Community-Acquired Pneumonia (CAP): Severity Assessment, Antibiotic Selection, and ICU Management

1️⃣ SITE OF CARE

CURB-65	Decision
0–1	✅ OPD
2	✅ IPD
≥3	🚨 ICU eval

2️⃣ OPD MANAGEMENT

Patient	Antibiotic
Healthy	Amoxicillin 1 g tid OR Doxy 100 mg bid
Comorbidity	Augmentin + Azithro/Doxy
Thailand note	❌ Avoid FQ mono (TB risk)

3️⃣ IPD (WARD)

Severity	Antibiotic
Non-severe	Ceftriaxone + Azithromycin
Alternative	Amp/Sulbactam + Macrolide

4️⃣ ICU (SEVERE CAP)

Rule	Regimen
🔥 Always combo	β-lactam + macrolide
Example	Ceftriaxone + Azithro

5️⃣ ADD-ON COVERAGE

Condition	Add
MRSA risk	Vancomycin / Linezolid
Pseudomonas	Pip-Tazo / Cefepime / Meropenem
Influenza	Oseltamivir 75 mg bid

6️⃣ VIRAL PCR POSITIVE

Setting	Action
OPD mild	± no ABx
OPD comorb	✅ ABx
IPD/ICU	✅ ABx
All	Reassess 48–72 hr

7️⃣ SUPPORT + DURATION

Item	Plan
O₂	If SpO₂ <92%
Duration	≥5 days
Reassess	48–72 hr

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Community-acquired pneumonia, or CAP, is pneumonia acquired outside the hospital. In real clinical practice, the important question is not only “Does this patient have pneumonia?” but also:

Where should the patient be treated — OPD, IPD ward, or ICU?Which empiric antibiotic should be started?Should we cover MRSA, Pseudomonas, or influenza?

CAP management becomes easy when we follow one rule:

Severity first, antibiotic second.

This article focuses on adult, non-immunocompromised CAP. Immunocompromised patients, active chemotherapy patients, transplant patients, and advanced HIV patients need a separate approach. The IDSA CAP clinical pathway is also designed for adults without immunocompromising conditions.

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1. Step One: Decide OPD, IPD, or ICU

Before choosing antibiotics, classify severity.

The main tools are:

Tool	Best use
CURB-65	Fast bedside tool, practical for exams and wards
PSI / Pneumonia Severity Index	More comprehensive, guideline-preferred for site-of-care decision
IDSA/ATS severe CAP criteria	Decide whether ICU-level care is needed

The IDSA pathway uses severity assessment tools such as CURB-65 or PSI to help decide outpatient versus inpatient treatment, then applies severe CAP criteria to identify ICU-level disease.

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2. CURB-65: Quick Admission Score

CURB-65 components

Each item = 1 point.

Letter	Meaning
C	Confusion or new disorientation
U	Uremia: BUN ≥20 mg/dL
R	Respiratory rate ≥30/min
B	Blood pressure: SBP <90 mmHg or DBP ≤60 mmHg
65	Age ≥65 years

CURB-65 interpretation

CURB-65 score	Risk	Suggested site of care
0–1	Low risk	OPD
2	Moderate risk	IPD ward
3–5	High risk	IPD + ICU evaluation

Exam pearl

CURB-65 tells you “Admit or not?”IDSA/ATS severe CAP criteria tell you “ICU or not?”

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3. ICU Criteria: Severe CAP

A patient has severe CAP if there is:

1 major criterion OR ≥3 minor criteria

Major criteria

Major criteria

Septic shock requiring vasopressors

Respiratory failure requiring mechanical ventilation

Minor criteria

Minor criteria

RR ≥30/min

PaO₂/FiO₂ ≤250

Multilobar infiltrates

Confusion/disorientation

BUN ≥20 mg/dL

WBC <4,000/µL

Platelet <100,000/µL

Core temperature <36°C

Hypotension requiring aggressive fluid resuscitation

The IDSA CAP clinical pathway defines severe CAP using one major criterion or three or more minor criteria, with the same major and minor criteria above.

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4. How to Guess the Possible Pathogen in Real Life

In the real world, we cannot know the pathogen immediately. We estimate it from:

1. Host setting

   • Age

   • Comorbidities

   • Immunity

   • Structural lung disease

2. Local epidemiology

   • Pneumococcal resistance

   • TB prevalence

   • Influenza season

   • Hospital antibiogram

3. CAP severity

   • OPD

   • IPD ward

   • ICU

4. Specific risk factors

   • Prior MRSA or Pseudomonas isolation

   • Recent hospitalization with IV antibiotics

   • Post-influenza pneumonia

   • Bronchiectasis or advanced structural lung disease

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5. OPD CAP Antibiotic Management

About many CAP cases can be managed as outpatient if severity is low and the patient has no hypoxemia, unstable vital signs, inability to take oral drugs, or serious social risk.

