We examined a patient with abnormal laboratory data whose serum retinal-binding protein (RBP) and transthyretin (TTR) values by nephelometric immunoassay (NIA) were markedly differed from the values by latex turbidmetric immunoassay (LTIA) and turbidmetric immunoassay (TIA). The RBP and TTR values of the patient serum determined by NIA were 18.5 mg / dl and 68.9 mg / dl, respectively. However, the RBP value by LTIA was 2.4 mg / dl, and TTR was 14.9 mg / dl by TIA. White precipitates were observed when the RBP- and TTR-assay reagents of NIA were added to the patient’s serum.
Immunofixation electrophoresis showed that these precipitates were the IgM-λ, IgA-λ and IgG-κ type M-protein. Moreover, it was confirmed that these precipitates had the rheumatoid factor (RF) activity. The precipitates were also formed by the interaction between the sera from patients with rheumatoid arthritis and RBP- and TTR-assay reagents of NIA. The precipitate also had the IgM-RF activity.
From these facts, it should be considered that the nonspecific reaction may occur by the interaction between the IgM with RF activity and the RBP- and TTR-assay reagents of NIA.

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