SIST EN ISO 17994:2004
(Main)Water quality - Criteria for establishing equivalence between microbiological methods (ISO 17994:2004)
Water quality - Criteria for establishing equivalence between microbiological methods (ISO 17994:2004)
The method is intended for a concentration range to meet the demands of the EC Drinking Water Directive.
Wasserbeschaffenheit - Kriterien für die Feststellung der Gleichwertigkeit von mikrobiologischer Verfahren (ISO 17994:2004)
Diese Internationale Norm legt ein Bewertungsverfahren fest, um zwei Verfahren zu vergleichen, die für den Nachweis oder die Quantifizierung derselben Zielgruppe oder Spezies von Mikroorganismen gedacht sind.
Diese Internationale Norm liefert die mathematische Grundlage für die Bewertung der durchschnittlichen relativen Leistungsfähigkeit von zwei Verfahren unter Heranziehung festgelegter Gleichwertigkeitskriterien.
Zwei beliebige quantitative Nachweisverfahren, die auf Zählungen (von Kolonien oder positiven Röhrchen) beruhen oder zwei beliebige qualitative Nachweisverfahren (Anwesenheit-/Abwesenheit-Verfahren), die dieselbe Zielsetzung haben, können verglichen werden.
Diese Internationale Norm liefert keine Lösung, um ein quantitatives Verfahren (Koloniezählung oder MPN) direkt mit einem qualitativen Nachweisverfahren (P/A) zu vergleichen.
Qualité de l'eau - Criteres pour établir l'équivalence entre les méthodes microbiologiques (ISO 17994:2004)
L'ISO 17994:2004 définit une procédure d'évaluation qui permet de comparer deux méthodes visant à détecter ou à quantifier le même groupe cible ou la même espèce de micro-organismes.
L'ISO 17994:2004 fournit les bases mathématiques pour évaluer les performances relatives moyennes des deux méthodes par rapport à des critères d'équivalence déterminés.
Deux méthodes quelconques de dénombrement fondées sur des comptages (de colonies ou de tubes positifs) ou deux méthodes quelconques de détection [méthodes présence/absence (P/A)] visant au même objectif peuvent être comparées.
L'ISO 17994:2004 n'indique aucune solution qui permette de comparer directement une méthode quantitative (comptage de colonies ou NPP) avec une méthode de détection (P/A).
Kakovost vode - Merila za ugotavljanje enakovrednosti mikrobioloških metod (ISO 17994:2004)
General Information
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Standards Content (Sample)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Kakovost vode - Merila za ugotavljanje enakovrednosti mikrobioloških metod (ISO 17994:2004)Wasserbeschaffenheit - Kriterien für die Feststellung der Gleichwertigkeit von mikrobiologischer Verfahren (ISO 17994:2004)Qualité de l'eau - Criteres pour établir l'équivalence entre les méthodes microbiologiques (ISO 17994:2004)Water quality - Criteria for establishing equivalence between microbiological methods (ISO 17994:2004)07.100.20Mikrobiologija vodeMicrobiology of waterICS:Ta slovenski standard je istoveten z:EN ISO 17994:2004SIST EN ISO 17994:2004en,fr,de01-november-2004SIST EN ISO 17994:2004SLOVENSKI
STANDARD
SIST EN ISO 17994:2004
EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN ISO 17994May 2004ICS 07.100.20; 13.060.70English versionWater quality - Criteria for establishing equivalence betweenmicrobiological methods (ISO 17994:2004)Qualité de l'eau - Critères pour établir l'équivalence entreles méthodes microbiologiques (ISO 17994:2004)Wasserbeschaffenheit - Kriterien für die Feststellung derGleichwertigkeit von mikrobiologischen Verfahren (ISO17994:2004)This European Standard was approved by CEN on 9 April 2004.CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this EuropeanStandard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on application to the Central Secretariat or to any CEN member.This European Standard exists in three official versions (English, French, German). A version in any other language made by translationunder the responsibility of a CEN member into its own language and notified to the Central Secretariat has the same status as the officialversions.CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,Slovenia, Spain, Sweden, Switzerland and United Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMITÉ EUROPÉEN DE NORMALISATIONEUROPÄISCHES KOMITEE FÜR NORMUNGManagement Centre: rue de Stassart, 36
B-1050 Brussels© 2004 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN ISO 17994:2004: ESIST EN ISO 17994:2004
EN ISO 17994:2004 (E)
2
Foreword
This document (EN ISO 17994:2004) has been prepared by Technical Committee ISO/TC 147 "Water quality" in collaboration with Technical Committee CEN/TC 230 "Water analysis", the secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by November 2004, and conflicting national standards shall be withdrawn at the latest by November 2004.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
Endorsement notice
The text of ISO 17994:2004 has been approved by CEN as EN ISO 17994:2004 without any modifications.
