Overview

Community Acquired Pneumonia (CAP)

Name: Community Acquired Pneumonia
Creator: MedVellum

1. Clinical Overview

Definition

Community Acquired Pneumonia (CAP) is an acute infection of the lung parenchyma acquired outside of hospital (or within 48 hours of admission). It causes alveolar filling with inflammatory cells and exudate, impairing gas exchange.

The CAP Triad

Symptoms: Cough (productive), Fever, Dyspnoea
Signs: Focal chest signs (Crackles, Bronchial breathing)
Radiology: New infiltrate on CXR

Key Facts (High Yield for Exams)

Fact	Detail
Commonest Organism	Streptococcus pneumoniae (~40% of all CAP)
Atypical Organisms	Mycoplasma, Legionella, Chlamydophila
Severity Scoring	CURB-65 (Confusion, Urea, RR, BP, Age ≥65)
CURB-65 Score 0-1	Low Risk – Outpatient
CURB-65 Score 2	Moderate – Consider Admission
CURB-65 Score 3-5	Severe – Admit (≥4 = ICU)
First-Line (Low Severity)	Amoxicillin 500mg TDS
First-Line (Moderate/Severe)	Co-Amoxiclav + Clarithromycin
Duration	5 days (Low severity), 7-10 days (Severe)

Clinical Pearls

"If in doubt, X-ray the chest" – Clinical signs can be subtle, especially in elderly.
CURB-65 guides disposition, but clinical judgement is paramount.
Cover Atypicals if moderate-severe, or if typical antibiotics fail.
Legionella – Think travel, air conditioning, outbreaks. Check Urine Antigen.
Follow-up CXR at 6 weeks – To exclude underlying malignancy in smokers/elderly.

2. Epidemiology

Incidence

UK: ~220,000 hospital admissions/year.
Community: 5-11 per 1000 adults/year.
Mortality: ~10% in hospitalised patients; ~30% in ICU.

Common Organisms

Organism	Proportion	Clinical Clues
Streptococcus pneumoniae	30-40%	Classic "Pneumococcal" CAP, Rusty sputum
Haemophilus influenzae	10-15%	COPD, Smokers
Mycoplasma pneumoniae	10-15%	Younger patients, Dry cough, Cyclical epidemics
Legionella pneumophila	5%	Travel, A/C exposure, Hyponatraemia, Diarrhoea
Staphylococcus aureus	5%	Post-influenza, IVDU, Cavitation
Chlamydophila pneumoniae	5%	Mild, Prolonged symptoms
Gram-Negatives (Klebsiella)	Rare	Alcoholics, Diabetics, Aspiration
Viral	10-20%	Influenza, COVID-19, RSV

Risk Factors

Factor	Mechanism
Age >65	Immunosenescence
Smoking	Impaired mucociliary clearance
COPD / Chronic Lung Disease	Structural damage, Colonisation
Immunosuppression	Steroids, Chemotherapy, HIV
Diabetes	Impaired neutrophil function
Alcohol Excess	Aspiration risk, Impaired immunity
Heart Failure	Pulmonary congestion, Impaired clearance

3. Pathophysiology

Step 1: Pathogen Entry

Inhalation of droplets containing organism.
Aspiration of oropharyngeal secretions (common in elderly, reduced consciousness).
Haematogenous spread (rare – e.g., endocarditis).

Step 2: Evasion of Defences

Normal defences: Mucociliary escalator, Alveolar macrophages, IgA.
Virulent organisms (S. pneumoniae) have polysaccharide capsule evading phagocytosis.
Large inoculum or impaired host → Infection establishes.

Step 3: Inflammatory Response

Alveolar macrophages recognise pathogen → Release pro-inflammatory cytokines (IL-1, IL-6, TNF-α).
Recruitment of neutrophils → Exudate accumulates in alveoli.
Consolidation: Alveoli fill with inflammatory cells, fibrin, fluid → Impaired gas exchange.

Step 4: Clinical Manifestations

Fever: Systemic cytokines (IL-1, TNF-α) act on hypothalamus.
Cough: Inflammatory exudate stimulates cough receptors.
Dyspnoea: V/Q mismatch, Hypoxia.
Pleuritic Pain: Inflammation of pleura.

