Total Viable Count (TVC) analyses are very useful for trend analysis whereby changes in the numbers of microbes present in a system can be monitored over time and multiple samples. This can reveal useful information regarding factors having an effect on TVC counts, e.g. If a count is substantially higher during summer months it is likely that the organisms in the system are flourishing in higher temperatures and a suitable control measure may include reducing the temperature of the system in the summer.
A TVC test is only an estimate and results should be considered as an order of magnitude, e.g. less than 10, less than 100, less than 1000. A count of 50 cfu/ml obtained one month followed by a count of 60 cfu/ml would not be considered as significant based on the limitations of the procedure however a count of 50 one month followed by a count of 5000 the following month is clearly indicating a dramatic change to the system.
Our analytical service provider has provided us with the following guidelines for TVC samples issued by the Drinking Water Inspectorate (DWI).
Method: 2 days @ 37°C maximum 50 cfu/ml
Method: 3 days @ 22°C maximum 500 cfu/ml
Method: 2 days @ 37°C maximum 500 cfu/ml
Method: 3 days @ 22°C maximum 5000 cfu/ml