Serum antibody levels were measured in all patients using both an INFERGEN-binding radioimmunoassay and an IFN (α)-2b-binding ELISA. A patient was considered to have developed binding antibodies if, using serum samples from 2 consecutive time points, a positive response was detected in either assay. The number of patients developing positive binding antibody responses in either assay was similar in the 9 mcg INFERGEN (11%) and 3 mIU IFN (α)-2b groups (15%). The titer of neutralizing antibodies to interferon was not measured. Sustained ALT response rates in patients treated with INFERGEN who developed binding antibodies (4/25) were similar to sustained ALT response rates in patients who did not develop detectable antibody titers (40/195). The most frequently observed time to first antibody response was week 16 of interferon treatment. Following cessation of interferon therapy, the number of patients with a positive antibody response declined during posttreatment observation.

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