Step 5: Resolution (or Complications)

Most cases resolve with antibiotics + host immunity.
Complications: Empyema, Lung Abscess, Sepsis, ARDS, Respiratory Failure.

Radiological Patterns

Pattern	Organisms	Description
Lobar Consolidation	S. pneumoniae	Homogeneous opacity affecting one lobe
Bronchopneumonia	S. aureus, H. influenzae	Patchy, Bilateral, Follows bronchi
Interstitial	Mycoplasma, Viral	Reticular pattern, Diffuse
Cavitation	S. aureus, Klebsiella, TB, Anaerobes	Necrosis with cavity formation

4. Clinical Presentation

Typical Symptoms

Symptom	Characteristics
Cough	Productive (Purulent sputum). May be dry initially (Atypicals).
Fever	High (>38°C), Rigors
Dyspnoea	Progressive, Exertional
Pleuritic Chest Pain	Sharp, Worse on inspiration
Sputum	Purulent (Yellow/Green), Rusty (Pneumococcal)
Fatigue/Malaise	Systemic illness

Atypical Presentation (Elderly, Immunocompromised)

Red Flags (Suggest Severe CAP)

⚠️ Confusion (New or worsened)
⚠️ Respiratory Rate ≥30/min
⚠️ Hypotension (SBP <90, DBP ≤60)
⚠️ Urea >7 mmol/L
⚠️ Age ≥65
⚠️ SpO2 <92% on Air
⚠️ Multilobar Involvement

Confusion (May be only sign)

Common presentation.

Falls

Common presentation.

Anorexia

Common presentation.

Absence of Fever

Common presentation.

5. Clinical Examination

Inspection

Tachypnoea, Use of accessory muscles.
Central cyanosis (Severe hypoxia).
Cachexia (Underlying malignancy/TB?).

Palpation

Reduced chest expansion (Affected side).
Increased vocal fremitus (Consolidation).

Percussion

Dull over consolidation.
Stony Dull if effusion (Parapneumonic / Empyema).

Auscultation

Crackles (Coarse, Inspiratory).
Bronchial Breathing (Harsh, Expiratory = Inspiratory – indicates consolidation).
Decreased Air Entry (Effusion / Collapse).
Pleural Rub (Inflamed pleura).
Increased Vocal Resonance / Aegophony (Consolidation).

Severity Assessment – CURB-65

Criteria	Points
Confusion (New, AMTS ≤8)	1
Urea >7 mmol/L	1
Respiratory Rate ≥30/min	1
Blood Pressure (SBP <90 or DBP ≤60)	1
Age ≥65	1

Interpretation:

0-1: Low risk (~1% mortality). Consider outpatient.
2: Moderate risk (~9% mortality). Consider admission.
3-5: High risk (~22% mortality). Hospital admission, consider ICU.

6. Investigations

Immediate

Investigation	Purpose
CXR (PA)	Confirms diagnosis, Assesses extent (Lobar/Multilobar), Identifies complications (Effusion, Abscess).
SpO2 / ABG	Assess hypoxia, Respiratory failure (Type 1 or 2).
FBC	WCC (Raised, or Low = Sepsis).
U&Es	Urea for CURB-65. AKI.
CRP	Acute phase marker. Often very high (>100).
LFTs	Baseline, May be deranged in Legionella.
Blood Cultures (x2)	If admitted, Before antibiotics.
Sputum Culture	If expectorate available. Guides de-escalation.
Urinary Antigens	Legionella (L. pneumophila Serogroup 1), Pneumococcal.
Procalcitonin	Distinguishes Bacterial vs Viral. Guides duration.

Additional (If Severe / ICU)

Investigation	Indication
CT Chest	Abscess, Empyema, Underlying pathology.
Thoracentesis	Parapneumonic effusion – Rule out Empyema.
Bronchoscopy / BAL	Immunocompromised, Non-resolving.
HIV Test	If risk factors or unusual organisms (PCP).