SIST EN ISO 17994:2004
Reference numberISO 17994:2004(E)© ISO 2004
INTERNATIONAL STANDARD ISO17994First edition2004-05-15Water quality — Criteria for establishing equivalence between microbiological methods Qualité de l'eau — Critères pour établir l'équivalence entre les méthodes microbiologiques
SIST EN ISO 17994:2004
ISO 17994:2004(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat accepts no liability in this area. Adobe is a trademark of Adobe Systems Incorporated. Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.
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SIST EN ISO 17994:2004
ISO 17994:2004(E) © ISO 2004 – All rights reserved iii Contents Page Foreword.iv Introduction.v 1 Scope.1 2 Normative references.1 3 Terms, definitions and symbols.1 4 Principle.4 5 Basic requirements for an equivalence experiment.4 6 Calculations.8 7 Evaluation.10 8 Test report.12 Annex A (informative)
Flowchart.13 Annex B (informative)
Collaborative equivalence trials.14 Annex C (informative)
Example.16 Bibliography.18
SIST EN ISO 17994:2004
ISO 17994:2004(E) iv © ISO 2004 – All rights reserved Foreword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. ISO 17994 was prepared by Technical Committee ISO/TC 147, Water quality, Subcommittee SC 4, Microbiological methods. SIST EN ISO 17994:2004
ISO 17994:2004(E) © ISO 2004 – All rights reserved v Introduction This International Standard presents the criteria and procedures for assessing the average quantitative equivalence of the results obtained by two microbiological analytical methods one of which may but need not be a standard or reference method. The methods considered are based on counts of colonies or of positive and negative liquid enrichment tubes (MPN and presence/absence methods). NOTE It is possible that a method that is not quantitatively equivalent with a reference method would be accepted, especially if it appears “better” than the reference either quantitatively or otherwise.
SIST EN ISO 17994:2004
SIST EN ISO 17994:2004
INTERNATIONAL STANDARD ISO 17994:2004(E) © ISO 2004 – All rights reserved 1 Water quality — Criteria for establishing equivalence between microbiological methods WARNING — Persons using this International Standard should be familiar with normal laboratory practice. This International Standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user to establish appropriate safety and health practices and to ensure compliance with any national regulatory conditions. 1 Scope This International Standard defines an evaluation procedure for comparing two methods intended for the detection or quantification of the same target group or species of microorganisms. This International Standard provides the mathematical basis for the evaluation of the average relative performance of two methods against chosen criteria of equivalence. Any two enumeration methods based on counts (of colonies or positive tubes) or any two detection methods [presence/absence (P/A) methods] intended for the same purpose can be compared. This International Standard provides no solution to directly compare a quantitative method (colony count or MPN) with a detection method (P/A). 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. ISO/TR 13843:2000, Water quality — Guidance on validation of microbiological methods 3 Terms, definitions and symbols 3.1 Terms and definitions For purposes of this document, the following terms and definitions apply. 3.1.1 General terms 3.1.1.1 reference method prescribed analytical method to analyse a given group or species of microorganisms NOTE As a rule, the reference method is a standard or a commonly used method. 3.1.1.2 trial method any method which is to be tested for equivalence with a reference method SIST EN ISO 17994:2004
ISO 17994:2004(E) 2 © ISO 2004 – All rights reserved 3.1.1.3 equivalent method method considered quantitatively equivalent with another method when the average relative difference of their confirmed counts is found “not different” when following the calculations specified in this International Standard 3.1.1.4 standard uncertainty uncertainty of the result of a measurement expressed as a standard deviation [GUM] 3.1.1.5 expanded uncertainty quantity defining an interval about the result of a measurement that may be expected to encompass a large fraction of the distribution of values that could reasonably be attributed to the measurand NOTE The fraction may be viewed as the coverage probability or level of confidence of the interval. To associate a specific level of confidence requires explicit or implicit assumptions regarding the probability distribution. The level of confidence may be attributed to this interval only to the extent to which such assumptions may be justified. [GUM] 3.1.1.6 coverage factor numerical factor used as a multiplier of the (combined) standard uncertainty in order to obtain an expanded uncertainty NOTE The coverage factor, k = 2 is chosen for this International Standard because the distribution of the relative difference is unlikely to be normal; the expanded uncertainty corresponds only approximately to the 95 % confidence interval. 