An enzyme-linked immunosorbent assay (ELISA) was developed and evaluated to determine whether separate measurement of immunoglobulin (Ig) M and G antibodies to L. pneumophila serogroups 1, 3 and 6 as single antigens can assist in the diagnosis.

Scientists at the Statens Serum Institut (Copenhagen, Denmark) compared the ELISA with an in-house indirect Legionella immunofluorescence antibody test (IFAT) measuring Total Ig. A total of 193 sera from 128 patients with confirmed L. pneumophila infections were used to assess the sensitivity of the developed ELISA. The sensitivity was assessed in different time-periods after onset of symptoms. L. pneumoniae infection was confirmed by one or more of the following methods: culture by standard method, Legionella urinary antigen EIA (Biotest AG, Dreieich, Germany), and an in-house polymerase chain reaction (PCR) method.

It was found that the sensitivity of the ELISA increased during the first month of infection, IgM being the most sensitive; ranging from 13% in the first week after onset of symptoms, 45% in the second week to 84% in the third week; in the fourth week and beyond, a drop to 67% was discerned. The IFAT detecting L. pneumophila subgroups 1–6 had a sensitivity of 11%, 27%, 80%, and 59%, respectively, during these time-periods. The test with the lowest sensitivity during these time-periods was the IgG ELISA (0%, 21%, 36%, and 52%) respectively, but by combining the IgG results with the IgM results, the overall sensitivity of the assay was improved (13%, 48%, 88%, and 70%), respectively.

The in-house methods IgM ELISA and IFAT both had a low false positive rate: 7% and 4% among 75 samples. In contrast, the IgG ELISA had a false positive rate of 14%, which was high compared to the other tests. The sera which were positive for Helicobacter pylori, Salmonella typhimurium/enteritidis, and Mycoplasma pneumoniae showed the highest rate of false positive in the IgG subgroup1 ELISA. The sera, which caused the false positive results in the IFAT and IgM ELISA were the sera with antibodies to Proteus species and S. typhimurium/enteritidis.

The study confirmed that detection of IgG and IgM antibodies by ELISA is an important diagnostic tool especially during the initial phase of the disease, when supported by other tests like the urinary antigen test, PCR, or culture of the organism. The authors concluded that specific antibodies against L. pneumophila subgroups 1, 3, and 6 can be detected in patients early in the course of infection. Furthermore, positive IgM level can be used as a good indicator of an ongoing or recent Legionella infection. The study was published in September 2011, in the Journal of Microbiological Methods.

Related Links:
Statens Serum Institut
Biotest AG