The Potential Use of Quality Control Data to Validate Pesticide Residue Method Performance

William Honvitz

CENTER FOR FOOD SAFETY AND APPLIED NUTRITION HFS-500, US FOOD AND DRUG ADMINISTRATION, WASHINGTON DC 20204, USA

Full scale interlaboratory (collaborative) studies are becoming too expensive and time-consuming to support their use as the only way to validate methods of analysis. Furthermore, reliable estimates of method performance parameters, such as accuracy, precision, and limits of applicability, cannot be achieved by individual collaborative studies at the concentration levels of 0.01-1 mg. kg-1 the region of interest for residue analysis, when the expected random error among laboratories is of the order of 20-30% of the mean. Performance data from proficiency studies of tens of thousands of control determinations accumulated over the past decade from individual and multiple laboratories are being examined to determine their potential as a substitute for interlaboratory performance data. An analysis of variance indicates that as much as 80% of the total variability of pesticide residue analysis is "random error." If this is the case, proficiency data may be substituted for method-performance data, when recovery is acceptable, because the individual factors of analyte, method, matrix, laboratory, and time contribute little to overall variability.

1 INTRODUCTION

A full-scale method performance study, utilizing the IUPAC-AOAC harmonized protocol,' requires a minimum of 8 laboratories to analyze at least 5 materials related to the analyte-matrix-concentration combinations of interest by the proposed method. Although such a study is probably the most pertinent and reliable way of demonstrating the performance of a method with a specific test sample, several alternatives are also available. The Youden pair technique (split level design) conducted at several relevant concentration levels is the model favored by the extensive water analysis program of the U.S. Environmental Protection Agency (EPA). In Europe, the "uncertainty" budget approach has been advocated, particularly in conjunction with laboratory accreditation, although this design has been modified during the past few years to become more similar to the method-performance model.

All of these models are expensive and time-consuming. What few calculations that have been conducted independently suggest that they do not provide the same performance parameters with the same method from the same data and information. Proficiency testing, also advocated by the FAO/WHO Codex Alimentarius Program as a way of demonstrating equivalency in laboratory results, has been suggested as still another potential substitute for the method-performance trials. The purpose of this paper is to explore this possibility.

2 AVAILABLE PROFICIENCY STUDIES

Data from several large scale proficiency programs have been made available to us in order to examine their potential use as a substitute for method performance in the determination of pesticide residues in food. As of late 1999, data from three programs have been examined. These include the following:

2.1 Total Diet Program (FDA-TD)

The Total Diet Program of the U.S. Food and Drug Administration (FDA), Kansas City Laboratory, has been in continuous operation for almost 40 years. The recoveries of approximately 7000 control determinations of analytes added to individual foods in the pesticide residue portion of the 4 annual market baskets analyzed during 1991-1997 were available as databases in EXCEL4. This is a within-laboratory program.

2.2 Food Analysis Performance Assessment Schemes (FAPAS)

This is an international fee-based program conducted by the Ministry of Agriculture, Fisheries, and Food of the United Kingdom. Each test material is thoroughly homogenized to a smooth paste and aliquots of stock standard solutions of pesticides of known purity are added with additional mixing. The fortified test material is measured into screw top glass jars, tested for homogeneity, and stored at -20° until shipped. More than 100 laboratories participated in many of the tests. Participants were required to submit their results within 8 weeks for their values to be included in a report that was distributed. The pesticide residue reports (Nos. 1901-1905), uncorrected for recovery, and percent recovery of about 1100 concurrent controls that were submitted, were the data used in the present examination.

2.3 State of California Quality Assurance Program (CA)

The State of California has been monitoring the quality of their Residue Enforcement Program since 1988, utilizing their headquarters and field...