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close this bookBasic Laboratory Procedures in Clinical Bacteriology (WHO; 1991; 128 pages)
View the documentPreface
View the documentIntroduction
open this folder and view contentsQuality assurance in microbiology
close this folderPart I. Bacteriological investigations
open this folder and view contentsBlood
open this folder and view contentsCerebrospinal fluid
open this folder and view contentsUrine
open this folder and view contentsStool
open this folder and view contentsLower respiratory tract infections
open this folder and view contentsUpper respiratory tract infections
open this folder and view contentsSexually transmitted diseases
open this folder and view contentsPurulent exudates, wounds, and abscesses
open this folder and view contentsAnaerobic bacteriology
close this folderAntimicrobial susceptibility testing
View the documentIntroduction
View the documentGeneral principles of antimicrobial susceptibility testing
View the documentClinical definition of terms “resistant” and “susceptible”: the three-category system
View the documentIndications for routine susceptibility tests
View the documentChoice of drugs for routine susceptibility tests in the clinical laboratory
View the documentThe modified Kirby-Bauer method
View the documentDirect versus indirect susceptibility tests
View the documentTechnical factors influencing the size of the zone in the disc diffusion method
View the documentQuality control
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
 

Direct versus indirect susceptibility tests

In the standardized method outlined above, the inoculum is prepared from colonies on a primary culture plate or from a pure culture. This is called an “indirect sensitivity test”. In certain cases, where a rapid answer is important, the standardized inoculum may be replaced by the pathological specimen itself, e.g., urine, a positive blood culture, or a swab of pus. For specimens of urine, a microscopical examination of the sediment should first be made in order to see if there is evidence of infection, i.e., the presence of pus cells and/or organisms. The urine may then be used as the inoculum in the standard test. Likewise, susceptibility tests may be performed on incubated blood cultures showing evidence of bacterial growth, or a swab of pus may be used as a direct inoculum, when a Gram-stained smear shows the presence of large numbers of a single type of organism. This is called a “direct susceptibility test”; its advantage over the indirect test is that a result is obtained 24 hours earlier. The main disadvantage is that the inoculum cannot be properly controlled. When the susceptibility plate shows too light or too heavy growth, or when the culture is a mixture, the results should be interpreted with caution, and the test repeated on pure cultures.

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