Legionella Infection Workup

Updated: Jun 05, 2023
  • Author: Mobeen H Rathore, MD, CPE, FAAP, FIDSA; Chief Editor: Russell W Steele, MD  more...
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Laboratory Studies

General laboratory testing in patients with Legionnaires disease (LD) reveals several nonspecific abnormalities. [11]

Hematologic studies may reveal leukocytosis or leukopenia with left shift, thrombocytosis, or thrombocytopenia with disseminated intravascular coagulopathy (DIC).

Erythrocyte sedimentation rate and C-reactive protein levels are elevated.

Chemistry studies may reveal elevated aminotransferase levels, hyponatremia (more commonly associated with Legionnaires disease than with other pneumonias in adults), hypophosphatemia, elevated creatine kinase levels, and other abnormalities. These abnormalities are not as common in children as they are in adults.

Urinalysis commonly reveals proteinuria and hematuria.

Diagnosis cannot be excluded when one or more of the following results are negative (a combination of tests increases the probability of confirming the diagnosis):

  • Urinary antigen test

    • This is the preferred initial test for Legionnaires disease.

    • The urine antigen test is a rapid, relatively inexpensive, and practical test for the detection of L pneumophila antigen excreted in the urine or present in pleural fluid.

    • The primary disadvantage of urinary antigen testing is that it detects only L pneumophila serogroup 1. However, because this serogroup causes most cases of Legionnaires disease, the test is recommended strongly as part of the workup.

    • Urine antigen testing has 70% sensitivity and approaches 100% specificity. [16]

    • Sensitivity improves if urine samples are concentrated by ultrafiltration and obtained within 7 days of the onset of pneumonia.

    • Test results may remain positive for weeks, even after appropriate antibiotic therapy.

  • Gram stains and cultures of sputum, lower respiratory tract secretions, tissue, or blood

    • In children, the best samples are obtained by bronchoscopy. [16] Samples obtained by bronchoalveolar lavage (BAL) are better than those from bronchial washings. In addition, fluid or pus from normally sterile sites (eg, cerebrospinal fluid [CSF], pleural fluid, peritoneal fluid) should be cultured.

    • Gram stain may show small, pleomorphic, weakly staining, gram-negative bacilli or no organisms with a large number of polymorphonuclear leukocytes. L micdadei may stain with acid-fast stain.

    • Culturing, considered the criterion standard, requires the use of special media (buffered charcoal yeast extract [BCYE] agar with L-cysteine and ferric ions to support growth, antibiotics to prevent overgrowth of other organisms, and dyes to impart a distinctive color to the organisms).

    • Specialized techniques may require 2-7 days to isolate Legionella organisms. Routine sputum cultures provide 80% sensitivity and 100% specificity. Culture from BAL specimens has a sensitivity of 90% or greater.

    • Since co-infection with other pathogens often occurs, isolation of other pathogens does not exclude the possibility of concomitant Legionella infection.

    • Legionella species can also be isolated from blood cultures; yield can be optimized by blind subculture (before sample results turn positive) onto BCYE agar from radiometric culture bottles.

  • Direct fluorescent antibody staining for Legionella species

    • Direct fluorescent antibody (DFA) staining is a rapid test that can be performed on respiratory samples and tissue and requires only 2-4 hours for results.

    • DFA using a monoclonal antibody is highly specific.

    • A cross-reaction with Pseudomonas species rarely occurs, and patients with tularemia may have a false-positive DFA result for Legionella organisms.

    • DFA staining has a low sensitivity of 33-70%. [16] Sensitivity depends on the specimen quality, the number of organisms present, and the experience of the technician. A negative DFA result does not exclude Legionella infection.

  • Serologic tests for Legionella antibodies

    • Serologic assays are not helpful in clinical decision making but are valuable for epidemiologic studies.

    • Confirmation of legionellosis requires a 4-fold or greater rise in antibody titer in paired acute and convalescent indirect fluorescent antibody (IFA) tests obtained 4-8 weeks apart.

    • A single elevated titer greater than 1:256 does not confirm a diagnosis of Legionnaires disease; titers of 1:256 or more are found in 1-16% of healthy adults and children.

  • Polymerase chain reaction (PCR) test

    • PCR assays to detect Legionella DNA in urine, BAL fluid, and serum samples have been used. Although commercial kits are available, the test is not widely used.

    • In adults, these assays appear highly specific but are no more sensitive than culture.

    • Published studies of Legionella diagnosis using PCR assays in children are limited but anecdotal cases continue to be reported. [12, 13, 31]


Imaging Studies

Pneumonia is the predominant clinical syndrome of Legionnaires disease. Chest radiography findings vary and are nonspecific and indistinguishable from those observed with other pneumonias.

Although initial chest radiography findings may be normal, especially in patients with nosocomial disease, the usual progression of findings on serial studies is from patchy areas or nodular appearance to multilobar, almost homogeneous, infiltrates. [17, 32]

Unilateral involvement is more common than bilateral involvement. Purely interstitial infiltrates are rare. Pleural effusion, present in at least one third of patients, may be the only abnormality. In adults, cavitation is more common in patients who are immunocompromised but has been described both in immunocompromised and immunocompetent children. [33, 34, 35]

Radiographic findings usually progress despite appropriate antibiotic therapy; infiltrates may take as long as 4 months to completely resolve. Permanent bullous emphysema requiring lobectomy has been reported. [36]



Perform BAL in pediatric patients for DFA and culture. Consider PCR if index of suspicion is high.

Perform thoracocentesis if the extent of pleural effusion is significant. Test fluids for Legionella antigen and perform DFA and culture. Consider PCR if suspicion is high.


Histologic Findings

An intense intra-alveolar inflammation without clinically significant intrabronchial exudate is the histologic hallmark finding. Alveoli usually contain a large number of polymorphonuclear leukocytes, alveolar macrophages, and necrotic debris.

Organisms are detected both intracellularly and extracellularly.

Microabscesses may be present in the lung parenchyma.

Fibrin formation and a predominance of histiocytes occur in more advanced stages of disease.