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

Reporting Name

Legionella Pneumophila Ab, S

Useful For

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

Clinical Information

Legionella pneumophila may cause pulmonary disease in normal and immunocompetent individuals. The disease may occur sporadically in the form of community acquired pneumonia or as an epidemic. Pneumonia (often referred to as Legionnaires disease) occurs more frequently in individuals who are severely immunosuppressed; a milder form of the illness, 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.


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.

Report Available

Same day/1 to 5 days

Day(s) Performed


Clinical Reference

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: Legionnaires' 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

Method Name

Enzyme-Linked Immunosorbent Assay (ELISA)

Specimen Type


Specimen Required

Collection Container/Tube:

Preferred: Serum gel

Acceptable: Red top

Submission Container/Tube: Plastic vial

Specimen Volume: 0.5 mL

Collection Instructions: Centrifuge and aliquot serum into plastic vial.

Specimen Minimum Volume

0.4 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Refrigerated (preferred) 14 days
  Frozen  14 days

Reference Values


Reference values apply to all ages.

Test Classification

This test has been modified from the manufacturer's instructions. Its performance characteristics were determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the US Food and Drug Administration.

CPT Code Information


LOINC Code Information

Test ID Test Order Name Order LOINC Value
SLEG Legionella Pneumophila Ab, S 7947-5


Result ID Test Result Name Result LOINC Value
SLEG Legionella Pneumophila Ab, S 7947-5


If not ordering electronically, complete, print, and send Infectious Disease Serology Test Request (T916) with the specimen.

Mayo Clinic Laboratories | Microbiology and Infectious Disease Catalog Additional Information: