The detection limit of any diagnostic test is defined as the lowest quantity of the target pathogen detectable with a 95% confidence level. When the quantity of the pathogen falls below the detection limit of the test, it can still be detected, albeit with a much lower probability.
When the quantity of the target pathogen excreted by the animal is low, it may be detected in one clinical sample but not in another sample produced from the same animal, at the same time, and on the same instrument. This explains how when a given animal is sampled multiple times, some samples have a positive diagnosis when others negative. It may also be observed on a given clinical sample tested multiple times.
Some pathogens have an intermittent shedding pattern, leading to high microbial load on a given day and low microbial load the day after. Factors such as vaccination, medication, or chronic infection all contribute to reduction of pathogen shedding. Shedding often occurs in low quantity at the beginning and towards the end of an infection.