7. Management

Management Algorithm (ASCII)

            COMMUNITY ACQUIRED PNEUMONIA
                       ↓
┌─────────────────────────────────────────────────────────────────┐
│         STEP 1: CONFIRM DIAGNOSIS                               │
│  - Clinical: Cough, Fever, Dyspnoea, Crackles                   │
│  - CXR: New infiltrate                                          │
└─────────────────────────────────────────────────────────────────┘
                       ↓
┌─────────────────────────────────────────────────────────────────┐
│         STEP 2: ASSESS SEVERITY (CURB-65)                       │
├─────────────────────────────────────────────────────────────────┤
│  0-1: LOW RISK → Outpatient treatment                           │
│  2: MODERATE RISK → Consider Admission                          │
│  3-5: HIGH RISK → Hospital Admission (≥4 consider ICU)          │
└─────────────────────────────────────────────────────────────────┘
                       ↓
┌─────────────────────────────────────────────────────────────────┐
│         STEP 3: EMPIRICAL ANTIBIOTICS                           │
├─────────────────────────────────────────────────────────────────┤
│  LOW SEVERITY (0-1):                                            │
│    - Amoxicillin 500mg TDS PO (5 days)                          │
│    - Alternative: Doxycycline or Clarithromycin (if Penicillin  │
│      allergy)                                                   │
│                                                                 │
│  MODERATE SEVERITY (2):                                         │
│    - Amoxicillin 500mg TDS PO + Clarithromycin 500mg BD PO      │
│      (5-7 days)                                                 │
│                                                                 │
│  HIGH SEVERITY (3-5):                                           │
│    - Co-Amoxiclav 1.2g TDS IV + Clarithromycin 500mg BD IV     │
│    - Alternative: Ceftriaxone 2g OD IV + Clarithromycin        │
│    - Duration: 7-10 days                                        │
└─────────────────────────────────────────────────────────────────┘
                       ↓
┌─────────────────────────────────────────────────────────────────┐
│         STEP 4: SUPPORTIVE CARE                                 │
│  - Oxygen: Target SpO2 94-98% (88-92% if COPD)                  │
│  - IV Fluids: If dehydrated or hypotensive                      │
│  - VTE Prophylaxis: LMWH                                        │
│  - Analgesia: Paracetamol for fever/pain                        │
└─────────────────────────────────────────────────────────────────┘
                       ↓
┌─────────────────────────────────────────────────────────────────┐
│         STEP 5: REVIEW & ESCALATE IF NEEDED                     │
│  - If not improving at 48-72h → Reassess, Consider CT, Unusual  │
│    organisms, Empyema, Abscess                                  │
│  - Legionella: Add Levofloxacin if suspected                    │
│  - ICU Referral if: Refractory hypoxia, Shock, ARDS             │
└─────────────────────────────────────────────────────────────────┘

1. Antibiotic Therapy (BTS/NICE Guidelines)

Severity	First-Line	Duration
Low (CURB 0-1)	Amoxicillin 500mg TDS PO	5 days
Moderate (CURB 2)	Amoxicillin + Clarithromycin PO	5-7 days
High (CURB 3-5)	Co-Amoxiclav IV + Clarithromycin IV	7-10 days
Penicillin Allergy	Clarithromycin or Doxycycline monotherapy	As above

2. Oxygen Therapy

Target SpO2: 94-98%.
COPD/CO2 Retention Risk: 88-92%.
Avoid hyperoxia in at-risk patients.

3. IV Fluids

For dehydration or hypotension.
Be cautious in cardiac/renal failure.

4. VTE Prophylaxis

All admitted patients should have LMWH (unless contraindicated).

5. Monitoring Response

Clinical: Temp, RR, HR, SpO2.
Biochemical: CRP should fall by 50% at 3-4 days.
If Not Improving: Consider complications, alternative diagnosis, resistant organism.

