Community-acquired pneumonia

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Pneumonia management links three decisions: whether the syndrome is pneumonia rather than an alternative, how sick the person is, and which pathogens/resistance risks require coverage. Severity scores organise but do not replace bedside physiology or trajectory.

NICE NG250, updated in 2025, standardises 5-day antibiotic review and adds corticosteroid consideration for high-severity hospital CAP. The regimen must still be adapted to allergy, pregnancy, renal/hepatic function, microbiology and current antimicrobial-resistance advice.

Failure to improve at 48-72 hours is a diagnostic event: reconsider resistant/uncovered organism, empyema, abscess, obstruction/malignancy, pulmonary embolism, heart failure, inflammatory disease or an immunocompromised-host infection before simply extending antibiotics.

Key points

CAP is an acute lower respiratory infection acquired outside hospital with compatible clinical features and, in hospital, usually new radiographic shadowing not explained by another cause.
Assess ABCDE, SpO2, sepsis physiology, ability to take oral medicines, frailty/comorbidity and social support before scoring severity.
CRB65 in the community and CURB65 in hospital support risk assessment: Confusion, urea above 7 mmol/L, respiratory rate at least 30/min, low BP, age 65 or over; each scores 1.
Scores supplement judgement. Profound hypoxaemia, shock, multilobar disease, immunosuppression or rapid deterioration can indicate high severity despite a low number.
Low-severity adult CAP: amoxicillin 500 mg orally three times daily for 5 days; doxycycline 200 mg day 1 then 100 mg daily or clarithromycin 500 mg twice daily are alternatives when appropriate.
Moderate severity: amoxicillin 500 mg TDS for 5 days, adding clarithromycin 500 mg BD if atypical pathogens are suspected; doxycycline or clarithromycin alone can be alternatives in penicillin allergy.
High severity: co-amoxiclav 500/125 mg orally TDS or 1.2 g IV TDS plus clarithromycin 500 mg BD (oral/IV as appropriate), review route and total duration at 5 days.
Use oral antibiotics first if the person can take them and severity does not require IV; review IV therapy by 48 hours and switch promptly when improving.
Stop antibiotics after 5 days unless microbiology requires longer or the person is not clinically stable; instability includes fever in the preceding 48 hours or more than one ongoing sign such as tachycardia, tachypnoea or low BP.
NICE 2025 recommends considering IV hydrocortisone in high-severity CAP in hospital for 4-7 days or until discharge sooner; monitor hyperglycaemia and secondary infection.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Typical bacteria

Streptococcus pneumoniae and other community respiratory bacteria reach the distal airways by microaspiration or inhalation, with risk increased by age, smoking and chronic disease.

Respiratory viruses

Influenza, SARS-CoV-2 and other viruses can directly cause pneumonitis or damage airway defences, permitting secondary bacterial infection.

Intracellular pathogens

Mycoplasma, Legionella and related organisms produce pneumonia with epidemiological or extrapulmonary clues, although symptoms alone do not reliably identify the pathogen.

Impaired host defence

Frailty, alcohol dependence, swallowing dysfunction, immune suppression and structural lung disease increase aspiration, microbial burden or failure of pulmonary clearance.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Lower-airway inoculation
Pathogens bypass or overwhelm mucociliary, cough and immune defences and multiply within bronchioles and alveoli, with effects that increase as the pathological process progresses.
2
Alveolar inflammation
Neutrophils, protein-rich fluid and cellular debris fill air spaces, producing consolidation and reducing local ventilation, thereby altering ventilation, gas transfer or respiratory mechanics.
3
Impaired oxygen transfer
Perfusion continues through poorly ventilated or fluid-filled lung, causing ventilation-perfusion mismatch, shunt physiology and increased work of breathing.
4
Systemic response
Cytokine release produces fever and malaise; an excessive or poorly contained response may cause sepsis, vasodilatation and multi-organ dysfunction.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Typical CAP

Acute cough, fever, breathlessness or pleuritic pain with focal crackles/bronchial breathing and new infiltrate in hospital. Older/frail adults may present with confusion, falls or functional decline rather than fever.

