It appears that addition of our patient's serum to the aqueous diluent or the enzyme reagent caused precipitation of his IgM monoclonal paraprotein in a manner akin to the Sia test
. Because the Cobas Fara is configured in such a way that dilution alone should not alter the measured absorbance and because the final measured acetaminophen was actually lower than the true value determined by HPLC, the precipitate may well have partially redissolved in the subsequent more alkaline medium of the color reagent.
The Sia test (Sia euglobulin precipitation test) was first described more than three-quarters of a century ago for evaluation of the euglobulin fraction in certain infectious diseases .
Although the Sia test is now largely thought of as being of only historical interest, these cases reveal the utility of understanding such "obsolete" tests and how they may be applied to the modern era of automated chemistry.
Immunochemical and physical studies of the Sia test. Am J Med Sci 1976;271:151-7.