## Sampling Strategies

How many animals to sample, how often to do it, what to look for, and which methods to use may be based upon scientific judgment rather than strictly obeying international guidelines.

Table 11.5 Calculation of Sample Sizes for Health Monitoring of Laboratory Animals.

Nosografic sensitivity(N1)

(Nj) = _ Infected animals reacting in the assay

Infected animals whether reacting or not

The estimated prevalence (p) in the colony p = Infected animals

Total number of animals

The risk ofa false negative result in an infected colony (C)

C = Number of infected colonies tested with a negative result Total number of infected colonies tested

Sample size (S) for colonies with more than 1000 animals

Sample size (S) for colonies with less than 1000 animals

D=Number of infected animals T=Total number of animals

The prevalence that a certain infection reaches depends on many factors, e.g., the contact between the animals, the resistance of the animals, but characteristics of the agent itself play a major role. The sample size needed to detect a specific agent in a colony therefore depends upon the acceptable risk of a false negative result, the sensitivity of the method applied and the estimated prevalence, that the infection as a minimum would reach if present in the colony (Table 1 1.5).249 As soon as a sample has been taken, it becomes historical. Curiosity only will dictate when to take the next sample. The result of one sampling visualizes whether changes have occurred since the last sampling.

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