The DAS reads the colors of the individual wells in a plate and assigns numerical values to them (this is the Z value). During the incubation the DAS measures how these values change over time. The DAS will stop the incubation when 10% of the individual wells reach the value of -10. However, the minimum number of wells with negative milk is 4.
A full plate has 96 wells. Thus 10% is 9.6=10 wells that need to have NEGATIVE milk. For a full plate, the lab needs to add 10 wells with negative control milk!
- The plate can also break to blocks of 16 wells. The lab can separate the plate carefully with scissors and use as many pieces they need per incubation.
See below the requirements:
- A strip with 16 wells: 4 negative controls (requires DAS as a minimum to determine control time) + 1 positive control for performance testing
- Two strips with 32 holes: 4 negative controls (see above) + 1 positive control for performance testing
- Three strips with 48 wells: 5 negative controls + 1 positive control for performance testing
- Four strips with 64 holes: 7 negative controls + 1 positive control for performance testing
- Five strips with 80 wells: 8 negative controls + 1 positive control for performance testing
- Entire plate with 96 wells: 10 negative controls + 1 positive control for performance testing.
- A positive control should always be added on each incubation. āTo optimize reliability of the test, DSM advises to verify the correct functionality of the test, operation and incubation with negative control and positive control milk samples available at DSM.ā
- Do not leave EMPTY wells. It is best to add negative raw milk to these wells. This will help the algorithm in the calculation of the control time.
In the graph below, you can see how the DAS estimates the control time and stops the incubation. It finds the 10% fastest samples and uses these curves to make sort of an average the Z (leader) value. Then this leader value reaches -10 the DAS stops and reads the results of all the other wells. By the default Delvotest T method, a sample that has a value <-4 (at the control time) is labelled negative and a sample with Z >-4 is labelled as positive.
SAQ:
- How does the DAS find the control time?
- How does the DAS work?
- How many Negative milk samples should I add on the plate/ Block?