accuracy antigen check checking colloidal coronavirus covid-virus detection diagnosis diagnostic-aid dropper drops home-use immunochromatographic-technology kit limitations limited lysis-buffer measuring-instrument medical nasal-secretion negative-result plastic quality-control-C-line rapid rectangular result sample sampling-tube self-testing test test-cassette testing extraction-reagent-tube nasal sterile-swab swab advertising black blue color colors colour colours free offer opportune opportunity potential promotion promotional red sale sign stress suggestive white activity acuity blind chart check eyes contact cutout Doctor etdrs eye eye test eyes glasses isolated letters measure optic see small table tablet vision visual visual acuity fire firework party sparklings sparks stick stone inspect lenses optometry sight abstract art ba1969 experiment fun grunge torn anti-tank armour bullet missile plate world war second cerfiy class educate education english grade learn note quiz rewrite student study task trial work business chair desk feather girl man office working write writer