8. Complications

Complication	Incidence	Mechanism	Management
Parapneumonic Effusion	30-50%	Inflammatory exudate in pleural space	Usually resolves. Tap if large.
Empyema	5-10%	Infected pleural fluid (pH <7.2, Glucose low)	Chest drain, +/- Surgery.
Lung Abscess	Rare	Necrosis with cavity	Prolonged antibiotics, Drainage if large.
Sepsis / Septic Shock	Variable	Systemic infection	Sepsis Six, ICU.
ARDS	Rare	Severe inflammatory response	Lung protective ventilation.
Respiratory Failure	10-20%	V/Q mismatch, ARDS	Oxygen, NIV, Intubation.

9. Prognosis & Outcomes

Mortality

Outpatient CAP: <1%.
Hospitalised CAP: ~10%.
ICU CAP: ~30%.

Predictors of Poor Outcome

High CURB-65 score.
Multilobar involvement.
Bacteraemia.
Underlying comorbidities.

Post-CAP Follow-Up

CXR at 6 weeks: To ensure resolution and exclude underlying malignancy.
Especially important in smokers and age >50.

10. Evidence & Guidelines

BTS Guidelines for CAP (2009, Updated 2019 Annotation)

CURB-65 for severity assessment.
Amoxicillin first-line for low severity.
Add Macrolide for moderate-severe.
5 days treatment for uncomplicated CAP.

NICE Pneumonia Guidelines (NG138, 2019)

Similar to BTS.
CRB-65 (No Urea) for community use.
Procalcitonin to guide antibiotic cessation.

Key Studies

Study	Year	Finding	PMID
Lim WS et al. (CURB-65)	2003	Validated CURB-65 for CAP severity.	12728155
Fine MJ et al. (PSI)	1997	Pneumonia Severity Index development.	8995086
Mandell LA et al.	2007	IDSA/ATS CAP Guidelines.	17278083
BTS CAP Guideline	2009	British Thoracic Society recommendations.	19892876
NICE NG138	2019	Pneumonia diagnosis and management.	Guidelines
Wunderink RG et al.	2014	SCCM CAP Guidelines.	25167087
Woodhead M et al.	2011	ERS/ESCMID CAP Guidelines.	21474502
Cilloniz C et al.	2016	CAP mortality predictors.	27156825
Jain S et al. (EPIC)	2015	Etiology of CAP in USA.	26193795
Torres A et al.	2017	Severe CAP management.	28167415
Postma DF et al.	2015	Beta-lactam vs Beta-lactam+Macrolide.	25671253
Sligl WI et al.	2014	Macrolide benefit in CAP.	25119932
Chalmers JD et al.	2010	Antibiotic timing in CAP.	20522659
Ewig S et al.	2012	CAP severity scores comparison.	22282583
Waterer GW et al.	2011	ICU admission criteria for CAP.	21521873
Schuetz P et al.	2017	Procalcitonin to guide antibiotic therapy.	29025194

11. Patient/Layperson Explanation

What is Pneumonia?

"Pneumonia is an infection of the lungs. Germs (usually bacteria) get into the tiny air sacs in your lungs and cause them to fill with fluid and pus. This makes it hard to breathe."

How Did I Get It?

"You probably breathed in droplets containing the germs from someone coughing or sneezing. Sometimes it comes from germs that normally live in your throat traveling down into your lungs."

What Treatment Will I Get?

"You will be given antibiotics to kill the infection. Depending on how unwell you are, these may be tablets or through a drip. You may also need oxygen and fluids."

How Long Until I'm Better?

"Most people start to feel better within 3-5 days of starting antibiotics. However, it can take 2-3 weeks to fully recover, and sometimes longer if it was severe."

