Lower respiratory tract infections (LRTIs) are among the most frequently encountered and difficult to diagnose infectious syndromes. Symptoms of most LRTIs overlap with other clinical conditions, such as congestive heart failure and obstructive lung disease, such asthma and chronic obstructive pulmonary disease (COPD). Diagnostic tests are not specific to LRTIs, and even invasive studies have a low yield in terms of confirming the diagnosis. This means that a thorough understanding of the epidemiology, clinical presentation, and patient factors is critical to providing care for those suffering from respiratory tract infections.
Broadly speaking, LRTIs can be divided into different categories based on patient exposures and characteristics. The most common of these would be community-acquired pneumonia (CAP) followed by health care–associated pneumonia (HAP). Additionally, there are a subset of patients and risk factors that merit special consideration. These pertain to endemic illnesses and respiratory tract infections in immunocompromised hosts. This chapter will discuss each of these groups.
Community-Acquired Pneumonia
Pneumonia, and CAP in particular, is a leading cause of death in the United States, particularly in the elderly. This is in part due to the nonspecific presentation and lack of a practical diagnostic gold standard to confirm CAP; therefore the diagnosis frequently relies upon symptom assessment and clinical gestalt. Several critical decisions must be made early in the course of CAP: first, whether CAP is present; second, if the patient has risk factors for multidrug-resistant organisms (MDROs); third, which clinical setting (outpatient, inpatient, or intensive care unit [ICU]) is appropriate for management; and fourth, which antimicrobial agents and what duration of therapy are most appropriate. The Infectious Disease Society of America and the American Thoracic Society have issued joint guidelines on the management and diagnosis of CAP, which integrate evidence-based approaches to these dilemmas and have been proven to reduce mortality.
Diagnosis is based on respiratory and infectious symptoms such as cough, fever, and malaise associated with radiographic infiltrate suggestive of pneumonia. Although it is possible for a chest radiograph to be clear in pneumonia, this is highly unusual, and typically further imaging (e.g., computed tomography) is not required to establish a clinically significant pneumonia.
Other testing can support the diagnosis or help with risk stratification, but normal results do not rule out pneumonia. For example, sputum microbiology, although helpful if positive, is negative in greater than half of patients with pneumonia. Renal function, leukocyte count, and hematocrit are all useful in risk stratification schemes, such as the Pneumonia Severity Index, but do little for establishing or refuting a diagnosis. Novel biomarkers, such as procalcitonin, have shown promise in study settings, but practical implementation trials to date have shown no known diagnostic benefit, and biomarkers are currently not part of any guideline-driven CAP treatment protocols.
The next decision is where to place the patient for clinical management; the outpatient setting is appropriate for the mildly to moderately ill, the inpatient setting is needed for the moderately to severely ill, and the intensive care setting for the severely ill. Tools like the Pneumonia Severity Index (a complex score typically requiring a calculator to use, but reasonably accurate) or the CURB-65 (a simple, but less accurate, score) can be used to assist such risk stratification, but ultimately, this is a clinical decision based on the patient’s appearance, baseline level of health, and risk for progression of disease.
Once the patient’s risk factors are categorized and a working diagnosis of pneumonia is established, the next step is to obtain additional diagnostic tests based on the most likely etiologies and risk factors for the patient. This is shown in Table 9.1 .
| Setting | Blood Culture | Sputum Culture | Urine Testing | Other Testing |
|---|---|---|---|---|
| ICU admission | X | X | Legionella and pneumococcal antigens | Multiplex PCR |
| Failure of outpatient treatment | X | Legionella and pneumococcal antigens | Multiplex PCR | |
| Asplenia or leukopenia | X | X | Pneumococcal antigen | Multiplex PCR Consider bronchoscopy |
| Alcohol abuse | X | X | Legionella and pneumococcal antigens | Multiplex PCR |
| Recent travel | Legionella antigen | |||
| Pleural effusion | X | X | Legionella and pneumococcal antigen | Multiplex PCR, thoracentesis |
| Cavitary disease | X | X | None | Fungal and mycobacterial blood cultures, consider bronchoscopy |
Once diagnostics are underway, appropriate empiric treatment should be initiated based on the patient’s clinical situation. Empiric therapy is targeted at common organisms, knowing that the most commonly isolated pathogen in CAP is Streptococcus pneumoniae , with other typical organisms, including Haemophilus influenzae, Moraxella catarrhalis , and atypical organisms such as Legionella spp. and Mycoplasma pneumoniae . Viral causes of illness, such as influenza and parainfluenza, are also common, but given the propensity for bacterial superinfections to occur in the critically ill, antibacterial therapy is often given when empirically treating a severe viral pneumonia. Of note, H. influenzae is thus named because of being mistakenly attributed as the cause of influenza in the 1890s due to this organism frequently being isolated from influenza patients during a pandemic.
Conversely, it should be noted that viruses can mimic bacterial pneumonia and, particularly during influenza season, testing for influenza and other viral pathogens should be performed in hospitalized patients, both for accurate diagnosis of viral, bacterial, or coinfections causing pneumonia and in order to initiate infection prevention strategies to limit the risk of nosocomial spread of viral infections.
CAP patients may be at risk for infections with MDROs in certain settings. Patients who were recently hospitalized, on antibiotics, or chronically ill with immune or pulmonary disease or chronic illness may lead to colonization with organisms such as Pseudomonas aeruginosa , notorious for developing resistance to first-line antimicrobial agents.
For common organisms (not the potential MDROs), empiric treatment is targeted at these organisms. For mild cases, a macrolide antibiotic or, as an alternative, doxycycline may be appropriate. As patients become more severely ill, options diverge to either be fluoroquinolone based or a combination of a β-lactam and macrolide, as this provides common and atypical bacteria coverage, and dual therapy carries potential morbidity and mortality benefits for those with severe S. pneumoniae pneumonia. In the ICU setting, a combination of a β-lactam (or cephalosporin) plus a macrolide or fluoroquinolone antibiotic is appropriate empiric coverage ( Fig. 9.1 ). In patient at risk for MDRO, coverage for specific organisms (methicillin-resistant Staphylococcus aureus [MRSA], resistant Pseudomonas , etc.) may be indicated.
