Almost 90% of older patients with serum B12 < 150 pmol/L show evidence of tissue vitamin B12 deficiency. Deficiency becomes manifest in older patients at relatively higher concentrations of serum B12 than in younger subjects, possibly because of lower levels of holotranscobalamin II in the older patients.

Filter paper discs saturated with dried blood can be used in the Immunotube solid-phase thyroxine radioimmunoassay. This assay utilizes polypropylene tubes to which antibody to thyroxine is covalently bound. The filter paper standards and samples are placed in the tubes, followed by an assay buffer that contains I125-labeled thyroxine and compounds to displace thyroxine from its binding proteins. After incubation, bound and free thyroxine are separated by aspirating or decanting the disc and buffer from the tube. The test can be used with 0.32 cm (1/8 inch) or 0.64 cm(1/4 inch) discs, and gives quantitative results that correlate well with those for serum samples. The intra-assay coefficient of variation is less than 10%. The assay may readily be mechanized with existing disc-punching equipment, and results of its use in mass screening programs are described.