Development and clinical evaluation of an amplified flow cytometric fluoroimmunoassay forClostridium difficile toxin A

Abstract

A rapid (2 h) amplified flow cytometric fluoroimmunoassay (AFCF) for Clostridium difficile toxin A was developed and compared with the cytotoxin assay (CTA) and culture of the organism from stool specimens from patients with suspected C. difficile‐associated gastrointestinal disease (CAD). For this assay polyclonal antitoxin A was attached to 10‐μm diameter and monoclonal antitoxin A was attached to fluorescent 0.1 μm‐diameter polystyrene microspheres. The microspheres and sample were reacted together as in a conventional double‐antibody sandwich assay. However, laser flow cytometric measurement allowed the omission of separation and washing steps by gating on light scattered by the larger microspheres and measuring only the fluorescence associated with these particles. The amount of fluorescence from the attached 0.1 μm microspheres was dependent on the concentration of toxin A in the sample. The AFCF detected purified toxin A at levels of 1 pg/ml and was linear from 1 to 40 pg/ml. The AFCF was compared with the CTA and culture of C. difficile for clinical use by comparing results from 198 stool specimens from patients with suspected CAD. The AFCF was 85.7% sensitive and 95.8% specific relative to the CTA, and 85.2% sensitive and 98.3% specific compared to the culture assay. If the isolation of toxigenic C. difficile or the patients clinical course was considered indicative of CAD, the sensitivities of the AFCF, CTA, and culture assay were 77.4%, 67.7% and 96.8%, respectively. The AFCF demonstrated a specificity of 98.8%, while both CTA and culture had a specificity of 100%. The AFCF for toxin A is a sensitive and rapid test, and in laboratories with a flow cytometer offers an alternative method for the laboratory diagnosis of CAD.