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What is a Bioassay?

By Danielle DeLee
Updated May 17, 2024
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A bioassay, a shortened form of biological assay, is a type of test in which scientists measure the effects a substance has on living things. Sometimes, they try to predict what a substance will do to humans by observing other creatures’ reactions to it. In other cases, the reaction of a certain organism or type of cell is used to identify an unknown substance. Bioassay tests are often criticized by animal rights activists and by the producers of the products being tested, who claim that they sometimes give skewed results.

There are two main reasons for using a bioassay approach rather than an inorganic test. First, when developing a new drug or addressing the effects of toxins that have not been studied before, it is difficult to approximate the effects that the substance will have on living creatures without testing it on living creatures. Inorganic tests can tell scientists what molecules are in a substance, but life forms, and cells from living creatures, react in unpredictable ways. The different components of a substance may interact with each other, for example, so the finding that is most relevant when predicting the effect that something will have on a person is the effect that it has had on another being, rather than what its individual components are.

The second reason is that inorganic tests may be unavailable. Sometimes, a substance causes a reaction, but scientists are unable to identify the precise compound that caused it. In these cases, it is often impossible to develop an inorganic test because scientists do not know which molecule they are testing for. A scientist may think that a plant is toxic, for example, but he has no way of knowing which toxin could be active. Thus, he might perform a bioassay to see if ingesting the plant is toxic to a mouse.

A bioassay is any test in which scientists apply a substance to living material and quantify the results. There are two categories of these tests. In vitro tests use living matter, like cells. In vivo tests use the entire organism.

Scientists often use in vitro bioassays to test for the presence of certain toxins. One common in vitro bioassay is the Limulus test, named for the genus of the horseshoe crab. Scientists mix a test culture with blood cells from the horseshoe crab. They clot in the presence of endotoxins, which are poisons found inside some gram-negative bacteria that are released when the bacterial cells break down. This test can help scientists identify the bacteria they are dealing with.

In vivo testing is often used for predictive tests, and the most commonly used animal in bioassays is the mouse. For example, scientists may inject medicines into pregnant mice to see if those particular drugs cross over from the mother to the fetus. They can then make recommendations about whether pregnant women should take the medication.

In France, in vivo testing was used every season to test oysters for toxins. Mice were injected with oyster juice, and the product could not be shipped if two thirds of the mice died within one day. Oyster fishermen objected on the grounds that mice often died from other causes, condemning safe products. France recognized their objections by announcing the end of this practice in January 2010. This means, however, that future oyster testing will be limited to a certain number of toxins, at the thresholds that scientists think will cause illness.

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