Section 2 | Reading Livestock Medicines Labels

Withdrawal Times and Residue Testing

What is a Withdrawal Time?

The withdrawal time is the recommended time between the last drug treatment and the slaughter of the animal or sale of milk, eggs, or honey. It is the time required for the drug residue to be reduced to safe levels in the animal’s body before the food produced by that animal may be eaten.

Withdrawal times only apply when the drug is used:

• At the labeled dose
• By the recommended route
• In the labeled animal species
• For the recommended number of treatments

Label withdrawals are not valid when the products are used:

• Extra-label
  • Different dose
  • Different route
  • Different species
  • Different frequency
• In combination with other drugs or routes of administration
• In a severely sick animal

When the drug is prescribed for extra-label use, the veterinarian is responsible for determining the withdrawal time to ensure there are no illegal residues.

How is a Withdrawal Time Calculated?

To determine the withdrawal time, the drug company uses data on the metabolism of the drug in specific species. The drug is administered over progressively longer slaughter or collection points, and the tissue or fluid is tested to determine if there is a residue. The withdrawal time is set until drug residues in tissues or fluids are well below a specified limit.

Withdrawal times aren’t the same for every animal!

Some animals may require longer to clear the drug residues from their tissues or fluids. This is especially important in sick animals who will take longer to clear the drug. It can be difficult to confirm the drug has cleared in many animals without invasive testing; although in dairy cattle, the milk can be tested to ensure there are no drug residues.

Calculating the Withdrawal Time

Withdrawal times can either be provided in hours or days. A withdrawal day is a full 24 hours starting at the time of treatment.

An example of a five day withdrawal time is as follows:

• A drug is administered at 6:00 am on Friday morning and with a 5 day withdrawal time, should be clear of the product by Wednesday at 6:00 am

An example of a 48-hour withdrawal time is as follows:

• An animal is administered a drug on Monday at 10:00 am and with a 48-hour withdrawal time, will be clear of the product by Wednesday at 10:00 am

Residue Testing

Food safety monitoring programs use residue-screening tests in normally-appearing animals or carcasses at slaughter, as well as in animals or carcasses they suspect might have drug residues. Issues that might cause an inspector to test meat for residues include signs of illness in live animals and abnormalities (e.g. abscesses, pneumonia, evidence of mastitis) in the carcass.

There are some classes of animals that are targeted with more intensity, based on historic drug residue issues. These classes of animal include veal calves and barbecue pigs.

What Samples are Tested?

Kidneys and muscle are the two primary tissues tested. The kidneys are responsible for getting rid of drugs from the body, and many antimicrobial medications are likely to be found there. Sampling is used to estimate levels of all other internal organs people may eat, such as liver and heart. The muscle tissue that is often chosen for sampling is the diaphragm, the internal muscle that helps the animal breathe. If residues are found here, likely residues are present in all other skeletal muscle that people typically consume from carcasses.

Residue Tests: A two-stage approach

Testing for residues in animal products involves two phases:

1. The primary test – this test tells us that there is a drug substance present and what it is, but it doesn’t tell us how much
2. The secondary test – this test confirms the results of the first test, further identifying the drug and telling us how much drug residue is present in the fluid or tissue. By comparing to the standard, it can be assessed whether the tissue has more residue than is safe for people to consume

The Primary Screening Test

When looking at an optimal screening test for residues, the following characteristics are important:

• Relatively inexpensive
• It catches the greatest range of possible drug residues
  • Very sensitive – this means that there might be some false-positive tests
• Simple to perform
• Gives rapid results

Positive screening tests are considered “suspect” and are sent for additional testing (secondary test).

The Secondary Screening Test

This test is more accurate, labor-intensive, and costly than the primary screening test.

It is used on suspect samples and has the following features:

• Identifies true positives among suspect samples
• Normally takes longer to complete than the primary test
• Used to make a final decision on whether any part of the carcass is suitable for human consumption