Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (580 page)

   Diagnostic testing depends on the severity of disease and specific risk factors. Healthy outpatients may be managed without additional laboratory testing. For patients with significant infection or risk for complications of respiratory tract infection, CBC, blood culture, sputum Gram stain, and culture are recommended. High-resolution CT scanning may be requested in patients with negative CXR.

   The diagnosis of tuberculosis should be considered and ruled out as appropriate. Special culture techniques (e.g.,
Legionella
culture) or urine antigen testing (e.g.,
Legionella, S. pneumoniae
) might be considered. Additional respiratory pathogens may be considered on the basis of epidemiologic risk and clinical presentation.

   Sputum specimens should be collected prior to initiation of antibiotic treatment. Patients should be instructed on how to produce a “deep” specimen and avoid mixing saliva with the specimen. Abstaining from eating for several hours and rinsing the mouth prior to collection may improve the quality of expectorated sputum specimens.

   The value of lower respiratory culture is limited by the quality of the specimen submitted, and results must be carefully interpreted. Criteria for accepting sputum, based on the presence of PMN and bacteria, and the absence of SECs should be established for routine bacterial cultures in order to avoid inoculation of contaminated specimens. Quantitative culture of BAL specimens may improve diagnosis for patients who are unable to provide a good-quality expectorated sputum sample or when unusual pathogens are suspected.

   Informative cultures should show moderate to heavy growth of a bacterial pathogen as the predominant growth in culture. Cultures that yield growth of three or more species in comparable quantities are more likely to be contaminated and are of limited value for managing patients. A specific pathogen can only be identified by culture in about half of patients with communityacquired pneumonia that requires hospitalization. Blood culture is positive in approximately 20% of these patients; pneumococcal urinary antigen is positive in approximately 50%.

   PCR may demonstrate improved sensitivity, but the impact on patient management has not been demonstrated.

   
Core laboratories
: Leukocytosis (>15,000 with left shift) is typical for acute bacterial pneumonia. Leukopenia is associated with poor prognosis. Serial measurement of ABGs, electrolytes, and other analytes should be collected to monitor the respiratory and metabolic status of patients with severe infection. Abnormalities typical for underlying medical conditions or severity of disease should be evaluated.

Suggested Readings

Mandell LA, Wunderink RG, Anzueto A, et al. Infectious Diseases Society of America/American Thoracic Society consensus guidelines on management of community-acquired pneumonia in adults.
Clin Infect Dis.
2007;44:S27–S72.

Reimer LG, Carroll KC. Role of the microbiology laboratory in the diagnosis of lower respiratory tract infections.
Clin Infect Dis.
1998;26:742–748.

van der Eerden MM, Vlaspolder F, de Graaff CS, et al. Value of intensive diagnostic microbiological investigation in low- and high-risk patients with community-acquired pneumonia.
Eur J Clin Microbiol Infect Dis.
2005;24:241–249.

PNEUMOCYSTIS PNEUMONIA (PCP)

   Definition

Pneumocystis jirovecii
(formerly
Pneumocystis carinii
) infection is almost exclusively restricted to pulmonary disease in immunocompromised patients. Its role as an opportunistic pathogen was described after World War II in malnourished children affected by atypical pneumonia and subsequently as a rare cause of pneumonia in patients with hematologic malignancies. The incidence of PCP increased dramatically in the 1980s in association with HIV infection. Though the incidence of
P. jirovecii
pneumonitis has decreased in recent years, due to the use of highly active antiretroviral therapy and prophylaxis in susceptible patients, PCP remains an important cause of pulmonary disease in immunocompromised patients. PCP is an opportunistic infection in patients with HIV infection and is an AIDS-defining illness in these patients. The incidence of PCP has fallen dramatically in patients compliant with highly active antiretroviral treatment.

   Who Should Be Suspected?

Radiology
: Most patients with
Pneumocystis
pneumonitis show bilateral, diffuse interstitial infiltrates on CXR. Some patients with PCP have no abnormality on CXR. In such patients, high-resolution CT scans have high sensitivity for detecting the characteristic ground-glass abnormalities of PCP.

HIV-infected patients
: The onset of PCP is usually slowly progressive with fever, shortness of breath, tachypnea, and nonproductive cough. Fatigue, weight loss, and other symptoms are common. Chest x-rays most commonly demonstrate diffuse, bilateral abnormality usually consisting of interstitial infiltrates; other patterns may be seen. Gallium scanning shows intense diffuse uptake. The risk of PCP is inversely related to CD4 counts; patients with HIV infection are at highest risk when the CD4 count falls below 200 cells/mm
³
.