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Radial immunodiffusion (RID) is a classic methodology for antigen quantification that relies on the development of a distinct precipitin ring from precipitated antigen-antibody complex. As the precipitin ring grows, the precipitate at the inner edge of the ring constantly dissolves due to excess antigen flooding from the point of application, while new precipitate forms at the leading edge of the ring. RID plates with anti-human alpha-1-antitrypsin are prepared in our lab to measure fecal alpha-1 antitrypsin (A1AT) to aid in the diagnosis and monitoring of Protein Losing Enteropathy (PLE). The procedure has routinely produced precipitin rings with small radii and distinct edges after incubating at room temperature for 48 hours. Unexpectedly, a fecal specimen from a patient produced an extremely weak and large precipitin ring that could have been easily overlooked. Dilution studies confirmed a highly elevated A1AT level of 67 mg/g dry stool. The very weak and large precipitin ring was reproduced with a spiked specimen with similar A1AT concentration and kept expanding for several days until a distinct ring was formed eventually. Our data highlights a rare example of high-dose hook effect in RID and calls for meticulous attention and caution when reading and interpreting gel-based immunoassays with unexpected markedly elevated results to avoid additional confirmatory testing. In these cases, we recommend repeat testing with diluted specimens.

Weak Precipitin Ring from A Fecal Specimen with Markedly Elevated Alpha-1 Antitrypsin Level Measured by Radial Immunodiffusion