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close this bookBasic Laboratory Procedures in Clinical Bacteriology (WHO; 1991; 128 pages)
View the documentPreface
View the documentIntroduction
close this folderQuality assurance in microbiology
View the documentIntroduction
View the documentDefinitions
View the documentInternal quality control
View the documentExternal quality assessment
open this folder and view contentsPart I. Bacteriological investigations
open this folder and view contentsPart II. Essential media and reagents for isolation and identification of clinical pathogens
View the documentSelected further reading
View the documentSelected WHO publications of related interest
View the documentBack Cover

External quality assessment

This section gives information on what is involved in participation in an external quality assessment scheme (sometimes known as a “proficiency testing scheme”).


The purposes of a quality assessment programme are:


• To provide assurance to both physicians and the general public that laboratory diagnosis is of good quality.

• To assess and compare the reliability of laboratory performance on a national scale.

• To identify common errors.

• To encourage the use of uniform procedures.

• To encourage the use of standard reagents.

• To take administrative measures (which may include revocation of the operating licence) against substandard laboratories.

• To stimulate the implementation of internal quality control programmes.


A quality assessment programme consists of a number of surveys in which coded specimens are distributed by mail to participating laboratories. These specimens should be incorporated into the laboratory routine, and handled and tested in exactly the same way as routine clinical specimens.

The surveys should be conducted in accordance with the following recommendations:


• Surveys should be carried out 12 times per year ideally, but at least 4 times per year.

• A minimum of 3 specimens should be included in each survey.

• The reporting period should be short, for example I week following receipt of specimen.

• Instructions and report forms should be included with each survey. The report sheet should be in duplicate, with a clearly stated deadline.


Cultures should be included for identification and for susceptibility testing against a limited range of antibiotics; they may be pure cultures or mixtures of two or more cultures.

Cultures should represent at least the first 3 of the following 5 categories:

1. Bacterial species that are of great public health potential, but which are not often seen in routine practice, for example Corynebacterium diphtheriae, Salmonella paratyphi A.


NOTE: Brucella and Salmonella typhi should not be used for quality assessment schemes, since they may give rise to serious accidental infections.

2. Abnormal biotypes that are often misidentified, for example H2S-positive Escherichia coli, lactose-negative E. coli, urease-negative Proteus.

3. Newly recognized or opportunistic pathogens, for example Yersinia enterocolitica, Vibrio parahaemolyticus, Pseudomonas cepacia.

4. A mixture of Shigella, Citrobacter, E. coli, and Klebsiella may be used to test the skill of a laboratory in isolating pathogenic microorganisms from a number of commensal organisms.

5. A mixture of nonpathogenic organisms may be used to test for ability to recognize negative specimens.


Serological tests for the following infections should be part of an external quality assessment programme:


• syphilis
• toxoplasmosis
• rubella
• brucellosis
• hepatitis
• streptoccocal infections
• typhoid (Widal test, where still in routine use)
• HIV infection.

Rating and reporting of results

As soon as all reports are received from participating laboratories, the correct answers should be sent to the laboratories. Within one month after that, a final report should be sent to the laboratories with an analysis of the results. A performance score is given to each laboratory. Each laboratory should have a code number known only to itself. Thus it can recognize its own performance in relation to others, but the other laboratories remain anonymous.


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