| Suspicion of high-risk cause (Staphylococcus aureus, gram-negative bacilli, aspiration, or postobstructive process) |
| Age > 50 years |
| Prior episode of pneumonia |
| Consolidation, multilobe involvement, or pleural effusion on chest radiograph |
| Abnormalities on physical examination: Temperature ≤ 95ºF (35ºC) or >104ºF (40ºC) Systolic or diastolic blood pressures ≤90 mm Hg or ≤60 mm Hg, respectively Respiratory rate ≥30 breaths/minute Heart rate >125 beats/minute Extrapulmonary areas of infection |
| Laboratory factors Abnormal renal function (BUN >20 mg/dL or serum creatinine >1.2 mg/dL) Sodium ≤130 mg/dL Glucose ≥250 mg/dL Hematocrit ≤30% WBC count ≤4000/mm³ or >30 000/mm³ Metabolic acidosis (pH ≤7.35) PaO2 ≤60 mm Hg breathing room air |
| Comorbid conditions Renal insufficiency Congestive heart failure Liver disease Diabetes mellitus Altered mental state Neurologic disease Alcoholism Immunosuppression Malignancy Splenectomy |
| No responsible person in the home to assist the patient |
Abbreviations: BUN = blood urea nitrogen; WBC = white blood cell.
Accurate history, including occupation; travel; exposure to animals, birds, and insects; sick contacts; recent dental work; and history of alcohol or drug abuse may suggest a causative agent (Table 32.2). Bacteria and respiratory viruses cause the majority of CAP with a minority of cases caused by fungi, such as the endemic mycoses. Geographic variations affect the spectrum and proportion of causative agents. A significant proportion of cases may be polymicrobial, most commonly bacteria in combination with either atypical bacteria or respiratory viruses. Most cases are treated without identification of a specific cause, especially in the outpatient setting.
| Anaerobes (oral) | Alcoholism, aspiration, lung abscess, recent dental work, endobronchial obstruction |
| Bordetella pertussis | Cough ≥2 weeks with whoop or vomiting after cough |
| Burkholderia cepacia | Bronchiectasis |
| Chlamydia pneumoniae | COPD, smokers, biphasic illness |
| Chlamydoia psittaci | Bird exposure |
| Coccidioides immitis | Travel to Southwest United States |
| Coronaviruses (SARS and MERS) | Travel to or residence in East Asia or the Middle East or with outbreak in other countries |
| Coxiella burnettii | Farm animal or pregnant cat exposure, hepatosplenomegaly |
| Francisella tularensis | Exposure to wild mammals, esp. rabbits and ticks in endemic areas |
| Haemophilus influenzae | COPD, smokers, HIV, postinfluenza |
| Hantavirus pulmonary syndrome | Pulmonary edema, hemoconcentration, thrombocytopenia esp. after travel to Southwest United States |
| Histoplasma capsulatum | Bat or bird droppings, cave exploration |
| Influenza | Seasonal outbreak. Travel to or residence in Asia: avian influenza |
| Klebsiella pneumoniae | Alcoholics |
| Legionella species | Hotel or cruise ship |
| Moraxella catarrhalis | COPD, smokers |
| Mycobacterium tuberculosis | Alcoholics, HIV, elderly, injection drug use |
| Mycoplasma pneumoniae | Prominent cough, hyperreactive airways, hemolytic anemia |
| Pneumocystis jirovecii | HIV, chronic corticosteroid use |
| Pseudomonas aeruginosa | COPD, bronchiectasis |
| Staphylococcus aureus | Postinfluenza, endobronchial obstruction, injection drug use |
| Streptococcus pneumoniae | Most common through all age groups, alcoholics, postinfluenza |
Bacillus anthracis (anthrax), Yersinia pestis (plague), and Francisella tularensis (tularemia) would be the most likely bacterial bioterrorism agents to cause pneumonia.
Abbreviations: COPD = chronic obstructive pulmonary disease; SARS = severe acute respiratory syndrome; MERS = Middle East respiratory syndrome; HIV = human immunodeficiency virus.
