Is It Infection?

A careful history and physical examination are essential. No universally effective approach is recognized, however, and good clinical judgment often will be needed (Table 23-1). Typically, the patient with pulmonary infection will be pyretic and will have a cough characterized by purulent sputum. Rigors are even more specific to this disorder but occur only in more severe infections. An abrupt onset with pleuritic chest pain is a classical presentation in pneumonia but often is lacking. Wheeze may be present in airway infections, and focal chest signs are common in pneumonia. The more specific pneumonia signs of bronchial breathing, egophony, and whispering pectoriloquy are rare. Unfortunately, in real life, the presence of advancing age and immunosuppression can mean that infection is present despite the absence of any of these signs, and underlying lung disease may mean that some of them are present in the absence of infection.

Table 23-1 Features of Respiratory Infection and Alternative Causes

For these reasons a confident diagnosis of pneumonia is difficult outside the hospital, where the cardinal feature of consolidation on the chest radiograph may be difficult to obtain. Even this feature may have other causes, but in the context of symptoms and signs suggesting infection, pneumonia is the likely diagnosis.

Very often the most important clinical considerations are whether an antibiotic should be prescribed (i.e., is this a bacterial infection?) and whether the patient should be admitted to the hospital. Historically, sputum purulence and the raised peripheral blood white cell count were considered to be suggestive of bacterial infection. Inflammatory markers such as C-reactive protein may be more specific and sensitive than the white cell count, and the hormokine procalcitonin may be an even more accurate marker of the presence or absence of bacterial infection. Available data are inconclusive, however, to allow recommendation of its routine use. The decision about hospital admission should be guided mainly by illness severity (see further on), but instability of comorbid diseases (e.g., diabetes mellitus) and social issues also are relevant to this decision.

Identification of a microorganism should be the best way to confirm infection, but this approach too has a number of limitations, most important of which is the insensitivity of available tests.

What Type of Infection is It?

Classification of Pulmonary Infections

The traditional anatomic classification of lung infection (e.g., upper respiratory, lower respiratory, pleural) forms a useful but far from perfect template on which to base a discussion of this topic. Although many infections are limited by anatomic boundaries (e.g., a lobar pneumonia), others do not fit easily into this classification schema (e.g., influenza virus infection).

The two main groups of acute adult pulmonary infections encountered in hospital practice are acute exacerbations of chronic obstructive pulmonary disease (COPD) (see Chapter 43) and pneumonia (Chapters 24 and 25). A chest radiograph is the key to separating these two, because it is the most sensitive screening method for detecting pulmonary consolidation (Figure 23-1, A and B). The presence of focal crackles is the most common feature of underlying consolidation. A pleural rub, or features of pleural effusion, may coexist. Other less common features include diarrhea, hypotension, and, in the elderly, mental confusion, hypothermia, and urinary incontinence. When pneumonia is suspected on clinical grounds and the chest radiograph is normal in appearance, a chest computed tomography (CT) scan may show changes in the lung parenchyma. However, other diseases may mimic pneumonia or be complicated by pneumonia (Figures 23-2 to 23-4).

Figure 23-1 Chest radiographs obtained in two patients with underlying chronic obstructive pulmonary disease (COPD) who presented with cough, purulent sputum, and dyspnea. A, Uncomplicated COPD exacerbation with no lung consolidation. B, COPD exacerbation caused by pneumonia with right middle zone consolidation.

Figure 23-2 Pulmonary eosinophilia. Right upper lobe consolidation may mimic pneumonia, as in this case.

Figure 23-3 Drug-induced lung disease. Bilateral, patchy, predominantly basal consolidation was secondary to bleomycin administration.

Figure 23-4 A, Appearance of left basal lung consolidation on chest radiograph. B, Contemporary computed tomography (CT) scan shows that consolidation is more extensive and is secondary to pulmonary embolism (arrow).

It is important to identify the likely type of pneumonia according to the setting in which the infection was acquired (Table 23-2). The spectrum of pathogens differs for the five pneumonia types—community-acquired, early, and late nosocomial; immunosuppression-related; and aspiration.

Table 23-2 Classification of the Pneumonias According to Likely Origin and Immune Status

Pneumonia Group	Likely Pathogens
Community-acquired	Gram-positive bacteria Mycoplasma, Chlamydia, Coxiella Common viruses (e.g., influenza viruses)
Nosocomial, early	As for community-acquired pneumonias
Nosocomial, late	Gram-negative enterobacteria Staphylococcus aureus Antibiotic-resistant bacteria
Immunocompromised	Opportunistic organisms
Aspiration	Oropharyngeal anaerobic bacteria, chemical pneumonitis

Other roles for plain chest radiography include assessing the extent of disease, for which it is especially useful; detecting complications (e.g., cavitation, abscess, pneumothorax, pleural effusion); identifying the lesions of additional or alternative diagnoses (e.g., bronchiectasis, pulmonary fibrosis, bronchial neoplasm); and, in some cases, guiding invasive investigation.

How Severe is the Illness?

The severity of clinical illness guides decisions about the optimal setting in which to manage the patient—home versus hospital, general ward or intensive care unit (ICU)—and also may guide specific investigations and treatment strategies.

In the patient with an exacerbation of COPD, measures of respiratory distress (e.g., respiratory rate) and gas exchange are the best markers of illness severity.

Among patients with community-acquired pneumonia (CAP), those who die usually are severely ill at presentation, so the correct interpretation of presenting features is important. This information should be supplemented by the results of investigations as they emerge. Scoring systems have been developed as an aid to clinical judgment to assess CAP severity. The Pneumonia Severity Index (PSI) is a two-step severity score built from 20 clinical and laboratory features (Figure 23-5). It has been validated in a number of studies mainly as a tool for keeping low-risk patients out of the hospital. The simpler CURB-65 index also is well validated and is based on just five clinical features (Figure 23-6), whereas CRB-65, which may be used outside the hospital setting, is based on four. Comparisons of the scores confirm the slightly greater accuracy of the PSI but simpler application of CURB-65 and CRB-65. Use of these scoring systems is recommended in the most recent CAP management guidelines.

Figure 23-5 The Pneumonia Severity Index.

Figure 23-6 The CURB-65 score. DBP, diastolic blood pressure; MSQ, mental status questionnaire; SBP, systolic blood pressure.

In nosocomial pneumonia as well as pneumonia in the immunocompromised patient, the importance of presenting features as opposed to features that develop during the course of the illness is less clearly defined than for patients who have CAP. Previous or inappropriate antibiotic therapy, renal failure, prolonged mechanical ventilation, coma, shock, and infection with Pseudomonas aeruginosa, Acinetobacter spp., and methicillin-resistant Staphylococcus aureus (MRSA) are additional markers for severe nosocomial pneumonia (Box 23-2).

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