Pneumonia and Lung Infections

In depth

The one-page summary above is what the PDF prints. These sections go further.

Severity and site of care in CAP

Two decisions are made at the door. Whether to admit at all is helped by a prognostic rule; ATS/IDSA 2019 prefers the Pneumonia Severity Index over CURB-65, always with clinical judgment. Neither rule was designed to choose the level of care inside the hospital. For that the guideline uses its severe CAP criteria: one major criterion, or three or more of the nine minor criteria.

Patients with hypotension needing a vasopressor or respiratory failure needing mechanical ventilation are admitted directly to the ICU (strong recommendation). For everyone else the guideline suggests using the minor criteria together with clinical judgment to decide who needs a higher level of care (conditional recommendation). The reason is timing: patients admitted to a ward and transferred to the ICU later have higher mortality than those admitted to the ICU directly, and unrecognized severe pneumonia is part of that.

In pooled studies, one major or three minor criteria predicted ICU admission with a sensitivity of 84% and a specificity of 78%; three or more minor criteria alone were 56% sensitive and 91% specific. A patient with no vasopressor and no ventilator can therefore still have severe CAP: a respiratory rate of 30/min, a PaO₂/FiO₂ of 250 or less and multilobar infiltrates already make three. Leukopenia counts only when it is due to the infection, not to chemotherapy.

Finding the organism: cultures, antigens and Legionella

The Legionella urinary antigen test detects Legionella pneumophila serogroup 1 only, so a negative result does not exclude other species and serogroups. CDC therefore advises using it together with culture or molecular testing of a lower respiratory specimen, collected before antibiotics when possible but without delaying treatment. In an intubated patient that specimen is a tracheal aspirate.

CDC's reasons to test a patient with pneumonia for Legionnaires' disease include failure of outpatient antibiotics, severe pneumonia (especially when intensive care is needed), an immunocompromised host, overnight travel away from home or an overnight stay in a healthcare facility during the incubation period, and a link to a setting with known Legionella. Besides fever and cough the illness can bring myalgia, headache, nausea, diarrhea, abdominal pain and altered mental status, and about 10% of patients die. Macrolides and respiratory fluoroquinolones are the Legionella-directed antibiotics, and both recommended severe CAP regimens contain one of them.

Antibiotics, influenza and treatment length

Severe CAP without risk factors for MRSA or Pseudomonas aeruginosa is treated with a β-lactam plus a macrolide, or a β-lactam plus a respiratory fluoroquinolone. Empiric antibiotics start once CAP is clinically suspected and confirmed on imaging, whatever the first procalcitonin level. The old category of healthcare-associated pneumonia is no longer used to choose broad coverage: MRSA or Pseudomonas is covered only when locally validated risk factors are present, with cultures sent so that the extra drugs can be stopped if neither organism grows. Options the guideline names include vancomycin 15 mg/kg every 12 hours or linezolid 600 mg every 12 hours for MRSA, and piperacillin-tazobactam 4.5 g every 6 hours, cefepime 2 g every 8 hours or meropenem 1 g every 8 hours for Pseudomonas.

A positive influenza test does not end the search for bacteria. Bacterial pneumonia can occur together with influenza or appear later as a relapse during recovery; Staphylococcus aureus is one of the most common organisms, followed by Streptococcus pneumoniae, Haemophilus influenzae and group A Streptococcus, and autopsies from the 2009 H1N1 pandemic found bacterial coinfection in about 30% of deaths. ATS/IDSA 2019 therefore strongly recommends standard antibacterial treatment at the start for adults with clinical and radiographic CAP who test positive for influenza, and an anti-influenza drug such as oseltamivir for inpatients however long they have been ill. When no bacterial pathogen is found, the procalcitonin is low and the patient is stable early, stopping the antibacterial drugs at 48 to 72 hours can be considered.

The 2025 ATS update kept this for hospitalized patients: with a positive respiratory virus test it still suggests empiric antibiotics for inpatients with nonsevere or severe CAP, and suggests withholding them only in outpatients without comorbidities.

Treatment length follows clinical stability: heart rate, respiratory rate, blood pressure, oxygen saturation and temperature back to normal, able to eat, and thinking clearly. ATS/IDSA 2019 set a minimum of 5 days, and 7 days for MRSA or Pseudomonas. The 2025 ATS update suggests less than 5 days, with a minimum of 3, for outpatients and for inpatients with nonsevere CAP who reach stability, and keeps 5 or more days for severe CAP.

