Proficiency-testing false positives

Analytical Methods Committee, Handling false negatives, false positives and reporting limits in analytical proficiency tests. Analyst, 122, 495, 1997. 

False-positive results with bDNA have been observed with proficiency testing specimens for HTV-1 in the College of American Pathologists HIV-1 viral load survey and HCV in the viral quality control program administered by the Netherlands Red Cross. The reason for the false-positive results with these proficiency testing specimens is not known but may be sample matrix effects. The extent to which this problem occurs with clinical samples has not been determined. However, both the HIV-1 and HCV bDNA assays were designed to have a false-positive rate of 5%. 

Handling False Negatives, False Positives and Reporting Limits in Analytical Proficiency Tests , Analyst Cambridge, 1997, 122, 495. 

Handling and preparation of authentic/proficiency test samples is preferably performed in a laboratory room dedicated to this purpose. The key advantage of a separate room is limitation of the possibilities of sample contamination. In previous tests, several participants have experienced cross-contamination, often with the consequence of false positive identifications and subsequent failure of the test. 

Figure 16. Overview of irrelevant and false positive chemicals, reported by participants in proficiency tests 1-10...

OPCW proficiency tests currently require the identification of between six and nine analytes (scheduled compounds or their degradation products) in matrices such as water, soil, and organic extracts. Identification must be confirmed by two independent techniques, and both identification and reporting must be in accordance with strict criteria. Although not mandatory, most laboratories rely on comparison with authentic compounds to confirm identification in order to minimize any chance of a false positive. If the authentic compound requires synthesis, then these tests are demanding in terms of time as well as in terms of detection and identification. 

A total of 102 process flavors were analyzed representing the variety of process flavors commonly added to foods in the United States. The study included an analysis of PHA levels in spiked and unspiked samples of process flavors to verify the accuracy and precision of the analytical method. Results of this proficiency test indicate that the likelihood of false positives or false negatives was low, and that measurement of the PHA levels would be precise and accurate (Knize and Salmon, 1998). 

False positive results must not occur. Any chemical that is not contained in or that could not be formed in the sample matrix will constitute a false positive result. Reporting any false positive result will constitute failure of the Proficiency Test. 

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