High-severity CAPRed flag

CURB65 3-5 and/or clinical shock, profound hypoxaemia, rapid progression or need for ventilatory/vasopressor support. Seek critical-care input early and treat promptly.

Atypical pathogen clue

Legionella may cause high fever, confusion, diarrhoea, hyponatraemia or liver-enzyme rise; Mycoplasma can cause extrapulmonary features. Epidemiology and severity guide testing/coverage, not a single sign.

Parapneumonic effusion/empyemaRed flag

Persistent fever, pleuritic pain, dullness or a pleural collection on imaging requires ultrasound and pleural-fluid assessment/drainage pathway; antibiotics alone may fail.

Immunocompromised hostRed flag

Neutropenia, transplant, advanced HIV or substantial immunosuppression expands pathogen range and invalidates routine uncomplicated CAP assumptions; obtain early infection/respiratory specialist input.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
CRB65/CURB65 plus clinical judgementFirst step
Why
Estimate mortality risk and guide place/intensity of care.
Interpretation and limitations
0-1 is usually low risk, 2 moderate, 3-5 high in hospital; in community CRB65 0 often community care, 1-2 consider hospital assessment, 3-4 urgent admission. Override for physiological instability or frailty.
02
Chest radiograph
Why
Confirm hospital CAP and identify extent/effusion or alternatives.
Interpretation and limitations
Obtain promptly in hospital when pneumonia is suspected. Early films can be normal in dehydration/neutropenia; repeat imaging/CT if clinical suspicion persists or course is atypical.
03
SpO2 and ABG when severe or hypercapnic risk
Why
Quantify oxygenation/ventilation and guide support.
Interpretation and limitations
Target 94-98% for most, 88-92% if risk of hypercapnic failure pending gases. A low PaO2 on oxygen or acidosis needs urgent escalation.
04
FBC, U&E, CRP, LFT, glucose and lactate
Why
Calculate CURB65, establish organ dysfunction and a baseline trend.
Interpretation and limitations
Urea above 7 scores 1. CRP can support trend but cannot confirm/exclude bacterial pneumonia alone; hyponatraemia/LFT changes may support Legionella testing in context.
05
Blood cultures, sputum culture and urinary antigens
Why
Identify pathogen in moderate/high severity, sepsis, unusual exposure or treatment failure.
Interpretation and limitations
Take cultures before antibiotics if this causes no delay. Use pneumococcal/Legionella urinary antigen and viral PCR according to severity/epidemiology; narrow treatment when reliable results arrive.
06
Pleural ultrasound and diagnostic aspiration
Why
Characterise a significant effusion and detect complicated infection.
Interpretation and limitations
Ultrasound guides safe sampling. Purulence, low pleural pH or loculation supports urgent chest-drain/pleural-team management; do not perform blind aspiration.
07
Follow-up imaging
Why
Ensure resolution or exclude underlying malignancy in selected adults.
Interpretation and limitations
Consider CXR at about 6 weeks for persistent symptoms/signs or higher malignancy risk such as smokers and age over 50; routine imaging is not necessary for every fully recovered low-risk adult.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Pulmonary embolism

Pleuritic pain, haemoptysis and hypoxaemia overlap, but thromboembolic risk and a probability-led imaging pathway distinguish PE from infection.

Cardiogenic pulmonary oedema

Orthopnoea, oedema, cardiac signs and bilateral congestion favour heart failure, although fever or focal opacity may coexist.

Lung cancer

Persistent or recurrent focal consolidation, haemoptysis, weight loss or obstructive change should prompt follow-up for an underlying tumour.

Acute exacerbation of airway disease

Widespread wheeze and variable obstruction without a new infiltrate favours asthma or COPD exacerbation, while either may coexist with pneumonia.

Organising or inflammatory pneumonitis

Subacute symptoms, migratory opacities, exposure or medicine timing and poor antimicrobial response suggest a non-infective inflammatory process requiring specialist reassessment.

Additional chapter-specific clues

Alternative diagnosisRed flag

Pulmonary embolism, oedema, malignancy/obstruction, alveolar haemorrhage, organising pneumonia or aspiration can mimic CAP. Reconsider with atypical imaging, haemoptysis, recurrent same-lobe disease or treatment failure.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01First hourAssess severity and treatFirst stepSuspected CAP with systemic illness or hospital presentation.

