Test Catalog

Test ID: SLEG    
Legionella pneumophila (Legionnaires Disease), Antibody, Serum

Useful For Suggests clinical disorders or settings where the test may be helpful

Evaluation of possible legionellosis (Legionnaires disease, Pontiac fever, extrapulmonary legionella infection caused by Legionella pneumophila)

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Legionella pneumophila may cause pulmonary disease in both normal and immunocompetent hosts. The disease may occur sporadically in the form of community acquired pneumonia and in epidemics. Pneumonia (often referred to as Legionnaires disease) occurs more frequently in severely immunosuppressed individuals; a milder form of the illness, referred to as Pontiac fever, is more prevalent in normal hosts. Extrapulmonary infection with L pneumophila is rare. Legionnaires disease, Pontiac fever, and extrapulmonary infection have been collectively referred to as legionellosis.

Approximately 85% of the documented cases of legionellosis have been caused by L pneumophila. Serogroups 1 and 6 of L pneumophila, by themselves, account for up to 75% of cases of legionellosis.


The definitive diagnosis of L pneumophila is made by isolation of the organism on specialized culture medium (buffered charcoal yeast extract agar) or detection by a nucleic acid amplification test. In the absence of invasive procedures (eg, bronchial alveolar lavage), evaluation of patient urine samples for L pneumophila serotype 1 antigen may be useful. Testing for antibodies to L pneumophila may be helpful to establish prior exposure or infection, however, does not differentiate between acute and past infection.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.


Reference values apply to all ages.

Interpretation Provides information to assist in interpretation of the test results

A negative result indicates that IgG, IgA, and IgM antibodies to Legionella pneumophila serogroups 1-6 were not detected. Negative results do not exclude Legionella infection. It may require 4 to 8 weeks to develop a detectable antibody response; serum specimens taken early in the course of infection may not yet have significant antibody titers. Furthermore, antibody levels can fall to undetectable levels within a month of infection, early antibiotic therapy may suppress antibody response, and some individuals may not develop antibodies above detectable limits.


Some culture-positive cases of Legionella do not develop Legionella antibody.


Positive results are suggestive of Legionella infection. A positive result only indicates immunologic exposure at some point in time. It does not distinguish between previous or current infection. The level of antibody response may not be used to determine active infection. Other laboratory procedures or additional clinical information are necessary to establish a diagnosis.


Specimens with equivocal results are retested prior to reporting. Repeat testing on a second specimen should be considered in patients with equivocal results, if clinically indicated.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

A diagnosis should not be made on the basis of positive Legionella antibody results alone. Test results for Legionella antibodies should be interpreted in conjunction with the clinical evaluation and the results of other diagnostic procedures.


A positive result suggests infection with one or more of the groups 1-6 species; however, it is not possible to distinguish between species with the results of this enzyme-linked immunosorbent assay (ELISA) test alone.


Use of serogroups 1-6 for assessing antibody responses to different Legionella species and serogroups has not been established.


Cross-reactivity may occur in sera with infections due to other Legionella species.


Positive results may be due to cross-reactivity with antibody generated as a result of non-Legionella infection. Serologic cross-reactions have been reported with Pseudomonas aeruginosa, several Rickettsia species, Coxiella burnetii, enteric gram-negative rods, Bacteroides species, Haemophilus species, Citrobacter freundii, and Campylobacter jejuni. Additionally, some reports indicate that a number of apparently healthy individuals may carry antibodies to legionellae; however, a positive result, along with clinical signs and symptoms may indicate possible Legionella infection. Additional testing to directly detect the organism, either through culture of nucleic acid amplification tests is recommended to make a diagnosis of current infection.


The assay performance characteristics have not been established for matrices other than sera.


Although the conjugate is designed to detect human IgG, IgM, and IgA, it is not possible to determine which antibody is present with this assay.


The use of hemolytic, lipemic, bacterially contaminated, or heat-inactivated specimens should be avoided as erroneous results may occur.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Koneman EW, Allen SD, Janda WM, eds. Color Atlas and Textbook of Diagnostic Microbiology. 5th ed. Lippincott-Raven Publishers 1997

2. Edelstein PH, Roy CR: Legionaires' disease and Pontiac fever. In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Elsevier; 2020