This article explains what Control Time (CT) is, why it is important, and when it should be established. It also explains why manual colour interpretation can sometimes be challenging and which automated solutions can help determine the optimal reading moment.
What Is Control Time?
Delvotest® performance and sensitivity depend on reading the test at the correct colour-changing moment, known as Control Time (CT).
The reference sample acts as the test's stopwatch. A reference sample is milk free from inhibitors that is expected to produce a negative result. When this reference sample reaches the correct yellow-green endpoint shown on the colour card, Control Time has been reached and the results can be evaluated.
The most appropriate reference sample depends on where Delvotest® is being used:
- Laboratory users: ✅ Negative Reference samples for Laboratory Use 🔗 for guidance on reference materials, routine laboratory controls, and performance verification.
- Farm users: ✅ Negative Reference sample for Farm Testing 🔗 for practical guidance on selecting suitable milk for establishing Control Time on the farm.
Why Control Time Matters
Reading the test at Control Time helps ensure that results are interpreted according to the validated method and supports consistent testing in both laboratory and farm settings.
- Reading too early means Control Time has not yet been reached and the test should not be interpreted.
- Reading too late may reduce the sensitivity of the test.
To understand why reading the results at the right time matters, take a look at the example below:
Once the reference sample reaches the correct endpoint, Control Time has been reached and all samples can be evaluated. In this example, the samples containing penicillin G and oxytetracycline are positive at Control Time and can therefore be correctly identified.
If incubation continues beyond Control Time, colour development also continues and the oxytetracycline sample turns negative. This effect is particularly relevant for bacteriostatic antibiotics such as tetracyclines, which slow bacterial growth rather than kill bacteria.
When incubation continues for too long, the bacteria may recover and continue growing. This may cause the test colour to change, resulting in reduced test sensitivity.
Test the Test
When using Delvotest® routinely, it is important to verify that the test system is performing as expected. Establishing Control Time is an important part of this process.
For every box of Delvotest®, run a reference sample alongside your samples and use it to establish Control Time. The negative control helps confirm that the test, incubation conditions, and testing procedure are working correctly.
For laboratory use, we also recommend including a positive control sample to verify test performance.
When Should Test Performance Be Verified?
We recommend verifying Delvotest® performance in the following situations:
-
Upon receipt of a new box
To confirm that the test kit performs as expected before routine use. -
Before first use after prolonged storage
To verify that storage conditions have not affected test performance. -
When implementing Delvotest® under new conditions
To confirm that local equipment, procedures, and operators can achieve the expected performance. -
When testing milk from a different animal species
To establish the appropriate Control Time for the type of milk being tested.
Differences in milk composition and microstructure can influence colour development, resulting in different Control Times for cow, goat, and sheep milk. Seasonal variation may also affect Control Time.
In practice, goat and sheep milk often require a Control Time that is 10 to 30 minutes longer than cow milk. Therefore, always include a reference control in each test run to establish the correct CT under the actual testing conditions.
The Challenge of Reading Colours
Control Time is reached within a relatively narrow colour window. Although Control Time can be established manually, interpreting colour changes consistently can be challenging.
Ambient lighting, reflections, viewing angle, and individual colour perception can all influence how the endpoint is interpreted.
Automated Solutions
Automated reading solutions can support consistent endpoint determination and reduce the influence of visual interpretation.
- Delvotest® Accelerator Smart (DAS) 🔗 automatically determines Control Time for plates and removes the need for manual colour assessment.
- Delvotest® Go 🔗 supports automated reading and interpretation of Delvotest® ampoules.
Manual Establishment of Control Time
Automated solutions provide the most consistent approach to establishing Control Time. However, Control Time can also be determined manually using a reference control and the Delvotest® colour card by following the procedures below:
- 🚧 How to Establish Control Time Using Delvotest® Plates 🔗
- ✅How to Establish Control Time Using Delvotest® Ampoules 🔗
💬 Got any questions? Need help? Contact us at support@delvotest.com