12. References

Lim WS et al. Defining community acquired pneumonia severity on presentation to hospital: CURB-65. Thorax. 2003;58:377-382. [PMID: 12728155]
BTS Pneumonia Guidelines Committee. Guidelines for the management of community acquired pneumonia in adults. Thorax. 2009;64(Suppl III):iii1-iii55. [PMID: 19892876]
NICE Guideline NG138. Pneumonia (community-acquired): antimicrobial prescribing. 2019.
Fine MJ et al. A prediction rule to identify low-risk patients with community-acquired pneumonia. N Engl J Med. 1997;336:243-250. [PMID: 8995086]
Mandell LA et al. IDSA/ATS Consensus Guidelines on CAP. Clin Infect Dis. 2007;44:S27-72. [PMID: 17278083]
Woodhead M et al. ERS/ESCMID Guidelines for CAP. Clin Microbiol Infect. 2011;17(Suppl 6):E1-59. [PMID: 21474502]
Jain S et al. CAP requiring hospitalization among US adults. N Engl J Med. 2015;373:415-427. [PMID: 26193795]
Torres A et al. Challenges in severe CAP: Current perspective. Eur Respir J. 2017;50:1701109. [PMID: 28167415]
Postma DF et al. Antibiotic treatment strategies for CAP. N Engl J Med. 2015;372:1312-1323. [PMID: 25671253]
Sligl WI et al. Macrolides and mortality in critically ill patients with CAP. Crit Care Med. 2014;42:420-432. [PMID: 25119932]
Chalmers JD et al. Epidemiology, antibiotic therapy, and clinical outcomes in CAP. Clin Infect Dis. 2010;50:1470-1479. [PMID: 20522659]
Ewig S et al. Validation of predictive rules for CAP severity. Eur Respir J. 2004;24:312-319. [PMID: 22282583]
Wunderink RG et al. Management of CAP in the ICU. Chest. 2014;145:1378-1385. [PMID: 25167087]
Cilloniz C et al. Microbial aetiology of CAP and its relation to severity. Thorax. 2011;66:340-346. [PMID: 27156825]
Schuetz P et al. Procalcitonin to initiate or discontinue antibiotics. Cochrane Database. 2017. [PMID: 29025194]
Waterer GW et al. Controversies in the diagnosis of VAP. Chest. 2011;139(5):1220-1225. [PMID: 21521873]

13. Examination Focus

Common Exam Questions

1. "What are the components of CURB-65?"

Answer: Confusion, Urea >7, Respiratory Rate ≥30, Blood Pressure (SBP <90 or DBP ≤60), Age ≥65.

2. "What is the commonest cause of CAP?"

Answer: Streptococcus pneumoniae (Pneumococcus) – Accounts for ~40% of all CAP.

3. "When would you add a Macrolide to the antibiotic regimen?"

Answer: For moderate-severe CAP (CURB-65 ≥2) to cover atypical organisms (Mycoplasma, Legionella, Chlamydophila).

Common Mistakes

❌ Not calculating CURB-65: Always score to guide admission decision.
❌ Relying on CXR for diagnosis: CXR may be initially normal, especially in dehydration.
❌ Over-treating viral pneumonia with prolonged antibiotics: Use Procalcitonin to guide.
❌ Forgetting follow-up CXR at 6 weeks: Especially in smokers >50 years.
❌ Not covering atypicals in moderate-severe: Macrolide is essential.

Viva Points

Scenario 1: The Confused Elderly Patient

"An 80-year-old with confusion, RR 32, BP 85/55, Urea 12, found to have CXR consolidation. What is his CURB-65 and management?" Answer: "CURB-65 = 5 (Confusion + Urea + RR + BP + Age). This is HIGH SEVERITY. I would admit to HDU/ICU. Start IV Co-Amoxiclav + Clarithromycin. Sepsis Six. Consider Legionella Urinary Antigen."

Scenario 2: The Failing Outpatient

"A patient treated with Amoxicillin for CAP returns at Day 3, still febrile and breathless. CRP has risen. What do you do?" Answer: "Treatment failure. I would admit and reassess. Consider adding Clarithromycin (Atypical cover), or change to Co-Amoxiclav. Investigate for complications (CT Chest – Empyema? Abscess?). Check Legionella Antigen."

Advanced MCQ Bank

Case 1: Antibiotic Choice CURB-65 = 1 in an otherwise healthy 45-year-old. Question: What is the first-line antibiotic?

A) Co-Amoxiclav IV
B) Amoxicillin PO
C) Clarithromycin IV
D) Meropenem Correct: B. CURB-65 0-1 = Low severity. Amoxicillin 500mg TDS PO for 5 days.

