Agglutination methods for rapid analysis

A two-particle turbidometric latex immunoassay for the detection of specific IgM antibodies

A simple two stage assay using latex particles as a reaction indicator has been developed for the detection of IgM antibodies to Trichinella spiralis. In the first stage, magnetic polystyrene beads (Dynabeads) coated with T. spiralis antigen were incubated for 30 min with the test serum. After washing, in the second stage, the assay was developed for 1 h using anti-mu-coated latex particles. After sedimentation of the Dynabeads the turbidity of the resultant latex suspension was measured spectrophotometrically at a wavelength of 400 nm. A decrease in turbidity of more than 20% from that of the control, unreacted, suspension was considered positive. Using an IgM phosphorylcholine-binding monoclonal antibody which was reactive with T. spiralis, the sensitivity of the assay was determined to be 110 ng/ml of antibody. This was 20-fold less than the sensitivity achieved in an indirect enzyme-linked immunosorbent assay (ELISA). When the assay was applied to sera obtained from CBA/N or BALB/c mice, which were either normal or immunized against T. spiralis, the expected results were obtained with titers up to 1/640 observed, and confirmed (r = 0.93, P less than 0.001) in the ELISA.

A spectrophotometric method for evaluating a latex agglutination assay of Salmonella typhi lipopolysaccharide

A spectrophotometer set at a wavelength of 400 nm was used to read reaction mixtures containing a 0.01% suspension of antibody-sensitized latex particles and different amounts of soluble antigen, after incubation in tubes for 30 min. The antigen used was Salmonella typhi lipopolysaccharide, and the antibody was an O-9-specific monoclonal antibody. Agglutination was indicated by a fall in the turbidimetric measurement compared to control, unagglutinated latex. It was observed that agglutination increased with increasing concentrations of antigen to a maximum at 0.25-1.0 micrograms/ml, after which, the amount of agglutination decreased at a similar rate as far as 1 mg/ml of antigen, when no agglutination occurred at all. Thus, a bi-symmetrical curve was obtained, suggestive of a precipitation reaction. At the 'equivalence point', the turbidity of the reaction mixture compared to control fell from OD400 1.1 to OD400 0.3. A similar inhibition of reaction at 'antigen excess' was observed visually in the reaction mixtures. Conventional slide tests performed in parallel could also be inhibited, but at higher antigen levels. Inhibition could be achieved with a commercially available latex used for the detection of Neisseria meningitidis antigen.

A comparison of immunological methods for the detection of Trichinella spiralis antigen

Eight immunological methods all using the same monoclonal antibody reagent were compared for the detection of Trichinella spiralis antigen. These were based on: (1) the direct adsorption of the antigen to the immunoadsorbent (nitrocellulose membrane, polyvinyl chloride strip or microplate); (2) capture of the antigen by antibodies pre-sensitized on the immunoadsorbent; and (3) latex agglutination. The methods found suitable were: (a) capture radioimmunoassay (capture-RIA) (sensitivity: less than 0.5 microgram/ml antigen); (b) direct enzyme immunoassay (direct-ELISA) (less than 0.5 microgram/ml); (c) tube latex agglutination test (2.2 micrograms/ml); and (d) direct immunodot assay (8.8 micrograms/ml). However, the performance of the direct-ELISA was greatly affected by the presence of each of three extraneous substances (bovine serum albumin (BSA), lipopolysaccharide (LPS), normal swine muscle homogenate (NSM) added to the antigen sample. The direct immunodot assay was also affected by the presence of BSA or LPS, whereas both the capture-RIA and the tube latex agglutination methods were affected by the presence of NSM only. The findings are discussed with a view of detecting T. spiralis larvae directly from pork samples by immunological means.

Detection of group D salmonellae in blood culture broth and of soluble antigen by tube agglutination using an O-9 monoclonal antibody latex conjugate

Latex particles sensitized with a Salmonella O-9 monoclonal antibody were used to detect group D salmonellae in blood culture broths. The system was found to be 100% accurate when used on 104 clinical specimens that included 13 S. typhi isolates. Enteric fever could thus be diagnosed as early as 1 day after the initiation of culture in some (18%) cases. The latex particles were also used in an experimental situation to detect soluble S. typhi lipopolysaccharide by a tube agglutination method. This was found to be more than four times as sensitive as the conventional slide agglutination method because at least 50 ng of antigen per ml in either buffer or urine was detected, albeit with less efficiency when serum was used.