3.1.2 Specific terms 3.1.2.1 count observed number of objects, e.g. colonies or cells of microorganisms, plaques of bacteriophages NOTE In this International Standard, the result of an MPN estimation is also considered a count. 3.1.2.2 presumptive count number of objects that according to their outward appearance should presumably be included in the count 3.1.2.3 confirmed count count corrected for false positive results by further testing of the presumptive objects 3.1.2.4 relative difference RD difference between two results, a and b, measured on a relative (natural logarithmic) scale NOTE The value of RD, x, expressed in percent, is given by x = [ln(a) − ln(b)] × 100 % SIST EN ISO 17994:2004
ISO 17994:2004(E) © ISO 2004 – All rights reserved 3 Essentially the same result is given by 2()100%()abxab−=×+ until the ratio between a and b is greater than about 3. This accounts for the usage of the term “relative difference” in both cases. 3.2 Symbols and abbreviated terms A the (symbol for the idea of) trial method a a test result by Method A ai the test result (confirmed count) of Method A in sample i B the (symbol for the idea of) reference method b a test result by Method B bi the test result (confirmed count) of Method B in sample i C coefficient for computing the number of samples, given the value of the experimental variance
D maximum acceptable deviation (value of confidence limit) in the case Methods A and B are “not different” i running index k coverage factor used for calculating the expanded uncertainty L smallest significant (i.e. maximum acceptable) relative difference between Methods A and B MPN most probable number quantitative method m number of parallel tubes per dilution in an MPN series n number of samples nA number of samples where for the P/A Method, A is positive and B negative nB number of samples where for the P/A Method, A is negative and B positive P/A presence/absence detection method s experimental standard deviation (standard uncertainty) s2 experimental variance xs standard deviation (standard uncertainty) of the mean U expanded uncertainty x relative difference xi value of the relative difference between ai and bi in sample i x arithmetic mean of xi (i = 1,2,…,n) xL value of the relative difference at the approximate lower 95 % confidence limit, derived by subtracting the value of the expanded uncertainty from the mean xH value of the relative difference at the approximate upper 95 % confidence limit, derived by adding the value of the expanded uncertainty to the mean X2 experimental Poisson index of dispersion y conditional variable used in computing the number of samples for equivalence testing and/or verification SIST EN ISO 17994:2004
ISO 17994:2004(E) 4 © ISO 2004 – All rights reserved 4 Principle The basic data are pairs of confirmed counts (ai, bi) obtained from the examination of two equal portions taken from the same vessel of a carefully mixed test sample, one determination (count) per method. The complete design consists of a large number of similar determinations. In this International Standard, two methods are considered quantitatively equivalent (“not different”) if the mean relative difference of the paired confirmed counts does not differ significantly from zero and the expanded uncertainty does not extend beyond the level of the stipulated maximum acceptable deviation. The decision rules based on the above principle are detailed in 7.2 and 7.3 and a flow chart is given in Annex A. NOTE 1 Fixing a value for the maximum acceptable deviation (D) implies indirectly that the smallest average difference (L) to be considered significant is one half of that value. NOTE 2 It has been suggested that in international and inter-laboratory method performance tests a limit of D = 10 % for the “confidence interval” be the maximum acceptable deviation for drinking water[2]. NOTE 3 For chemical methods, mean and precision are used as criteria for equivalence. In microbiology, equal precision (equal variance) is not an equivalence criterion. 