Paired sera desirable. Collect 2 weeks apart.
A titre of < 128 is normal
Sera demonstrating a titre of >=512 will be sent to ESR for confirmation and serotyping.
Performed 2-3 times a week depending on demand.
There are more than 30 species of Legionella however, L.pneumophilia is responsible for the bulk of human infection. Of the 14 sera groups of L.pneumophilia, 1, 4 & 6 account for the overwhelming majority of human infection.
The natural route for infection in most patients appears to be via the respiratory tract. The usual manifestation of legionella infection is of a pneumonic illness that cannot be readily distinguished from other bacterial pneumonias. Risk factors associated with infection include immunosuppression, diabetes, chronic lung disease, renal disease and alcohol excess although disease can occur in healthy young adults. Extrapulmonary infections have been observed in immunocompromised hosts.
The organisms are fastidious aerobic bacilli that are difficult to visualise by gram staining, require complex media for culture and are slow growing. Serological tests are therefore an important adjunct to microbiological diagnosis. As it usually takes 2-6 weeks for patients to develop an antibody response, serology is usually a retrospective diagnosis. Serology is also hindered by cross reactions amongst Legionella species and other bacteria. Single elevated titres of 1:256 or higher have generally been accepted as presumptive evidence of infection at an undetermined time, but even this cautious definition is not very useful because of the high background levels in some populations. 20% of the population have low legionella titres of < 1:128.
A single standing titre of 1:256 or more has generally been accepted as presumptive evidence of infection at an undetermined time, but even this cautious definition is not very useful because of the high background levels in some populations. (20%).
Seroconversion may be considered as evidence of some on-going activity although this is not necessarily evidence of an active Legionella infection. Further investigation into the patient's clinical background and consultation with the department's pathologist is necessary before any presumptive interpretation can be made.
Typing of the specific species and groups for Legionella may be useful in cases where serology has detected "early antibody production". In this situation, identification of a specific group or species is possible. In late infection (where there has been a large amount of antibody production) too much cross-reactivity occurs between the species and groups, which makes identification impossible.
A positive culture or DFA test (from fluid or biopsy) is conclusive evidence of a Legionella infection. Serology is only intended to be used as a "Guideline Tool".
See Also
For further information contact the laboratory (contact via Lablink: 22000 or (09) 307-8995 or 0800 522 7587) ,or:
the Virology team
virology@adhb.govt.nz