Rapid diagnostic device, assay and multifunctional buffer

A post-filter cap which is operably affixable to a test unit so as to be in vertical communication with a reaction zone after a fluid test sample has been applied to the reaction zone, the post-filter cap comprising:

• an outer sleeve comprising an outwardly extending flange, sidewalls depending from the outwardly extending flange and terminating at an open-ended base, the base further comprising a collar projecting inwardly from the sidewalls;
• an inner sleeve dimensioned to frictionally engage the inner surface of the outer sleeve, the inner sleeve comprising an outwardly extending flange, sidewalls depending from the outwardly extending flange and terminating at an open-ended base;
• and
• a removable post-filter unit supported by the collar of the outer sleeve and held in place by the open-ended base of the inner sleeve, the post-filter unit comprising a porous material having a direct label conjugated general marker protein complex embedded thereon,
• whereby application of a buffer to the porous material mobilizes the direct label conjugated general marker protein complex and liberates the complex from the porous material to flow-through the base of the outer sleeve.