Radio immune assays (competition assay) used in diagnosing FDH
:
Radio immune assays are useful in diagnosing FDH. In 1-step assay
labelled T4 (the tracer) competes with serum T4 for a fixed number of
anti-T4 antibody binding sites. Free harmone concentration is determined
by the fractional occupancy of antibody binding sites. Equilibrium
between T4 and its binding proteins is conserved during measurement, so
that the amount of tracer displaced reflects the ’free’ rather than
’total’ hormone concentration. It is protein-dependent and is prone to
under- or overestimate FT4.
In the 2 step assay wash step prior to tracer addition. It may reduce
but not completely eliminate such interference. If the problem persists,
hormone measurement following equilibrium dialysis (ED) remains the
gold-standard for eliminating FT4 assay interference.