Critical Comparison of Analytical Performances of Two Immunoassay Methods for Rapid Detection of Aflatoxin M1 in Milk

Aflatoxin B-1 (AFB(1)) is a secondary metabolite produced by some Aspergillus spp. fungi affecting many crops and feed materials. Aflatoxin M-1 (AFM(1)), the 4-hydroxylated metabolite of AFB(1,) is the main AFB(1)-related compound present in milk, and it is categorized by the International Agency for Research on Cancer (IARC) as a "group 1 human carcinogen". The aim of this work was to evaluate and compare the analytical performances of two commercial immunoassays widely applied for the detection of AFM(1) in milk, namely strip test immunoassay and enzyme linked immunosorbent assay (ELISA). Assay validation included samples at AFM(1) levels of 25, 50, 75 ng/kg and blank samples (AFM(1) < 0.5 ng/kg). With respect to a screening target concentration (STC) of 50 ng/kg the two assays showed cut-off values of 37.7 ng/kg and 47.5 ng/kg for strip test and ELISA, respectively, a false suspect rate for blanks <0.1% (for both assays) and a false negative rate for samples containing AFM(1) at levels higher than STC, of 0.4% (for both assays). The intermediate precision (RSDip) was <32% for the strip test and <15% for the ELISA. Method verification through long-term intra-laboratory quality control (QC) measurements confirmed the results from the validation study. Furthermore, a satisfactory correlation of the results obtained with both immunoassays and the AOAC Official Method 2000.08 was obtained for the analysis of cow milk samples naturally contaminated with AFM(1) at levels within "not detected" (< 0.5 ng/kg) and 50 ng/kg. Finally, the extension of the scope of the strip test method to goat and sheep milk was evaluated by applying the experimental design foreseen in the EU regulation.