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close this bookHealth and Environment in Sustainable Development - Five Years after the Earth Summit (WHO, 1991, 128 p.)
close this folderPart II. Essential media and reagents for isolation and identification of clinical pathogens
close this folderPriority grading of pathogens, culture media, and diagnostic reagents
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View the documentList of recommended culture media and diagnostic reagents for the intermediate microbiological laboratory


A certain degree of flexibility has been incorporated by adopting a graded priority code for pathogens, media, and diagnostic reagents, as follows:

Code 1: high priority
Code 2: intermediate priority
Code 3: low priority.

Pathogen priority

The categorization of pathogens is based on a number of factors: frequency of isolation, clinical relevance, severity of disease, epidemic potential, and the cost-benefit ratio of isolation and/or identification. As an example of the latter factor, chlamydiae, although considered as important pathogens, receive low priority, since there is no cheap laboratory technique for their detection. The grading is by no means absolute and will vary from country to country, or from laboratory to laboratory, depending on the local disease pattern, the laboratory capacity, and the resources available.

Media and reagents priority

The grading of media and reagents is linked to that of the pathogens for which they are used for isolation or identification. However, there may be differences - if the medium or reagent is used very broadly, it may score higher than any one pathogen for which it is used.

Code 1: High priority media and reagents should be available in all the laboratories that practise general diagnostic bacteriology. These are most often general-purpose, easy to prepare, and few in number.

Code 2: Intermediate priority media and reagents are additional useful substances that make laboratory diagnosis more complete and more useful for epidemiological studies, although they may not be essential for immediate patient care, e.g., the grouping antisera for meningococci.

Code 3: Low priority media and reagents are substances that are of value only occasionally for the patient, but which are useful in teaching, research, and specialized reference laboratories. This category applies to media and reagents that are too expensive for general use, or which are needed for organisms that rarely occur or are difficult to isolate and are therefore probably of low cost-effectiveness.

The pathogens involved, and the culture media and diagnostic reagents required, are listed below for some common bacteriological investigations, together with a suggested priority coding. The coding should be adapted for each laboratory according to local circumstances.