Abstract
Until recently there has been little formal guidance on procedures for adopting new methods in water microbiology. However, the European Union Drinking Water Directive of 1998 specified methods that were to be used for the microbiological parameters, most being ISO methods, but allowed the use of alternative methods that were “at least as reliable”. At that time, there were no published procedures for demonstrating equivalency of performance between methods. Work commissioned by the UK Drinking Water Inspectorate (DWI) developed suitable analytical and statistical protocols for comparing microbiological methods. The statistical aspects have been refined and recently published as ISO 17994. ISO has also recently published guidance on the validation of methods for water microbiology (ISO/TR 13843), which gives guidance for developers of new media on what performance information is required. These developments provide a framework for the enhancement of validation and verification procedures within a laboratory's quality system for evaluating new methods prior to their adoption. This paper overviews these developments in light of the author's experience in their use and discusses issues relating to the analytical procedures and the statistical rationale employed (including the concept of “equivalency” of performance between methods). Water SA Vol.31 (3) 2005: pp.393-396
Highlights
Many methods used widely in water microbiology have not undergone full validation of performance, having been widely used and accepted historically, and there has been little formal guidance on demonstrating equivalent or superior performance of a new method prior to its adoption by a laboratory
Verification of the claimed performance, and where appropriate, demonstration of equivalent or inferior/superior performance compared to the method in current use are required
While there has been some guidance on assessment of performance characteristics for water microbiology methods (Havelaar et al, 1993; Lightfoot and Maier, 1998; ISO, 1999a) that on comparing the performance of two methods has been very limited
Summary
Many methods used widely in water microbiology have not undergone full validation of performance, having been widely used and accepted historically, and there has been little formal guidance on demonstrating equivalent or superior performance of a new method prior to its adoption by a laboratory. This will impact on the response of target bacteria in quantitative methods requiring selective growth All these factors result in a significant natural variability in recovery of micro-organisms from water which must be taken into account when devising analytical and statistical protocols for comparing quantitative methods, when the numbers that are encountered in routine samples tend to be very low, as in the case of drinking water monitoring. Key information will relate to recovery efficiency, upper and lower working (detection) limits, selectivity and specificity (false-positives and false-negatives), counting uncertainty (methodological and analyst) and a general estimate of precision Since these data will provide the initial assessment of performance of a new or modified method it is strongly recommended that analysts with considerable experience in microbiological methods conduct the work. If the types of bacteria normally encountered by the laboratory with their current method are subject to seasonal variation, it may be appropriate to conduct the verification exercise over a period of time that would take that source of variability into account
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