A. OPD CAP: Healthy adult, no comorbidity

This means no chronic heart, lung, liver, or renal disease; no diabetes; no alcoholism; no malignancy; and no asplenia.

Preferred monotherapy options

Drug	Dose
Amoxicillin	Amoxicillin (1000 mg) PO tid for 5 days
Doxycycline	Doxycycline (100 mg) PO bid for 5 days
Azithromycin	Azithromycin (500 mg) PO day 1, then 250 mg PO OD day 2–5
Clarithromycin	Clarithromycin (500 mg) PO bid for 5 days

Macrolide monotherapy should only be used when local pneumococcal macrolide resistance is low; this is why many settings prefer amoxicillin or doxycycline first. The ATS/IDSA 2019 CAP guideline is the core reference for these outpatient and inpatient antibiotic strategies. (IDSA)

B. OPD CAP with comorbidities

Comorbidities include:

• Chronic heart disease

• Chronic lung disease

• Chronic liver disease

• Chronic renal disease

• Diabetes mellitus

• Alcoholism

• Malignancy

• Asplenia

Standard guideline option

Use:

Beta-lactam + macrolide/doxycyclineORRespiratory fluoroquinolone monotherapy

Practical Thai approach

In Thailand, we usually prefer:

✅ Dual therapy ❌ Avoid routine respiratory fluoroquinolone monotherapy

Why? Because Thailand has a high TB burden, and fluoroquinolones have activity against Mycobacterium tuberculosis. Empiric fluoroquinolone use for presumed pneumonia can partially treat TB, delay diagnosis, and contribute to fluoroquinolone-resistant TB. WHO-derived data report Thailand’s TB incidence at 146 per 100,000 population in 2024, and systematic review evidence notes concern that fluoroquinolones can delay TB diagnosis in respiratory infections. (World Bank Open Data)

Preferred OPD regimen with comorbidities in Thailand

Regimen	Dose
Amoxicillin/clavulanate + azithromycin	Amoxicillin/clavulanate (875/125 mg) PO bid for 5 days + Azithromycin (500 mg) PO day 1, then 250 mg PO OD day 2–5
Amoxicillin/clavulanate + doxycycline	Amoxicillin/clavulanate (875/125 mg) PO bid for 5 days + Doxycycline (100 mg) PO bid for 5 days

Alternative beta-lactams:

Drug	Dose
Cefpodoxime	Cefpodoxime (200 mg) PO bid
Cefuroxime	Cefuroxime (500 mg) PO bid

Then add:

• Azithromycin, or

• Clarithromycin, or

• Doxycycline

Exam phrase

In TB-endemic settings such as Thailand, CAP with comorbidities is preferably treated with beta-lactam plus macrolide or doxycycline rather than routine fluoroquinolone monotherapy.

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6. IPD CAP Antibiotic Management

Hospitalized CAP is divided into:

1. Non-severe CAP → general ward

2. Severe CAP → ICU

A. Non-severe CAP: IPD ward

Standard regimen

Beta-lactam + macrolide

This is the most practical and Thai-compatible approach.

Beta-lactam option	Dose
Ampicillin/sulbactam	Ampicillin/sulbactam (1.5–3 g) IV q6h
Ceftriaxone	Ceftriaxone (1–2 g) IV OD
Cefotaxime	Cefotaxime (1–2 g) IV q8h
Ceftaroline	Ceftaroline (600 mg) IV q12h

Add one macrolide:

Macrolide	Dose
Azithromycin	Azithromycin (500 mg) IV/PO OD
Clarithromycin	Clarithromycin (500 mg) PO bid

Common ward order example

Ceftriaxone (2 g) IV OD + Azithromycin (500 mg) IV/PO OD

What about respiratory fluoroquinolone monotherapy?

ATS/IDSA allows respiratory fluoroquinolone monotherapy for some hospitalized non-severe CAP patients, but in Thailand we avoid routine use because of TB concerns. This is a local stewardship adaptation, not because fluoroquinolones do not work for CAP.

B. Severe CAP: ICU

Severe CAP requires combination therapy.

❌ Do not use monotherapy in severe CAP.✅ Always combine a beta-lactam with atypical coverage.

Preferred severe CAP regimen

Regimen	Dose
Ceftriaxone + azithromycin	Ceftriaxone (2 g) IV OD + Azithromycin (500 mg) IV OD

Alternative:

Regimen	Dose
Cefotaxime + azithromycin	Cefotaxime (2 g) IV q8h + Azithromycin (500 mg) IV OD
Ampicillin/sulbactam + azithromycin	Ampicillin/sulbactam (3 g) IV q6h + Azithromycin (500 mg) IV OD

ATS/IDSA historically allows either beta-lactam + macrolide or beta-lactam + respiratory fluoroquinolone for severe CAP, but the 2023 ERS/ESICM/ESCMID/ALAT severe CAP guideline specifically suggests adding macrolides, not fluoroquinolones, to beta-lactams as empiric therapy in hospitalized severe CAP. (ERS Publications)

Severe CAP exam pearl

ICU CAP = combination therapy.Macrolide is preferred when possible.

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7. When to Add MRSA Coverage

Do not cover MRSA for every CAP patient. Add MRSA coverage only when risk is present.

MRSA risk factors

MRSA risk factors

Prior MRSA colonization or infection, especially respiratory isolation

Recent hospitalization with parenteral antibiotics within the last 90 days

The IDSA CAP pathway lists MRSA severe CAP risk factors including hospitalization with parenteral antibiotics in the last 90 days and prior MRSA colonization or isolation, especially from the respiratory tract.

MRSA drugs

Drug	Dose
Vancomycin	Vancomycin (15 mg/kg) IV q12h, adjust by renal function and drug monitoring
Linezolid	Linezolid (600 mg) IV/PO q12h

Clinical clues for MRSA pneumonia

• Post-influenza pneumonia

• Necrotizing pneumonia

• Cavitary lesion

• Rapid severe pneumonia

• Septic shock

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8. When to Cover Pseudomonas

Do not use anti-pseudomonal drugs for everyone.

Pseudomonas risk factors

Pseudomonas risk factors

Prior Pseudomonas respiratory isolation

Recent hospitalization with parenteral antibiotics within 90 days

Advanced structural lung disease, such as bronchiectasis or advanced COPD

The IDSA CAP pathway separates non-severe and severe Pseudomonas risk assessment and highlights recent hospitalization with parenteral antibiotics, prior respiratory colonization/infection, and advanced structural lung disease.

Anti-pseudomonal beta-lactam options

Replace the usual beta-lactam with one of the following:

Drug	Dose
Piperacillin/tazobactam	Piperacillin/tazobactam (4.5 g) IV q6h
Cefepime	Cefepime (2 g) IV q8h
Ceftazidime	Ceftazidime (2 g) IV q8h
Imipenem	Imipenem (500 mg) IV q6h
Meropenem	Meropenem (1 g) IV q8h
Aztreonam	Aztreonam (2 g) IV q8h

Then add atypical coverage:

• Azithromycin, or

• Respiratory fluoroquinolone if appropriate

Example

Piperacillin/tazobactam (4.5 g) IV q6h + Azithromycin (500 mg) IV OD

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9. Influenza Pneumonia and Oseltamivir

If influenza is suspected or confirmed, especially in severe CAP or during influenza season, start antiviral therapy early.

Drug	Dose
Oseltamivir	Oseltamivir (75 mg) PO bid for 5 days

CDC recommends starting oseltamivir as soon as possible for hospitalized patients with suspected or confirmed influenza. (CDC)

Practical rule

✅ Do not wait for PCR if the patient has severe CAP during influenza season. ✅ Give oseltamivir early, then adjust when PCR returns.

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10. CAP with Positive Respiratory Virus PCR: Do We Still Give Antibiotics?

This is a common exam and ward trap.

A positive viral PCR does not always mean “no antibiotics.” Viral infection can predispose to bacterial co-infection, especially in older patients, comorbid patients, and severe CAP.

Practical approach

Situation	Antibiotic decision
Healthy OPD, no comorbidity, strong viral picture, low bacterial suspicion	May withhold antibiotics with close follow-up
OPD with comorbidities	Give empiric antibiotics
IPD non-severe CAP	Give empiric antibiotics
Severe CAP / ICU	Give empiric antibiotics
Influenza suspected or confirmed	Add oseltamivir

The 2025 ATS CAP update suggests not prescribing empiric antibiotics for selected low-risk outpatients without comorbidities who have CAP and a positive respiratory virus test, but it suggests empiric antibiotics for outpatients with comorbidities, non-severe inpatients, and severe CAP because of concern for bacterial-viral coinfection.

Common bacterial co-infection organisms

• Streptococcus pneumoniae

• Staphylococcus aureus

• Haemophilus influenzae

• Streptococcus pyogenes

Exam phrase

Positive viral PCR does not automatically stop CAP antibiotics. In comorbid, IPD, or ICU patients, treat empirically first, then de-escalate.

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11. Supportive Management

Antibiotics are not the whole treatment. CAP management also needs respiratory support and general care.

