This invention relates to reagent comprising: any one of cells, viral particles, organelles, parasites, cells comprising organelles, cells comprising viral particles, cells comprising parasites, cells comprising bacterial cells and any combination thereof, the cells, viral particles, organelles or parasites comprising at least one nucleic acid sequence serving as an internal control (IC) target for nucleic acid testing (NAT) assay; wherein the reagent is suitable to be added to a test sample undergoing sample preparation to release, concentrate and/or purify nucleic acids and amplification and/or detection of nucleic acids so as to be used to verify: (i) the efficiency of sample preparation; and (ii) the efficiency of nucleic acid amplification and/or detection. The present invention also relates to a method to verify or validate the preparation and amplification and/or detection of a nucleic acid target sequence in a sample spiked with a reagent of the present invention.