Is the Urine Antigen test the exclusive test method for confirming or ruling out Legionnaires Disease?
The diagnosis of Legionnaires disease cannot be based on clinical or radiological evidence alone. There is no single satisfactory laboratory test for Legionnaires disease. Culture results, serology, and antigen detection methods should all be used in conjunction with clinical findings for diagnosis.
The Legionella pneumophila serogroup 1 will not detect infections caused by other serogroups, Legionella micdadei or Legionella longbeachae. Culture is recommended for suspected pneumonia to detect causative agents other than Legionella pneumophila serogroup 1, and to confirm infection.
Excretion of Legionella antigen in urine may vary among patients, depending on their underlying illness or treatment. Some individuals have been shown to excrete antigen for extended periods of time (up to 1 year after acute infection) and positivity may, therefore, indicate previous infection rather than current infection. Early treatment with appropriate antibiotics may also decrease antigen excretion in some individuals, and the use of diuretics may affect the ability of the test to detect antigen. Consequently, patient history (eg, a history of a recent respiratory illness compatible with Legionnaires disease) must be considered when evaluating results.
Is there a time limitation within which the Urine Antigen test is best performed before the test results become unreliable?
The presence of Legionella antigen in urine makes an ideal specimen for collection, transport, and subsequent detection in early, as well as later, stages of the disease.
Legionella antigenuria can be detected as early as 1 day after onset of symptoms and persists for days to weeks. In one instance, excretion of antigen was documented to occur for longer than 300 days.
Does a negative result for Legionnaires Disease from a Urine Antigen test confirm the lack of Legionnaires Disease?
Presumptive negative for Legionella pneumophila serogroup 1 antigen in urine, suggests no recent or current infection. Infection with Legionella cannot be ruled out because:
- Other serogroups (other than serogroup 1, which is detected by this assay) and other Legionella species (other than Legionella pneumophila) can cause disease
- Antigen may not be present in urine in early infection
- The level of antigen may be below the detection limit of the test
Legionella culture is recommended for cases of suspected Legionella pneumonia due to organisms other than Legionella pneumophila.
What are the alternative tests that can be performed in the event of a negative antigen test and should they be performed concurrently?
Legionella infection is undoubtedly under recognized. Diagnosis relies on the use of specialized tests, often in combination. Urinary antigen tests, sputum culture, and PCR (Polymerase Chain Reaction) testing of lower respiratory tract samples are the most important diagnostic tools for detection of Legionella infection early in the course of illness.
Understanding local Legionella epidemiology is important for making decisions about whether to test for Legionella infection and which diagnostic tests to use. In most situations, the use of both the urinary antigen test plus sputum culture is the best diagnostic combination. Further work needs to focus on the development of urinary antigen tests assays that detect a wider range of pathogenic legionellae.