OPD supportive care

• Rest

• Adequate oral hydration

• Antipyretic:

  • Paracetamol (500–1000 mg) PO q6h PRN

• Return precautions:

  • Worsening dyspnea

  • Persistent high fever

  • Confusion

  • Cyanosis

  • Hypotension

  • Poor oral intake

  • SpO₂ dropping

IPD supportive care

General IPD order style:

1. Admit

2. Monitor vital signs and oxygen saturation

3. Consider I/O monitoring if unstable, elderly, septic, renal disease, or heart failure

4. Oxygen therapy if hypoxemic

5. IV fluid if dehydrated or septic, but avoid overload

6. Blood culture/sputum culture if severe CAP or MRSA/Pseudomonas risk

7. Start antibiotics early

8. Reassess within 48–72 hours

Clinical stability in hospitalized CAP includes improvement in fever, heart rate, respiratory rate, oxygenation, blood pressure, and mental status; the IDSA pathway lists stability markers such as Tmax ≤38°C, HR ≤100, RR ≤24, oxygen saturation ≥90%, baseline mental status, and SBP ≥90 mmHg.

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12. De-escalation and Duration

Reassess at 48–72 hours

Ask:

• Is the patient clinically improving?

• Are cultures positive?

• Is MRSA nasal PCR negative?

• Is viral PCR positive with low bacterial suspicion?

• Is procalcitonin low or falling, if used by the hospital?

The IDSA CAP pathway supports stewardship reassessment and states that antibiotics may be discontinued when viral diagnostics are positive, procalcitonin is low or decreased substantially, WBC is <10,000/µL, and bacterial co-infection suspicion is low.

Duration

For many exam settings, write:

Treat for at least 5 days and until clinical stability.

Modern ATS guidance allows shorter courses in selected clinically stable outpatients and non-severe inpatients, with a minimum of 3 days, but severe CAP should receive ≥5 days.

Practical exam-safe answer

CAP antibiotics: usually 5 days, reassess after 48–72 hours, extend if unstable, severe CAP, MRSA, Pseudomonas, abscess, empyema, or bacteremia.

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13. Steroid Note

Steroids are not routine for ordinary non-severe CAP.

The 2025 ATS update recommends not administering systemic corticosteroids in non-severe CAP, while suggesting systemic corticosteroids only for severe CAP, excluding severe influenza pneumonia.

Exam pearl

❌ No routine steroid in non-severe CAP. ❌ Avoid steroid in influenza pneumonia unless another clear indication exists. ✅ Consider steroid only in selected severe CAP according to guideline/local protocol.

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14. Full CAP Management Flowchart

Suspected CAP
↓
Confirm pneumonia:
new infiltrate + respiratory symptom + fever/leukocytosis/hypoxia/abnormal lung sound
↓
Assess severity:
PSI or CURB-65
↓
CURB-65 0–1 / PSI I–III
→ OPD treatment
↓
CURB-65 2 / PSI IV–V
→ IPD ward treatment
↓
1 major or ≥3 minor IDSA/ATS severe CAP criteria
→ ICU treatment
↓
Then ask:
MRSA risk?
Pseudomonas risk?
Influenza season/PCR?
Positive viral PCR?
TB possibility?
↓
Choose antibiotic + supportive care
↓
Reassess at 48–72 hours
↓
De-escalate or continue

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15. Final High-Yield Summary

Clinical setting	Preferred treatment
OPD, healthy adult	Amoxicillin or doxycycline
OPD with comorbidities	Amoxicillin/clavulanate + azithromycin or doxycycline
OPD with comorbidities in Thailand	Prefer dual therapy; avoid routine fluoroquinolone monotherapy because of TB concern
IPD non-severe CAP	Beta-lactam + macrolide
ICU severe CAP	Beta-lactam + macrolide, always combination
MRSA risk	Add vancomycin or linezolid
Pseudomonas risk	Use anti-pseudomonal beta-lactam + atypical coverage
Influenza suspected/confirmed	Add oseltamivir
Viral PCR positive + comorbidity/IPD/ICU	Still give empiric antibiotics initially
Stable after 48–72 hours	De-escalate based on cultures and clinical response

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Take-home message

CAP treatment is not just “give ceftriaxone.” The correct approach is:

Score severity → decide OPD/IPD/ICU → choose empiric antibiotics → add MRSA/Pseudomonas/influenza coverage only when indicated → reassess and de-escalate.

For Thailand, the most important local adaptation is:

Avoid routine respiratory fluoroquinolone monotherapy when beta-lactam-based dual therapy is appropriate, because TB is common and fluoroquinolones can delay TB diagnosis.