Delvotest® is a range of microbial inhibition tests designed to detect antibacterial residues, like antibiotics, in milk. Read more details on how it works here: Understanding the Delvotest®.
There are various reasons a microbial inhibition test result is being not repetitive. This guide will help you identify common root causes that influence the test performance.
False Positives Results
Results that seem false can sometimes occur due to improper test use or contamination during preparation. In most cases, these issues arise from skipping the instructions or inadequate hygiene practices.
1) Sample deviations
read here - sample preparation Delvotest® Sample preparation - Delvotest - Help Center
| Sample deviations | Investigation |
| Not testing raw cow's milk | Delvotest® is developed and quality-controlled on raw cow milk. See here how to test with milk from other species or other dairy matrices |
| Milk sample is not representative | Read our guide here to learn how to milk individual animal and ensure a homogeneous sample. |
| Milk sample is not stored refrigerated (2*-7* °C) | *Check local/national required values |
| Milk tested some days after sampling |
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Solution:
- Take sample with correct homogenization and with a clean sampling device. Take at least 1 correct representative sample and measure. If in doubt at least test in duplicate to confirm equal/reproduceable test result of both duplicate samples.
- If in doubt, take a ‘reference control’ sample (example a known positive or negative sample) and analyze it along with the sample to be tested.
2) Handling/ testing method deviations
Possible influences on the performance of Delvotest® microbial inhibition test
Possible reasons:
- Incorrect / not consistent test execution/procedure:
- Inaccuracy of pipetting (incorrect use of pipette such as air bubbles in pipette stem), timing.
- In a Delvotest® (microbial) the agar medium must be un-damaged.
- Wells not sealed.
- After pipetting the milk into the Delvotest® not directly proceeded to incubation at 64 °C.
- Incorrect test result reading.
- Test result not read directly after test is ready.
Read ampoules on the side, reading the 2/3 lower part of the color. The upper 1/3 part may always show a bit of more dark color due to the milk on top. Plate format versions are read on the bottom side.
Investigation/Solution
- Read carefully the test kit and equipment instructions.
Test kit and equipment deviations
Possible reasons:
- Too cold test kit (reagents) directly taken from (cold) storage.
- Incubator/incubating device not pre-heated to 64°C.
- Incorrect storage/transport temperature of test kits (place test kits/reagents unused always directly after use back to cold storage.
- Never store test kits contacting ‘frozen‘ parts (such as the back of the refrigerator)
- Test kit over expiration date.
- Test kit and or equipment contaminated with antibiotics and/or cleaning/disinfecting reagents.
- Equipment calibration incorrect (incorrect/varying temperatures), reader calibration, pipette calibration.
- Fully insert ampoules into metal hole in block heater, check the block heater/incubator holes(bottom) are clean.
- Damaged aluminum foil sealing on top of ampoules and plates, causing test agar to dry out (water evaporation).
Investigation/Solution
- As comparison open a new test kit (tests to be used acclimated to room temperature) to verify correct results using a negative and positive control reference sample. Negative and positive control samples can be ordered via your DSM representative.
- Calibrate equipment.
False Negatives Results
Delvotest® false negative results are most caused by in-correct handling:
- Too late result reading due to too long incubation 9at 64 C) causing the test becoming less sensitive.
- Incorrect sample storage, antibiotic concentrations can decline over time
Most other defects, in-correct handling are causing (False) positive test results.
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