneumonia	7–14 days
Lung abscess	3–6 weeks
Necrotizing pneumonia	2–4 weeks
Empyema	2–6 weeks (with drainage)
Meningitis, endocarditis, metastatic infection	Pathogen-specific prolonged therapy

Adjunctive Therapies

Therapy	Recommendation
Corticosteroids(Controversial)	Not recommended routinely in CAP (ATS/IDSA 2019). No benefit in most patients and may cause hyperglycemia, secondary infections, delirium, and GI bleeding. Use only when another indication exists, such as refractory septic shock ,acute COPD exacerbation, asthma exacerbation, adrenal insufficiency, or other established indications. May Recommended in severe CAP(Hydrocortisone showed benefit in the CAPE-COD trial.hydrocortisone 50 mg IV q6hr was tapered off within 8-14 days, depending on whether the patient was improving after four days ). Avoid routine use in influenza,funal,T.B pneumonia.
Chest Physiotherapy	Not routinely recommended in uncomplicated CAP. May be considered in selected patients with copious secretions, neuromuscular weakness, ineffective cough, bronchiectasis, or mucus retention, especially in ICU patients. Includes postural drainage, percussion, vibration, assisted coughing, and airway clearance techniques.
Mucolytics (N-acetylcysteine, Carbocisteine, Ambroxol, etc.)	Routine use is not recommended because evidence for improved outcomes is limited. May be considered in selected patients with thick tenacious sputum causing difficulty in expectoration.
Oxygen Therapy	Recommended for hypoxemic patients. Target SpO₂ 92–96% in most patients. In chronic hypercapnic respiratory failure (e.g., COPD), target 88–92%.
Bronchodilators (Salbutamol, Levosalbutamol, Ipratropium)	Not routinely indicated in CAP. Use only when there is evidence of bronchospasm.
Nebulized/Inhaled Budesonide	No routine role in CAP treatment. May be used if the patient has coexisting asthma, COPD exacerbation, eosinophilic airway disease, or another established indication.

The FLORALI trial suggested improved mortality among patients with severe hypoxemia treated with HFNC.
HFNC should be considered in patients with significant work of breathing and/or tachypnea. The goal of HFNC is to reduce the work of breathing and thereby prevent patients from tiring out. For this to be effective, HFNC must be initiated before the patient becomes exhausted.
BiPAP doesn’t allow patients to clear their secretions. Patients treated on BiPAP often do well initially, but eventually may fail due to retained secretions and mucus plugging.

Complications

Pulmonary

Parapneumonic effusion
Empyema
Lung abscess
ARDS

Systemic

Sepsis
Septic shock
AKI
Multiorgan failure

Non-Resolving Pneumonia

Defined as:

No clinical improvement after 48–72 hrs
Radiological non-resolution after 6–8 weeks

Causes:

Wrong diagnosis
Resistant organisms
Tuberculosis
Malignancy
Pulmonary embolism

Negative procalcitonin (<0.25 ng/ml) after three days suggests the presence of a non-infectious complication, whereas persistently elevated procalcitonin suggests active infection.

Prevention

Vaccination

Pneumococcal vaccine

PCV13 + PPSV23 (as per age/risk)

Influenza vaccine (annual)
COVID-19 vaccination

Risk Factor Control

Smoking cessation
Alcohol moderation
Chronic disease control

REFERENCES

Irwin and Rippe’s Intensive Care Medicine 9TH edition
Washington manual 4th edition
ISCCM ICU Protocols 3rd edition
Emcrit-IBCC