5 Basic requirements for an equivalence experiment 5.1 General Both methods shall fulfil at least the minimum requirements of validity specified in ISO/TR 13843. The most important basic requirement of equivalence trials is a wide range of samples. Participation by a number of laboratories is usually necessary to expand the sample range over large geographical areas. Also the credibility of a general conclusion is commonly believed to require the participation of several laboratories. The result of the comparison is generally valid only within the range of sample types studied. Collaborative trials are detailed in Annex B. It is essential that all laboratories taking part in a collaborative study have recognized quality assurance systems in use and apply approved basic techniques of cultivation. 5.2 Types of samples The requirements for method comparisons differ somewhat from the daily routine situation. It is useful and often necessary to pre-select or prepare special samples. Samples for method comparisons should contain enough bacteria that the likelihood of scoring a zero count is small. Samples for method comparisons should represent types that are included in the scope of both methods. Natural samples are ideal. Appropriate samples may also be prepared by dilution, spiking, or mixing of different kinds of water to achieve the desired population in a suitable density. Spiking with pure cultures should be considered the last resort. It may be appropriate to stress the microbial population of some samples by controlled application of disinfectants[2] or by refrigerated storage in order to simulate situations encountered in routine laboratory practice. 5.3 Number of samples and participating laboratories 5.3.1 General It is not possible to determine beforehand the exact number of samples required for a valid comparison. The number depends on the actual difference observed, on the experimental standard deviation and on the difference considered significant. This International Standard includes an adequacy clause based on a SIST EN ISO 17994:2004
ISO 17994:2004(E) © ISO 2004 – All rights reserved 5 stipulated “maximum acceptable deviation” and the expanded uncertainty. If the data are found inadequate for deciding that the methods are “not different”, more samples are to be collected and examined. If the methods happen to differ markedly, a small number of samples might suffice to determine this fact. It is therefore advisable to proceed in stages. The first stage should be planned to detect large differences between the methods. If large differences are not found (result inconclusive), more samples are taken until the system is able to detect the average difference that corresponds with the maximum acceptable deviation chosen at the beginning of the trial. Tables are given in 5.3.3 and 5.3.6 to provide help for planning. 5.3.2 The number of laboratories There are no previous standards or rules about the number of laboratories in inter-laboratory equivalence trials. Six is tentatively suggested as minimum number. 5.3.3 Number of samples, two colony methods The total number of samples, n, sufficient for the detection of a given average relative difference at about 95 % confidence depends on the experimental variance according to the equation: n = Cs2 where s2 is the variance; C is a coefficient that depends on the chosen least significant difference. The value of C is derived from the relationship: C = 4/L2. The relationship between D (the maximum acceptable deviation) and L (the least significant difference) is: L = D/2 (see Table 1). Table 1 — Coefficients for determining the number of samples required for the detection
of a given relative difference (L) Da % L % C 60 30 0,004 4 40 20 0,010 0 30 15 0,017 8 20 10 0,040 0 10 5 0,160 0 a The corresponding maximum acceptable deviation (D) is shown for comparison.
EXAMPLE A rather frequently observed value for the experimental standard deviation of the relative difference is approximately s = 80. Inserting this value in the equation gives n = 6 400C. In order to detect an average relative difference of 10 % units (L = 10 %), n = 6 400 × 0,040 0 = 256 samples should be sufficient. 5.3.4 Number of samples, two MPN methods With MPN methods the number (n) of samples depends on the number (m) of parallel tubes according to the equation: n = 1 700/m With five parallel tubes per dilution, 1 700/5 = 340 samples should suffice for the detection of a 10 % relative difference. SIST EN ISO 17994:2004
ISO 17994:2004(E) 6 © ISO 2004 – All rights reserved 5.3.5 Number of samples, mixed comparisons When one of the methods is an MPN Method and the other a colony method the number of samples likely to be required is halfway between that in (5.3.3) and (5.3.4). NOTE With some recent methods based on chromogenic substrates, it is possible to estimate two bacterial groups s
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