Legionella Serotyping of Isolates (DFA)


Legionella Serotyping (DFA)


The direct fluorescent antibody (DFA) test is a rapid microscopic procedure used to identify Legionella pneumophila and other Legionella species. Antibodies specific for Legionella species and serogroups are tagged with a fluorescent dye. When Legionella is isolated by culture from an environmental source or a patient, the bacteria are placed on a slide and exposed to a panel of DFA reagents. A positive reaction confirms the identification. DFA is sensitive and highly specific when used for species/serotype confirmation.

DFA staining is not recommended as a method to be applied directly on an environmental sample due to false positive and false negative results. DFA staining will stain both viable (living) and non-viable (dead) Legionella cells.

There are more than 60 Legionella species and 16 serogroups of L. pneumophila, however only a select few species and serogroups are responsible for most illness. Therefore DFA testing is performed to identify or rule out only the most clinically relevant strains. If requested or when species identification is required such as in investigations of clinical cases, DNA sequencing can be performed for identification of unusual species or those not included in the DFA panels.

Test Method

  • Legionella Identification by direct fluorescent antibody staining.
  • Legionella pneumophila confirmed using specific species/serogroup reagents.
  • Legionella species, not pneumophila, are confirmed using specific reagents to include the following: L. anisa, L. bozemanii serogroups 1 and 2, L. gormanii, L. longbeachae serogroups 1 and 2, L. dumoffii, L. micdadei, L. wadsworthii, L. oakridgensis, L. feeleii serogroups 1 and 2, L. sainthelensi, L. jordanis, L. hackeliae serogroups 1 and 2, L. maceachernii, L. santicrucis, L. spirtensis, L. jamestowniensis, L. cherrii, L. steigerwaltii, L. rubrilucens /L. erythra.

Turn Around Time
2–3 days

Test Code

Sample Transport
See Sampling and Shipping page for instructions.


  1. Procedures for the Recovery of Legionella from the Environment. Centers for Disease Control publication. January 2005.
  2. DFA manufacturers’ instructions: Remel, Lenexa, KS;  Bio-Rad, Redmond, WA and m-Technologies, Alpharetta, Ga.

Accreditations and Proficiency
See Accreditations and Proficiency page for information on our accreditations and Legionella testing proficiency programs.