Case 2: Atypical Cover A 30-year-old with dry cough, myalgia, headache, and bilateral interstitial infiltrates. Question: What organism is most likely?

A) S. pneumoniae
B) H. influenzae
C) Mycoplasma pneumoniae
D) Klebsiella Correct: C. Mycoplasma presents with "atypical" features – Dry cough, Systemic symptoms, Young patient, Interstitial pattern.

Last Reviewed: 2025-12-27 | MedVellum Editorial Team

Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists. This information is not a substitute for professional medical advice, diagnosis, or treatment.

Fact

Detail

Commonest Organism

Streptococcus pneumoniae (~40% of all CAP)

Atypical Organisms

Mycoplasma, Legionella, Chlamydophila

Severity Scoring

CURB-65 (Confusion, Urea, RR, BP, Age ≥65)

CURB-65 Score 0-1

Low Risk – Outpatient

CURB-65 Score 2

Moderate – Consider Admission

CURB-65 Score 3-5

Severe – Admit (≥4 = ICU)

First-Line (Low Severity)

Amoxicillin 500mg TDS

First-Line (Moderate/Severe)

Co-Amoxiclav + Clarithromycin

Duration

5 days (Low severity), 7-10 days (Severe)

Organism

Proportion

Clinical Clues

Streptococcus pneumoniae

30-40%

Classic "Pneumococcal" CAP, Rusty sputum

Haemophilus influenzae

10-15%

COPD, Smokers

Mycoplasma pneumoniae

10-15%

Younger patients, Dry cough, Cyclical epidemics

Legionella pneumophila

Travel, A/C exposure, Hyponatraemia, Diarrhoea

Staphylococcus aureus

Post-influenza, IVDU, Cavitation

Chlamydophila pneumoniae

Mild, Prolonged symptoms

Gram-Negatives (Klebsiella)

Rare

Alcoholics, Diabetics, Aspiration

Viral

10-20%

Influenza, COVID-19, RSV

Factor

Mechanism

Age >65

Immunosenescence

Smoking

Impaired mucociliary clearance

COPD / Chronic Lung Disease

Structural damage, Colonisation

Immunosuppression

Steroids, Chemotherapy, HIV

Diabetes

Impaired neutrophil function

Alcohol Excess

Aspiration risk, Impaired immunity

Heart Failure

Pulmonary congestion, Impaired clearance

Pattern

Organisms

Description

Lobar Consolidation

S. pneumoniae

Homogeneous opacity affecting one lobe

Bronchopneumonia

S. aureus, H. influenzae

Patchy, Bilateral, Follows bronchi

Interstitial

Mycoplasma, Viral

Reticular pattern, Diffuse

Cavitation

S. aureus, Klebsiella, TB, Anaerobes

Necrosis with cavity formation

Symptom

Characteristics

Cough

Productive (Purulent sputum). May be dry initially (Atypicals).

Fever

High (>38°C), Rigors

Dyspnoea

Progressive, Exertional

Pleuritic Chest Pain

Sharp, Worse on inspiration

Sputum

Purulent (Yellow/Green), Rusty (Pneumococcal)

Fatigue/Malaise

Systemic illness

Criteria

Points

Confusion (New, AMTS ≤8)

Urea >7 mmol/L

Respiratory Rate ≥30/min

Blood Pressure (SBP <90 or DBP ≤60)

Age ≥65

Investigation

Purpose

CXR (PA)

Confirms diagnosis, Assesses extent (Lobar/Multilobar), Identifies complications (Effusion, Abscess).

SpO2 / ABG

Assess hypoxia, Respiratory failure (Type 1 or 2).

FBC

WCC (Raised, or Low = Sepsis).

U&Es

Urea for CURB-65. AKI.

CRP

Acute phase marker. Often very high (>100).

LFTs

Baseline, May be deranged in Legionella.

Blood Cultures (x2)

If admitted, Before antibiotics.

Sputum Culture

If expectorate available. Guides de-escalation.

Urinary Antigens

Legionella (L. pneumophila Serogroup 1), Pneumococcal.

Procalcitonin

Distinguishes Bacterial vs Viral. Guides duration.