Corticosteroids in severe CAP

This recommendation has changed. ATS/IDSA 2019 advised against routine corticosteroids in nonsevere CAP and suggested against them in severe CAP, apart from refractory septic shock. In 2023 the CAPE COD trial gave ICU patients with severe CAP intravenous hydrocortisone 200 mg daily for 4 or 7 days followed by a taper: death by day 28 fell from 11.9% to 6.2%, and among patients not ventilated at the start, intubation fell from 29.5% to 18.0%. The hydrocortisone group needed more insulin. The evidence is not all one way: in 2025 the REMAP-CAP trial stopped its hydrocortisone arm (50 mg every 6 hours for 7 days) for futility, with 90-day mortality of 15% with hydrocortisone and 9.8% without, and concluded that a large reduction in mortality was unlikely.

Who is left out matters as much as who is treated. The benefit was seen in severe pneumonia: in the ATS 2025 pooled analysis, mortality in severe CAP was 9.8% with corticosteroids and 15.1% without, and hyperglycemia was the main harm; the recommendation is conditional because the quality of the evidence was judged low. In influenza pneumonia, observational data suggest that corticosteroids may raise mortality, so ATS/IDSA 2019 and ATS 2025 both leave influenza out. The Merck Manual also advises against adjunctive corticosteroids in immunocompromised patients and when viral or fungal pneumonia is suspected. None of this overrides a corticosteroid given for another reason, such as a COPD or asthma exacerbation or refractory septic shock.

After recovery: follow-up imaging and screening

ATS/IDSA 2019 suggests no routine follow-up chest imaging for adults with CAP whose symptoms resolve within 5 to 7 days. Repeat imaging found something new in only 0.2% to 5% of patients, and many of those patients already met the criteria for lung cancer screening. Radiographic clearing can lag behind clinical recovery by several weeks, so a repeat film is for symptoms that persist or worsen, not for proof of cure.

Screening is a separate question with its own rule. The USPSTF (2021) recommends annual low-dose CT for adults aged 50 to 80 years with a smoking history of at least 20 pack-years who smoke now or quit within the past 15 years. Pack-years are packs per day multiplied by years smoked: one pack a day for 20 years and two packs a day for 10 years are both 20 pack-years.

Ventilator-associated pneumonia: sampling and empiric therapy

VAP is pneumonia arising more than 48 hours after endotracheal intubation. The 2016 IDSA/ATS guideline suggests diagnosing it with noninvasive sampling, meaning endotracheal aspiration, and semiquantitative culture, rather than invasive sampling (bronchoalveolar lavage, protected specimen brush or blind mini-lavage) with quantitative culture. Trials found no difference in mortality, ICU stay or ventilator days between the two approaches, and an aspirate is quicker, needs fewer resources and causes fewer complications. If an invasive quantitative culture has been done and is below the threshold (protected brush under 10³ CFU/mL, lavage under 10⁴ CFU/mL), the guideline suggests withholding antibiotics.

The decision to start antibiotics rests on clinical criteria alone: the guideline recommends against adding procalcitonin, and suggests against adding the Clinical Pulmonary Infection Score, for that decision. Procalcitonin with clinical criteria may be used later to help stop treatment. Delay and failure to cover the causative organism are both associated with higher mortality, so empiric treatment is not held for the culture result.

Risk factors for multidrug-resistant VAP are intravenous antibiotics within the past 90 days (the strongest, with an odds ratio of about 12), septic shock at the time of VAP, ARDS before VAP, 5 or more days in hospital before VAP, and acute renal replacement therapy before VAP. A patient with late VAP who received intravenous antibiotics within the past 90 days therefore starts on an MRSA drug plus two antipseudomonal drugs from different classes, chosen from the unit's own antibiogram, and is narrowed when the culture returns.

Inhaled antibiotics are an addition, not a substitute: for VAP caused by gram-negative bacilli susceptible only to aminoglycosides or polymyxins, the guideline suggests inhaled plus systemic antibiotics rather than systemic antibiotics alone. Prevention bundles and the surveillance definitions (VAE, IVAC, PVAP) are on the Preventing ICU Infections page.

