Falsely positive HIV test due to Interference by heterophile antibodies in a patient with Mantle cell lymphoma treated with Rituximab

Heterophilic antibodies can interfere with laboratory tests causing erroneous results. We report the case of a 55-year-old man with Mantle cell myeloma treated with Rituximab that caused false positive results of HIV serologic test. The cause of this interference was investigated, and very high levels (741 ng/ml) of human anti-mouse antibodies (HAMA) were demonstrated. Polyethylene glycol (PEG) precipitation of intact immunoglobulins was used to cross-check the antibody-related interference. The PEG treatment negativized the HIV test outcome, confirming that the cause of the false positive serologic HIV result was due to interference caused by HAMA. In conclusion, we suggest to cautiously interpreting laboratory findings in patients treated with Rituximab because of the possibility that HAMA may cause interference in serological assays.