1ABCDE, SpO2, sepsis assessment and CRB65/CURB65; obtain CXR and relevant cultures promptly without delaying antibiotics in unstable disease.
2Give oxygen to 94-98%, or 88-92% if hypercapnic risk. Give oral antibiotic first if feasible; use IV when high severity, vomiting, absorption concern or critical illness requires it.
3Assess fluids, VTE prophylaxis, analgesia/antipyresis and critical-care need. Use cautious fluid boluses with reassessment in older adults or cardiac/renal disease.
4Record antibiotic indication, allergy, route, start time, planned 48-hour IV review and 5-day stop/review date.

02AntibioticsSeverity-matched adult regimenCAP diagnosed or strongly suspected.

1Low severity: amoxicillin 500 mg PO TDS for 5 days. If penicillin allergy or atypical suspicion makes amoxicillin unsuitable: doxycycline 200 mg day 1 then 100 mg OD, or clarithromycin 500 mg BD, for 5 days.
2AlternativeModerate severity: amoxicillin 500 mg TDS for 5 days; add clarithromycin 500 mg BD if atypical pathogens are suspected. Doxycycline or clarithromycin alone is an alternative when penicillin allergic.
3High severity: co-amoxiclav 500/125 mg PO TDS or 1.2 g IV TDS plus clarithromycin 500 mg BD PO/IV for 5 days then review. Seek microbiology advice for severe beta-lactam allergy or resistant-organism risk.
4Review at 48 hours, switch IV to oral when improving and able, narrow to microbiology, and stop at day 5 unless a longer course is microbiologically required or clinical stability criteria are not met.

03High severityCritical care and adjunctive corticosteroidCURB65 3-5 with high clinical severity, shock, profound hypoxaemia or ventilatory need.

1Involve critical care early, deliver high-severity antibiotics promptly and investigate Legionella/other pathogens and pleural complications.
2AlternativeConsider IV hydrocortisone started in hospital in addition to antibiotics for 4-7 days, or until discharge if sooner. NICE specifies the treatment decision and duration but not a universal dose; a specialist critical-care regimen may use 200 mg/day, for example 50 mg IV every 6 hours or 200 mg by continuous infusion over 24 hours. Use the adopted critical-care protocol and an alternative corticosteroid only if hydrocortisone is unsuitable.
3Monitor glucose, sodium/fluid status, delirium, GI/infection risk and response. Corticosteroid does not replace source control, antibiotics or organ support.

04Non-responseReassess at 48-72 hoursPersistent fever/instability, rising oxygen need or no clinical improvement.

1Repeat clinical examination and imaging; review adherence, route, dose, cultures and whether a pleural collection/abscess requires source control.
2Reconsider PE, heart failure, malignancy/obstruction, TB, inflammatory lung disease, aspiration and immunocompromised pathogens. Use CT, bronchoscopy or specialist testing selectively.
3Change antibiotics only with a reason—microbiology, resistance risk, wrong spectrum or complication—and document a new duration. Seek respiratory/infection/pleural input rather than stacking broad agents blindly.

Key medicines and prescribing safety6 treatments · regimens, roles and cautions

First-line low-severity CAP and core moderate-severity treatment.

Amoxicillin

500 mg orally three times daily for 5 days; higher doses may be used for severe infection according to BNF/renal function.

Check immediate/non-immediate penicillin allergy, renal function and rash/diarrhoea; adjust when required. Does not cover atypical organisms.

Alternative in low/moderate CAP when amoxicillin is unsuitable, including atypical coverage.

Doxycycline

200 mg orally on day 1, then 100 mg once daily on days 2-5.

Avoid in pregnancy; take with plenty of water while upright and separate from iron/calcium/antacids. Photosensitivity and oesophagitis can occur.

Alternative in penicillin allergy or add-on for suspected atypical pathogens; paired with co-amoxiclav in high severity.

Clarithromycin

500 mg orally or IV twice daily for 5 days.

QT prolongation and major CYP3A4 interactions, including some statins; review hepatic function and pregnancy alternatives. Adjust in significant renal impairment.