Aspiration pneumonitis and aspiration pneumonia

After a witnessed aspiration the first actions are mechanical: position the patient to limit further aspiration (head turned to the side, then the head of the bed raised), suction the mouth and pharynx, and give oxygen. A nebulized bronchodilator treats bronchospasm. Intubation follows the usual indications (mental status, degree of hypoxemia, hemodynamic stability) and makes bronchoscopy easier when a large volume of solid material has been aspirated; ventilate with a lung-protective strategy and decompress the stomach to prevent a repeat.

Antibiotics are not needed for the chemical injury itself. ATS/IDSA 2019 notes that many patients who aspirate gastric contents improve within 24 to 48 hours with supportive treatment alone, and the Merck Manual finds no clear benefit from empiric antibiotics and does not routinely recommend systemic glucocorticoids; observe closely and treat if the course turns toward bacterial infection. When aspiration pneumonia is treated, ATS/IDSA 2019 suggests no added anaerobic coverage unless lung abscess or empyema is suspected.

Where the infiltrate appears depends on position at the time. Lying flat, the posterior segments of the upper lobes and the apical (superior) segments of the lower lobes are dependent; upright or semi-recumbent, the basal segments of the lower lobes. Risk factors are a depressed level of consciousness (sedation, overdose, alcohol, seizure), dysphagia, vomiting, and devices and procedures of the airway and gut. Prevention is elevation of the head of the bed above 30° and stopping sedating drugs where possible.

Pneumocystis pneumonia

Pneumocystis jirovecii is a fungus that causes pneumonia in immunocompromised patients. In HIV, about 90% of cases occurred at CD4 counts below 200 cells/µL; outside HIV the patients are those with hematologic or solid cancers, transplants, autoimmune disease, or corticosteroid and other immunosuppressive treatment. With HIV the illness is typically subacute: progressive dyspnea, fever, a nonproductive cough and chest discomfort that worsen over days to weeks. Without HIV the onset is more acute, progression is faster and severe hypoxemia is frequent.

Hypoxemia is the most characteristic finding. Exertion brings out tachypnea, tachycardia and diffuse dry crackles, and a fall in SpO₂ with walking signals a widened A–a gradient, though it is not specific. LDH is often raised, which is also nonspecific. The radiograph classically shows diffuse, bilateral, symmetric interstitial infiltrates but can be normal early, when thin-section CT shows patchy ground-glass opacities. A pneumothorax in a patient with HIV should raise the suspicion of Pneumocystis.

The organism cannot be grown in routine culture, so the diagnosis depends on seeing it or detecting its DNA in a respiratory sample. Induced sputum is tried first (sensitivity from under 50% to over 90%, depending on the laboratory), then bronchoalveolar lavage (90–99%). PCR is very sensitive but cannot by itself separate colonization from infection, and a raised serum β-D-glucan supports the diagnosis without proving it. Organisms persist for days to weeks after treatment begins, so treatment starts on suspicion and is not delayed for the procedure.

The therapist's part is early support of oxygenation and help with the specimens. For acute hypoxemic respiratory failure the ERS guideline suggests high-flow nasal cannula over conventional oxygen and over noninvasive ventilation, so high-flow oxygen is a reasonable first support for the hypoxemic immunocompromised patient while the workup proceeds. Watch closely for failure: in reported series 23–46% of patients with Pneumocystis pneumonia needed mechanical ventilation.

Trimethoprim-sulfamethoxazole (TMP-SMX) is the treatment of choice: 15–20 mg/kg/day of the trimethoprim component in 3 or 4 divided doses for 21 days. Patients often worsen in the first 3 to 5 days as dying organisms provoke inflammation, and treatment is not judged a failure until at least 4 to 8 days have passed.

Adjunctive corticosteroids are given for moderate to severe disease, defined on a room-air arterial gas: a PaO₂ below 70 mm Hg, or an A–a gradient of 35 mm Hg or more. Either one qualifies. Start as early as possible and within 72 hours of starting Pneumocystis treatment: prednisone 40 mg twice daily on days 1–5, 40 mg daily on days 6–10 and 20 mg daily on days 11–21, or intravenous methylprednisolone at 75% of the prednisone dose.

The gradient catches patients whose PaO₂ looks acceptable because they are hyperventilating. On room air at sea level, alveolar PO₂ is 0.21 × (760 − 47) − PaCO₂ ÷ 0.8. With a PaCO₂ of 30 mm Hg that is 150 − 37.5, about 112 mm Hg; if the PaO₂ is 72 mm Hg the gradient is about 40 mm Hg, and the patient qualifies for prednisone even though the PaO₂ is above 70. The criteria use the arterial gas, not the SpO₂.

