1.1 This test method describes a procedure that can be used in the field or in a laboratory to detect antigens indicative of microbial contamination in liquid fuels, including those blended with synthesized hydrocarbons or biofuels, with kinematic viscosities (at 40 °C) of ≤24 mm2s–1 (for example, Specifications D396, D975, and D1655) and in fuel-associated water. 1.1.1 This test method has been validated by an ILS for a range of middle distillate fuels meeting Specification D1655, EN590, Specification D975, and ISO 8217:2012. 1.2 This test method semi-quantitatively assesses the concentration of specific antigens generated by aerobic microorganisms during active growth in fuels. 1.2.1 A proprietary formulation of antibodies and antibody mixtures is used to detect three types of microbial antigen contamination: antigens generally found in aerobic bacteria, antigens generally present in common fungi (yeast and molds), and an antigen that is characteristic of Hormoconis resinae (the fungus most commonly associated with fuel biodeterioration). 1.2.2 Although the antibodies and antibody mixtures are characteristic of diverse types of bacteria and fungi, it is unlikely that they are universal. Recognizing that for every microbe that has been isolated and characterized, it is likely that there are a billion that have not. Consequently, as is the case with all microbiological test methods, this test method does not purport to detect 100 % of the microbes present in a fuel or fuel-associated water sample. 1.3 For each of the three sets of antigen detected (H. resinae, common fungi, and aerobic bacteria), the test detects whether the antigen concentration present is within set ranges representing negligible, moderate, or heavy microbial contamination. 1.3.1 For fuel specimens, the antigen concentration ranges detected are <150 µg/L (negligible), 150 µg/L