Q-253. Assessment of False Positive and False Negative Confirmation Rates for Methods 1680 and 1681 in Biosolid Matrices
Background: Under Subpart D of Title 40 Code of Federal Regulations (CFR) Part 503, sewage sludge (biosolids) are classified based on pathogen concentrations. The U.S. Environmental Protection Agency (US EPA) has validated Methods 1680 (LTB/EC)/1681(A-1) for fecal coliforms in Class A and B biosolids to support monitoring requirements. The objective of this study was to assess false positive and false negative confirmation rates for Methods 1680 and 1681 across multiple laboratories and biosolid matrices. Methods: Twenty three laboratories participating in the study streaked 996 LTB or A-1 tubes (767 typical and 229 atypical results) onto mEndo plates and shipped the plates to the verification laboratory for identification. Results: False positive rates for Method 1680 ranged from 0% - 17% for all six matrices (aerobically digested, alkaline-stabilized, anaerobically digested, composted, heat-dried, and thermophilically digested). False positive rates for Method 1681 were relatively low for three of the matrices (aerobically digested, alkaline-stabilized, and anaerobically digested) ranging from 3% - 9%. The false positive rate for composted matrices was considerably higher, at 37%. False negative rates were very low for Method 1680, ranging from 0% - 5% for all six matrices. In addition, false negative rates for Method 1681 were also very low, ranging from 0% - 3% for aerobically digested, alkaline-stabilized, anaerobically digested, and composted matrices. Although false negative rates were adjusted to include the number of “true” negatives [clear tubes (not submitted for confirmation)], a significant number of negative tubes (turbidity and no gas) submitted to verification verified as false negatives (82/152). Conclusion: Based on the results of this study, it is highly recommended that analysts demonstrate proficiency for each matrix type evaluated prior to analysis of field samples.