A CSF specimen from a suspected cryptococcal meningitis case gives a negative result on undiluted cryptococcal antigen lateral flow testing, yet India ink shows numerous encapsulated budding yeasts. Repeating the assay after serial dilution of the same CSF produces strongly positive bands. What phenomenon explains the initial false-negative result?

• A Hook effect (prozone) caused by massive excess of capsular antigen ✓
• B Post-zone failure from antibody excess binding all available antigen
• C Degradation of antigen by raised CSF protease activity
• D Cross-reactivity of the antibody with polysaccharide from commensal bacteria

Explanation

When capsular polysaccharide antigen concentration is enormously high, antigen saturates the capture antibody on both sides of the immunocomplex so that no lattice forms and the test reads falsely negative or weakly positive; this hook or prozone effect is reversed by dilution, exactly as seen here. Post-zone refers to antibody excess, which is irrelevant since the patient produces little anticryptococcal antibody. Proteolysis and bacterial cross-reactivity would not correct themselves on simple dilution of the same specimen.

Reference: Mandell, Douglas and Bennett's Principles and Practice of Infectious Diseases, 9th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.