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.