Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (582 page)

   Common causes include influenza (adults), parainfluenza (children), RSV (immunocompromised patients), human metapneumovirus (children), adenovirus, corona viruses, CMV (primarily in immunocompromised patients and children), HSV, measles virus, and VZV.

   Who Should Be Suspected?

   The clinical presentation is variable and depends on the patient’s age, immunocompetence, underlying medical conditions, and specific viral pathogen. Most patients have mild, self-limited disease, but viral pneumonia may present clinically with life-threatening disease, especially in high-risk patients. In immunocompetent hosts, disease is usually self-limited and mild, with resolution of symptoms within 7–10 days.

   The activity of viruses circulating in the community should be considered in the patient’s initial assessment.

   The presenting findings in viral pneumonia include acute illness with fever, showing signs of hypoxemia. Cough is usually nonproductive with scant mucoid sputum. Examination typically demonstrates tachypnea, rales, and wheezing. There may be signs of viral infection in other respiratory tract tissues, like conjunctivitis and acute rhinosinusitis. Underlying medical conditions may be exacerbated by viral pneumonia; the severity of viral pneumonia is often greater in patients with underlying illness.

   Imaging studies typically demonstrate diffuse, bilateral interstitial infiltrates, although the spectrum of abnormalities is broad and nonspecific.

   Bacterial superinfection is well described and represents a significant complication of viral pneumonia. Bacterial superinfection may be suspected in patients whose initial pneumonia resolves but develop fever, cough, and dyspnea 1–2 weeks later. Bacterial pathogens associated with superinfection of viral pneumonia include
S. pneumoniae
,
H. influenzae
, and
S. aureus
.

   Diagnostic and Laboratory Findings

Most patients with viral pneumonia have a relatively benign, self-limited illness. Specific diagnosis is usually not required unless severe disease or complication of infection is present.

Culture
: Most of the relevant viruses may be isolated by viral culture, but turnaround time is slow. Therefore, viral culture is usually not helpful for acute clinical management.

Direct antigen detection
: Antigen detection kits are commercially available for a number of the relevant viruses, such as influenza viruses A and B, RSV, and human metapneumovirus. Although the specificity of these assays is usually high, sensitivity may be <80%; they may be used to confirm but cannot exclude any specific viral infection. The use of specific DFA staining is useful for evaluation of specimen quality and has shown improved sensitivity.

Molecular testing
: FDA-approved assays are available for respiratory viral pathogens. These assays provide high sensitivity and specificity, a broad range of detectable viruses, and short turnaround time, but higher cost, compared to culture and antigen testing.

Serology:
Serologic testing is not useful for the acute management of patients.

Core laboratory findings
: ABGs, CBC, and other tests should be monitored in patients with severe or complicated viral pneumonia. Core laboratory tests are usually normal. In patients with severe respiratory distress, careful monitoring of ABGs is critical for patient management. Fluid status must be monitored carefully because of the risk of dehydration due to fever and tachypnea.

Suggested Reading

Treanor JJ. Chapter 2 Respiratory infections. In: Richman DD, Whitley RJ, Hayden FG (eds).
Clinical Virology
, 3rd ed. Washington, DC: ASM Press; 2009.

TUBERCULOSIS

Diagnosis of tuberculosis is suspected on clinical presentation, screening tests (e.g., IGRAs), and imaging studies and is confirmed by acid-fast smear and culture and other laboratory findings. See Chapter
11
, Infectious Diseases for a further discussion of mycobacteria and mycobacterial diseases.

   Definition

Tuberculosis refers to disease caused by infection with
Mycobacterium tuberculosis
(Mtb), or, rarely, related mycobacterial species. Tuberculosis is usually transmitted by inhalation of respiratory droplets. Transmission is not efficient, typically requiring prolonged exposure on multiple occasions. Other organs may be infected by lymphohematogenous spread.

   Who Should Be Suspected?