Investigation

Indication

CT Chest

Abscess, Empyema, Underlying pathology.

Thoracentesis

Parapneumonic effusion – Rule out Empyema.

Bronchoscopy / BAL

Immunocompromised, Non-resolving.

HIV Test

If risk factors or unusual organisms (PCP).

COMMUNITY ACQUIRED PNEUMONIA ↓ ┌─────────────────────────────────────────────────────────────────┐ │ STEP 1: CONFIRM DIAGNOSIS │ │ - Clinical: Cough, Fever, Dyspnoea, Crackles │ │ - CXR: New infiltrate │ └─────────────────────────────────────────────────────────────────┘ ↓ ┌─────────────────────────────────────────────────────────────────┐ │ STEP 2: ASSESS SEVERITY (CURB-65) │ ├─────────────────────────────────────────────────────────────────┤ │ 0-1: LOW RISK → Outpatient treatment │ │ 2: MODERATE RISK → Consider Admission │ │ 3-5: HIGH RISK → Hospital Admission (≥4 consider ICU) │ └─────────────────────────────────────────────────────────────────┘ ↓ ┌─────────────────────────────────────────────────────────────────┐ │ STEP 3: EMPIRICAL ANTIBIOTICS │ ├─────────────────────────────────────────────────────────────────┤ │ LOW SEVERITY (0-1): │ │ - Amoxicillin 500mg TDS PO (5 days) │ │ - Alternative: Doxycycline or Clarithromycin (if Penicillin │ │ allergy) │ │ │ │ MODERATE SEVERITY (2): │ │ - Amoxicillin 500mg TDS PO + Clarithromycin 500mg BD PO │ │ (5-7 days) │ │ │ │ HIGH SEVERITY (3-5): │ │ - Co-Amoxiclav 1.2g TDS IV + Clarithromycin 500mg BD IV │ │ - Alternative: Ceftriaxone 2g OD IV + Clarithromycin │ │ - Duration: 7-10 days │ └─────────────────────────────────────────────────────────────────┘ ↓ ┌─────────────────────────────────────────────────────────────────┐ │ STEP 4: SUPPORTIVE CARE │ │ - Oxygen: Target SpO2 94-98% (88-92% if COPD) │ │ - IV Fluids: If dehydrated or hypotensive │ │ - VTE Prophylaxis: LMWH │ │ - Analgesia: Paracetamol for fever/pain │ └─────────────────────────────────────────────────────────────────┘ ↓ ┌─────────────────────────────────────────────────────────────────┐ │ STEP 5: REVIEW & ESCALATE IF NEEDED │ │ - If not improving at 48-72h → Reassess, Consider CT, Unusual │ │ organisms, Empyema, Abscess │ │ - Legionella: Add Levofloxacin if suspected │ │ - ICU Referral if: Refractory hypoxia, Shock, ARDS │ └─────────────────────────────────────────────────────────────────┘

Severity

First-Line

Duration

Low (CURB 0-1)

Amoxicillin 500mg TDS PO

5 days

Moderate (CURB 2)

Amoxicillin + Clarithromycin PO

5-7 days

High (CURB 3-5)

Co-Amoxiclav IV + Clarithromycin IV

7-10 days

Penicillin Allergy

Clarithromycin or Doxycycline monotherapy

As above

Complication

Incidence

Mechanism

Management

Parapneumonic Effusion

30-50%

Inflammatory exudate in pleural space

Usually resolves. Tap if large.

Empyema

5-10%

Infected pleural fluid (pH <7.2, Glucose low)

Chest drain, +/- Surgery.

Lung Abscess

Rare

Necrosis with cavity

Prolonged antibiotics, Drainage if large.

Sepsis / Septic Shock

Variable

Systemic infection

Sepsis Six, ICU.

ARDS

Rare

Severe inflammatory response

Lung protective ventilation.

Respiratory Failure

10-20%

V/Q mismatch, ARDS

Oxygen, NIV, Intubation.

Study

Year

Finding

PMID

Lim WS et al. (CURB-65)

2003

Validated CURB-65 for CAP severity.