These steroid criteria come from patients with HIV. Without HIV the benefit is uncertain: a randomized trial of 226 patients found 28-day mortality of 21.5% with methylprednisolone and 32.4% with placebo, a difference that was not statistically significant, and observational studies conflict, so the decision is individualized.

Latent tuberculosis infection

• In latent TB infection the bacteria are alive but inactive. The person does not feel sick, cannot spread TB bacteria to others, usually has a positive TB blood test or skin test, typically has a normal chest radiograph, and has negative sputum smears and cultures.
• A positive interferon-gamma release assay (QuantiFERON-TB Gold Plus or T-SPOT.TB) or skin test usually means TB infection, and more tests are needed to rule out TB disease. The next steps are a medical history and symptom review, a physical examination and a chest radiograph, with sputum examination when indicated.
• Interferon-gamma release assays are blood tests that need one visit, can give a result within 24 hours and are unaffected by BCG vaccination. They are preferred for people who received BCG and for people unlikely to return to have a skin test read.
• Without treatment about 5–10% of infected people develop TB disease over their lifetimes, and untreated latent infection accounts for about 80% of TB cases in the United States, so latent infection is treated.
• Preferred regimens are short and rifamycin-based: 3 months of once-weekly isoniazid plus rifapentine (3HP), 4 months of daily rifampin (4R), or 3 months of daily isoniazid plus rifampin (3HR). Six or nine months of daily isoniazid is the alternative. TB disease must be excluded before any of them is started.

Inhaled anti-infectives: amikacin liposome and zanamivir

Amikacin liposome inhalation suspension (Arikayce) is an aminoglycoside for adults with Mycobacterium avium complex (MAC) lung disease who have limited or no other treatment options and whose sputum cultures are still positive after at least 6 consecutive months of a multidrug regimen. It is added to that regimen as part of combination treatment. The dose is one 590 mg/8.4 mL vial inhaled once daily. The vial is brought to room temperature, shaken well for at least 10 to 15 seconds and poured into the Lamira Nebulizer System, the only device the label allows.

Bronchospasm and hypersensitivity pneumonitis call for different answers. Wheeze or chest tightness with dosing is bronchospasm: patients who use a bronchodilator take it first, and pretreatment with a short-acting beta-2 agonist is considered in asthma, COPD or known hyperreactive airways. The label's hypersensitivity pneumonitis includes allergic alveolitis, pneumonitis and interstitial lung disease. Hypersensitivity pneumonitis in general brings cough, dyspnea and often fever, with diffuse crackles, usually no wheeze, and diffuse small nodules or ground-glass opacities on CT, so that picture during treatment should raise it. Hold further doses and notify the prescriber, because the label's instruction is to discontinue the drug. Other aminoglycoside cautions also apply: nephrotoxicity, and neuromuscular blockade in disorders such as myasthenia gravis.

Zanamivir (Relenza) is a dry powder: each blister holds 5 mg of drug in 20 mg of lactose, and it is inhaled only through the breath-activated Diskhaler supplied with it. Treatment is 10 mg (two inhalations) twice daily for 5 days in patients aged 7 years and older who have had symptoms for no more than 2 days. A patient due for an inhaled bronchodilator takes it before the zanamivir.

The label does not recommend zanamivir for patients with underlying airways disease such as asthma or COPD: serious bronchospasm, including deaths, has been reported in patients with and without airways disease, and the drug is stopped if bronchospasm or a decline in respiratory function develops; CDC's summary for clinicians calls inhaled zanamivir contraindicated in these patients. It is contraindicated in patients with an allergy to milk proteins, which the lactose carrier contains.

For hospitalized patients with influenza, CDC recommends oseltamivir by mouth or by feeding tube, started as soon as possible; treatment might still reduce illness and death when it is begun more than 48 hours after onset. The adult dose is 75 mg twice daily, and limited data suggest it is well absorbed through an orogastric or nasogastric tube in critically ill patients. Inhaled zanamivir, oral baloxavir and intravenous peramivir are not routinely recommended for hospitalized patients; peramivir may be considered when enteral oseltamivir cannot be tolerated or absorbed, as with gastric stasis, malabsorption or gastrointestinal bleeding. Severely ill patients may need more than 5 days of treatment.