Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1388 page)

   Interpretation

   
Positive:
Presumptive positive for
L
.
pneumophila
serogroup 1 antigen in urine, suggesting current or past infection.

   
Negative:
Presumptive negative for
L
.
pneumophila
serogroup 1 antigen in urine, suggesting no recent or current infection. Infection due to
Legionella
cannot be ruled out, since other serogroups and species may cause disease, antigen may not be present in urine in early infection, and the level of antigen present in the urine may be below the detection limit of the test.

   Limitations

   There is no single confirmatory laboratory test for Legionnaire disease. Culture results, serology, and antigen detection methods may all be useful, in conjunction with clinical findings, for diagnosis.

   The
Legionella
antigen test will not detect infections caused by other
L
.
pneumophila
serogroups and by other
Legionella
species. Culture is recommended for suspected pneumonia to detect causative agents other than
L
.
pneumophila
serogroup 1 and to recover
L
.
pneumophila
serogroup 1 when antigen is not detected.

   Excretion of
Legionella
antigen in urine may vary depending on the individual patient. Antigen excretion may begin as early as 3 days after onset of symptoms and persist for up to a year afterward.

   A positive
Legionella
urinary antigen test result can occur due to current or past infection and, therefore, is not definitive for infection without supporting evidence.

LEGIONELLA
CULTURE (RULE OUT)

   Definition

   This test is requested for diagnosis of legionellosis by culture of patient specimens, usually of the lower respiratory tract. Testing is recommended on patients with pneumonia who fail treatment, have severe pneumonia, are immunocompromised, or have epidemiologic risk for legionellosis. Special testing, usually performed outside of clinical microbiology laboratories, is needed for evaluation of environmental cultures for isolation of
Legionella
species.

   Special Collection and Transport Instructions

   Specimens should be submitted early in the acute phase of infection.