2.3 Specimen preservation and transport
The composition of the specimen’s bacterial flora begins to change from the moment of specimen collection, and this change may undermine the diagnosis.
Potential consequences of suboptimal specimen preservation include:
- lethal/sub-lethal injury of fragile organisms – false negative culture result
- overgrowth of normal flora or environmental contaminants – false positive culture result, incorrect diagnosis
- pathogen overgrowth during transport – false semi-quantitative estimates of pathogen numbers.
Specimens should be refrigerated as soon as possible after collection to reduce bacterial replication. This is critical if testing involves semi-quantitative estimations of viable bacterial numbers, for example, as used for the bacteriological diagnosis of cow
What might happen if specimens are not kept cold?
Answer
If the cold chain is not maintained, contaminants or co-infecting bacteria may overgrow the pathogen, hampering its detection in culture. This is a serious problem for fastidious pathogens such as Haemophilus parasuis, that are easily overgrown by contaminants.
Storage by freezing has detrimental effects on bacterial viability and should only be used if specimens cannot reach the laboratory within 24 hours. Repeated freezing and thawing should also be avoided as the number of viable bacteria may fall below the limit of detection of culture.
Swab specimens should be transported in transport media designed to avoid desiccation, while specimens for anaerobic culture should be preserved in sealed containers immediately after collection until they can be cultured.
2.2 The importance of proper labelling and form completion
