Admission to hospital <48 hours ago (no hospital admission in last 6 weeks):

Treat as community acquired pneumonia (CAP).

N.B. Residents of care homes or long-term care facilities in the community should not be regarded as having HAP if a lower respiratory tract infection is diagnosed in their usual environment or at the time of admission to hospital.

Mild HAP:

Doxycycline oral 200mg on day 1 then 100mg daily

N.B. In pregnant patients, doxycycline is contraindicated - discuss appropriate empirical choices with an infection specialist.

Also refer to the HAP algorithm.

Severe HAP without risk factors for P. aeruginosa:

Co-trimoxazole IV or oral 960mg 12 hourly

and

Gentamicin IV (dosing info here). For maximum of 72 hours only. Stop gentamicin after 72 hours of treatment following the 'IV Gentamicin Review after 72 hours of Treatment' algorithm (N.B. links within the document are only active if accessing via NHS network).

N.B. Co-trimoxazole contains trimethoprim and sulfamethoxazole. Do not use this agent in pregnancy. Trimethoprim is contraindicated in the first trimester of pregnancy due to a risk of neural tube defects; sulfamethoxazole can cause neonatal haemolysis and methaemoglobinaemia in the last trimester.

Gentamicin should be avoided in patients with decompensated liver disease (jaundice, ascites, encephalopathy, variceal bleeding or hepato-renal syndrome). Gentamicin must not be administered to patients with myasthenia gravis as it can precipitate a myasthenic crisis.

Also refer to the HAP algorithm.

Severe HAP in patients with contraindications to co-trimoxazole or gentamicin:

Amoxicillin IV adjusted for renal function

and

Temocillin IV adjusted for renal function

If true penicillin / beta-lactam allergy:

Levofloxacin oral/IV adjusted for renal function

Severe HAP with risk factors for P. aeruginosa:

• If P.aeruginosa has previously been cultured from respiratory samples, base antimicrobial choice on the released susceptibilities.
• The only orally available antimicrobials with possible anti-pseudomonal activity are quinolones. P. aeruginosa reported as "R" to ciprofloxacin cannot be treated with oral antimicrobials.
• P. aeruginosa is never reported susceptible ("S") to quinolones or piperacillin-tazobactam, but can only be susceptible at increased dose ("I") or resistant ("R"). 

If P. aeruginosa pneumonia is clinically suspected, but there are no previous respiratory isolates to guide antimicrobial choice, discuss therapy with an infection or respiratory specialist.

Also refer to the HAP algorithm.

Treat patients who were admitted to hospital for >48 hours in the last 6 weeks as HAP.

A diagnosis of HAP requires signs of new or worsening consolidation on clinical chest examination and at least 2 of:

• Fever >38°C
• WBC >11 or <4 x10⁹/L
• Decline in sO₂
• Purulent respiratory secretions

If any of the following are present, the patient should be treated as severe HAP:

• Respiratory rate >30 breaths per minute
• P_aO₂ ≤8kPa or sO₂≤93% on air
• New onset hypotension
• Multilobar infiltrates on chest radiograph

Do not use CURB65 for severity assessment; this score is only validated for use in community-acquired pneumonia. 

Risk factors for Pseudomonas aeruginosa pneumonia:

• Known colonisation with P. aeruginosa
• Recent invasive ventilation
• Transfer from healthcare facility abroad

There is often diagnostic uncertainty in the diagnosis of HAP, and other hospital acquired infections such as catheter-associated UTI (CAUTI) are often part of the differential diagnosis.

The addition of gentamicin for 72 hours provides additional gram negative cover during the period while a definitive diagnosis is established and culture results are awaited. E. coli resistance rates to gentamicin are around 8% in bloodstream isolates and 11% in hospital urinary isolates.