False positives may be observed in latex agglutination tests for the capsular antigen of Cryptococcus neoformans because of

False positives may be observed in latex agglutination tests for the capsular antigen of Cryptococcus neoformans because of

• A. the use of serum instead of CSF.
• B. the presence of rheumatoid factor in the specimen.
• C. cross-reactivity with other fungal antigens.
• D. the low specificity of the assay.

Answer: (B)

The test detects cryptococcal antigen by looking for agglutination of latex beads when the target polysaccharide is present. A key way false positives arise is through interference from rheumatoid factor, an IgM autoantibody that binds to the Fc portion of IgG. In these latex agglutination setups, RF can cross-link IgG present in the sample or antibodies coating the beads, causing beads to clump together even when cryptococcal antigen is absent. This non-specific bridging produces a positive readout, so rheumatoid factor in the specimen is a classic cause of false positives.

The test detects cryptococcal antigen by looking for agglutination of latex beads when the target polysaccharide is present. A key way false positives arise is through interference from rheumatoid factor, an IgM autoantibody that binds to the Fc portion of IgG. In these latex agglutination setups, RF can cross-link IgG present in the sample or antibodies coating the beads, causing beads to clump together even when cryptococcal antigen is absent. This non-specific bridging produces a positive readout, so rheumatoid factor in the specimen is a classic cause of false positives.