Streptococcus pneumoniae is the most commonly identified pathogen in all treatment settings, followed by atypical bacterial agents, Haemophilus influenzae, and respiratory viruses. Staphylococcus aureus causes only 1% to 3% of cases of CAP, but is associated with significant morbidity and mortality, especially in younger adults. There are no unique clinical features of any pathogen that allow a specific identification by history alone. Human immunodeficiency virus (HIV) disease should be a diagnostic consideration in most patients hospitalized with CAP.
Laboratory studies (Tables 32.3 and 32.4) may be useful in diagnosis and management. The extent of the evaluation should depend on the severity of illness and the likelihood that test results will influence therapy. Diagnostic studies are usually unnecessary for patients treated on an outpatient basis because empiric antimicrobial choices adequately treat most common etiologies. Hospitalized patients should have, at a minimum, routine laboratory studies including a complete blood count with differential, a chest radiograph, and arterial blood gases. Blood cultures are diagnostic in up to 14% of patients hospitalized for CAP. They are most useful in patients with severe CAP (Table 32.4). The Gram stain and culture of expectorated sputum is most useful in patients with severe CAP (Table 32.4). The sputum specimen should be grossly purulent, obtained by deep cough (or tracheal aspirate), and processed in less than 2 hours. Minimum criteria for a specimen suitable for culture are fewer than 10 squamous epithelial cells or more than 25 polymorphonuclear neutrophils (PMNs) per low-power field. Sputum studies can be diagnostic for Legionella, mycobacteria, fungi, and Pneumocystis jirovecii (Table 32.3). Molecular diagnostic tests such as polymerase chain reaction amplification assays may be performed on nasopharyngeal or lower respiratory tract specimens for many pathogens including S. pneumoniae, Mycoplasma pneumoniae, Chlamydia pneumoniae, Bordetella pertussis, and respiratory viruses. A parapneumonic pleural effusion is a common complication of pneumonia, and cultures obtained by thoracentesis will often give a positive result. The incidence of pleural effusion with pneumonia depends on the etiologic agent, accompanying ~95% of Streptococcus pyogenes infections but only 10% of S. pneumoniae infections. Sampling of the lower respiratory tract via bronchoalveolar lavage, bronchoscopic protected specimen brushings, or quantitative endotracheal aspirates may be useful in nonresolving pneumonia. Transthoracic needle aspiration and lung biopsy should only be considered in severely ill patients not responding to therapy for whom less invasive techniques are nondiagnostic.
| Chest radiograph (posterior and lateral) |
| Arterial blood gas values (for hospitalized patients. Pulse oximetry should be obtained for patients judged suitable for outpatient therapy.) |
| Complete blood count with differential |
| Chemistry panel, including electrolytes, glucose, blood urea nitrogen, and creatinine |
| Aminotransferases |
| Blood culture (2 sets drawn 10 minutes or more apart) Not necessary for all patients. See Table 32.4. |
| Pleural fluid stain, culture, leukocyte count with differential, pH |
| Sputum studies (for pneumonia unresponsive to usual antibiotics, see Table 32.4): Acid-fast stain and culture Fungal stains and culture Legionella spp. culture Immunofluorescent antibody, Gomori’s methenamine silver, or Giemsa stain for Pneumocystis jirovecii (carinii) A Gram stain (from an appropriately obtained specimen, examined by an expert within 2 h of collection before the patient has received antibiotics) |
| Urinary antigen Streptococcus pneumoniae Legionella spp. |
| Serology (for patients with appropriate epidemiologic history) HIV serology Legionella spp. Francisella tularensis Mycoplasma pneumoniae Chlamydia (pneumoniae and psittaci) spp. Coxiella burnetii |
Abbreviation: HIV = human immunodeficiency virus.
| Indication | Blood culture | Sputum culturea | Legionella UAT | Pneumococcal UAT |
|---|---|---|---|---|
| ICU admission | X | X | X | X |
| Failure of outpatient antibiotic therapy | X | X | X | |
| Cavitary infiltrates | X | Xb | ||
| Leukopenia | X | X | ||
| Active alcohol use | X | X | X | X |
| Chronic severe liver disease | X | X | ||
| Severe obstructive/structural lung disease | X | |||
| Asplenia (anatomic or functional) | X | X | ||
| Recent travel (within 2 weeks) | X | |||
| Positive Legionella UAT result | X
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