Cut-off on demand: adjustment of the threshold level of an immunochromatographic assay for chloramphenicol
Immunochromatographic assay of small molecules is carried out in a competitive format and is based on the disappearance of analytical zone colouration, as the analyte concentration exceeds a certain threshold level. Therefore, the assay of toxic contaminants has the highest information content if this threshold level (cut-off between negative and positive samples) corresponds to the maximum residue limit of the analyte. In this study, adjusting the threshold level was investigated for an immunochromatographic assay for chloramphenicol, which is a veterinary drug of concern in the food industry because of its toxic effects. Test strips were produced using hapten–(protein carrier) and antibody–(gold nanoparticle) conjugates, and the effects of the compositions and concentrations of these conjugates on the threshold level of the assay were investigated. Changing all of these parameters at once shifted the threshold level to values that could reach more than two orders of magnitude. Thus, varying the composition of chloramphenicol–bovine serum albumin conjugates causes a shift of the threshold level from 1500 to 2 ng mL−1, and varying the composition of chloramphenicol–soybean trypsin inhibitor conjugates causes a shift from 930 to 9 ng mL−1. Besides, the threshold shift is accomplished by the shift of the working range for quantitative immunochromatography based on measurements of the analytical zone colouration. In this way, the limit of instrumental detection could also be varied by more than two orders of magnitude. The proposed approach can be applied to different low-molecular-weight compounds and it allows the adoption of an immunochromatographic test of the desired levels to control the target analyte without screening of a series of antibodies.