Broad beta-lactam component for high-severity disease.

Co-amoxiclav

High-severity CAP: 500/125 mg orally TDS or 1.2 g IV TDS, with clarithromycin, for 5 days then review.

Penicillin allergy, renal/hepatic adjustment, cholestatic hepatitis and C. difficile risk; switch IV to oral promptly when stable.

Adjunct to antibiotics in adults with high-severity CAP in hospital under NICE NG250.

Hydrocortisone for high-severity hospital CAP

Consider for 4-7 days or until discharge if sooner. NICE does not specify a universal dose. A specialist critical-care regimen may use hydrocortisone 200 mg/day, for example 50 mg IV every 6 hours or 200 mg by continuous infusion over 24 hours; follow the adopted critical-care protocol.

Hyperglycaemia, delirium, fluid/sodium effects, GI and secondary infection risk. Evidence/regimens are specialist hospital care; do not use routinely in low/moderate CAP.

Corrects hypoxaemia while definitive infection treatment takes effect.

Controlled oxygen

Titrate to SpO2 94-98% for most adults; 88-92% pending gases if at risk of hypercapnic respiratory failure. Device and flow depend on severity.

Escalating oxygen requirement is a severity signal. Repeat gases when hypercapnia is possible; oxygen does not replace ventilation.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Sepsis and shock

Uncontrolled infection and systemic inflammation can cause hypotension, tissue hypoperfusion, renal injury and other organ failure.

Respiratory failure

Extensive consolidation and inflammatory lung injury may produce severe hypoxaemia, ventilatory fatigue or acute respiratory distress syndrome.

Parapneumonic effusion or empyema

Inflammation can extend into the pleural space, where bacterial invasion and loculation create a collection requiring drainage as well as antimicrobial treatment.

Lung abscess

Tissue necrosis or impaired drainage may form a cavitating, pus-filled lesion, particularly with aspiration, obstruction or virulent infection.

Cardiovascular events

Acute inflammation, hypoxaemia and physiological stress can precipitate arrhythmia, heart failure or myocardial injury in susceptible adults.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Track RR, SpO2/oxygen requirement, BP, pulse, temperature, mental state, urine output and ability to eat/mobilise; recalculate risk when the patient changes.
Review antibiotics at 48 hours for IV-to-oral switch, microbiological narrowing, allergy/adverse effects and confirmation of the 5-day stop plan.
At day 5 stop unless microbiology requires longer or instability remains; document the reason and new review date if continuing.
Monitor glucose and infection/delirium effects when using corticosteroid; monitor ECG/QT and interactions with macrolide therapy when relevant.
Arrange follow-up and selected 6-week CXR for persistent symptoms/signs or higher malignancy risk; give return advice for worsening breathlessness, confusion, chest pain or haemoptysis.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Score and physiology can disagree

A young adult with severe hypoxaemia can have a low CURB65. Clinical severity overrides the arithmetic.

Oral is not inferior by default

If absorption is reliable and severity permits, oral therapy avoids line complications and should be preferred; IV route is a severity/feasibility decision, not a marker of quality.

Five days is a review rule

Do not extend merely because infiltrate or cough has not fully disappeared. Continue only for microbiological need or defined clinical instability.

Pleural source control changes everything

Empyema and complicated effusion need ultrasound-guided drainage decisions; broader or longer antibiotics alone may not achieve control.

Recurrent same-lobe pneumonia has a reason

Consider an obstructing tumour, foreign body or structural lesion and investigate with CT/bronchoscopy rather than repeating identical community prescriptions.

11Common pitfallsFrequent interpretation and management errors.

01
Letting a low CRB65/CURB65 overrule shock, hypoxaemia or rapid deterioration.
02
Delaying antibiotics in unstable disease while waiting for radiology or cultures.
03
Using IV antibiotics when oral absorption and severity do not require them, then failing to review at 48 hours.
04
Continuing antibiotics beyond 5 days without microbiological or clinical-instability rationale.
05
Using corticosteroid routinely for low/moderate CAP or without glucose/infection monitoring.
06
Missing empyema, PE, heart failure or obstructing malignancy when treatment response is poor.