[Determination of cutoff value of serum anti-Legionella antibody titer--microplate agglutination test (MPAT)].

In order to promote the serological diagnosis of legionellosis in the clinical laboratory, the cutoff values of serum anti-Legionella antibody titers for microplate agglutination (MPAT) test were determined. Antibody levels were tested for 178 serum specimens including 98 healthy persons, 22 ordinally workers having either metabolic or renal failure, definitely diagnosed patients of 17 mycoplasmal and 9 chlamydial pneumonia, 32 patients of other bacterial pneumonia. Heat killed unstained cell suspension of each strain of Legionella pneumophila serogroup (SG) 1a, 1b, SGs 2 to 6, L. bozemanii, L. dumoffii, L. gormanii, and L. micdadei were used as antigens. Strains of L. pneumophila SG 1b were mainly isolated from environmental specimens. However, in some Legionella pneumonia cases, etiologic agents were determined as L. pneumophila SG 1 b. Thus the representative strain of SG 1b was used as an antigen for the determination of the patient's antibody titer. Quantitative agglutination was performed by using a 96-well U-bottom microplate for each antigen. Test sera were diluted from 1:16 to 1:256. Results were read after 20 h at 25 degrees C. Cutoff values for 11 antigens were determined, at this moment, as 4-fold or greater increase in level to > or = 1:128 in paired sera, and > or = 1:256 in single serum. However, final diagnosis should be given by over-all coordination of serological results and clinical symptoms together with other laboratory findings. Two culture-positive Legionella pneumonia cases due to either SG3 or 6 in which significant rise of serum antibody titers against organisms of corresponding SG estimated by MPAT method were discussed.