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Legionella Antigen DFA

Item Value
Approval req'd? No
Available Stat? No
Test code P124
Test group Legionella
Performed by Microbiology
In House Availability Monday-Saturday, day shift.
Method Direct immunofluorescent assay
Collection Instructions Due to potential cross reactivity with other organisms please contact Microbiology at x3-1268 before collecting upper respiratory tract samples.
Container type Clean container, urine cup
Sample type BAL, Sputum, Unfixed tissue
Normal range Negative
Synonyms Legionnaires disease; Legionella antigen
Stability Refrigerated 24 hours
Additional information Diagnosis of L. pneumophila can be challenging, particularly since existing tests lack good sensitivity. While this disease is usually considered in an otherwise healthy individual presenting with atypical pneumonia (possibly in an outbreak setting), our patient population often has significant comorbidities that make the diagnosis even more difficult.

1) Culture is the gold standard and 100% specific, but sensitivity can vary by lab from 25-75%. Inadequate specimens, poor growth characteristics and prior antibiotic use will decrease culture yield.
2) DFA is highly specific, but has relatively poor sensitivity (approx 50%), again, inadequate specimens and few organisms can be present in samples. Additionally, samples from the upper respiratory tract may contain cross reacting bacterial species.
3) Urine Ag can be more sensitive for L. pneumophilia, Serotype 1, however at UCSF only 28% of Legionella infections are due to this serotype.

Overall, diagnosis of Legionella pneumonia is first entertained and treated clinically, proper specimen collection is critical to the laboratory diagnosis, with culture and DFA being a reasonable first pass where there is moderate to high clinical suspicion that Legionella is the causative organism. Unfortunately, no test is adequate for ruling out the disease, and most testing is designed to identify outbreak situations for infection control and treatment of exposed individuals.
LOINC code 39541-8
Last Updated 10/24/2013 10:46:12 AM
Entry Number 597
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