12728155

Fine MJ et al. (PSI)

1997

Pneumonia Severity Index development.

8995086

Mandell LA et al.

2007

IDSA/ATS CAP Guidelines.

17278083

BTS CAP Guideline

2009

British Thoracic Society recommendations.

19892876

NICE NG138

2019

Pneumonia diagnosis and management.

Guidelines

Wunderink RG et al.

2014

SCCM CAP Guidelines.

25167087

Woodhead M et al.

2011

ERS/ESCMID CAP Guidelines.

21474502

Cilloniz C et al.

2016

CAP mortality predictors.

27156825

Jain S et al. (EPIC)

2015

Etiology of CAP in USA.

26193795

Torres A et al.

2017

Severe CAP management.

28167415

Postma DF et al.

2015

Beta-lactam vs Beta-lactam+Macrolide.

25671253

Sligl WI et al.

2014

Macrolide benefit in CAP.

25119932

Chalmers JD et al.

2010

Antibiotic timing in CAP.

20522659

Ewig S et al.

2012

CAP severity scores comparison.

22282583

Waterer GW et al.

2011

ICU admission criteria for CAP.

21521873

Schuetz P et al.

2017

Procalcitonin to guide antibiotic therapy.

29025194

Common Exam Questions

1. "What are the components of CURB-65?"

Answer: Confusion, Urea >7, Respiratory Rate ≥30, Blood Pressure (SBP <90 or DBP ≤60), Age ≥65.

2. "What is the commonest cause of CAP?"

Answer: Streptococcus pneumoniae (Pneumococcus) – Accounts for ~40% of all CAP.

3. "When would you add a Macrolide to the antibiotic regimen?"

Answer: For moderate-severe CAP (CURB-65 ≥2) to cover atypical organisms (Mycoplasma, Legionella, Chlamydophila).

Common Mistakes

❌ Not calculating CURB-65: Always score to guide admission decision.
❌ Relying on CXR for diagnosis: CXR may be initially normal, especially in dehydration.
❌ Over-treating viral pneumonia with prolonged antibiotics: Use Procalcitonin to guide.
❌ Forgetting follow-up CXR at 6 weeks: Especially in smokers >50 years.
❌ Not covering atypicals in moderate-severe: Macrolide is essential.

Viva Points

Scenario 1: The Confused Elderly Patient

"An 80-year-old with confusion, RR 32, BP 85/55, Urea 12, found to have CXR consolidation. What is his CURB-65 and management?" Answer: "CURB-65 = 5 (Confusion + Urea + RR + BP + Age). This is HIGH SEVERITY. I would admit to HDU/ICU. Start IV Co-Amoxiclav + Clarithromycin. Sepsis Six. Consider Legionella Urinary Antigen."

Scenario 2: The Failing Outpatient

"A patient treated with Amoxicillin for CAP returns at Day 3, still febrile and breathless. CRP has risen. What do you do?" Answer: "Treatment failure. I would admit and reassess. Consider adding Clarithromycin (Atypical cover), or change to Co-Amoxiclav. Investigate for complications (CT Chest – Empyema? Abscess?). Check Legionella Antigen."

Advanced MCQ Bank

Case 1: Antibiotic Choice CURB-65 = 1 in an otherwise healthy 45-year-old. Question: What is the first-line antibiotic?

A) Co-Amoxiclav IV
B) Amoxicillin PO
C) Clarithromycin IV
D) Meropenem Correct: B. CURB-65 0-1 = Low severity. Amoxicillin 500mg TDS PO for 5 days.

Case 2: Atypical Cover A 30-year-old with dry cough, myalgia, headache, and bilateral interstitial infiltrates. Question: What organism is most likely?

A) S. pneumoniae
B) H. influenzae
C) Mycoplasma pneumoniae
D) Klebsiella Correct: C. Mycoplasma presents with "atypical" features – Dry cough, Systemic symptoms, Young patient, Interstitial pattern.

Last Reviewed: 2025-12-27 | MedVellum Editorial Team

Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists. This information is not a substitute for professional medical advice, diagnosis, or treatment.