Bioassay is the name for a group of methods where the quantity of a compound is measured by a biological test system e.g. bacteria, yeast, algae. A precondition for the test is that the compound to be measured is known and that it is the growth limiting factor for the test micro-organism. Quantities of vitamins (especially B-group vitamins) and amino acids are generally measured by bioassay techniques. The test material may be either pure solutions or mixed preparations e.g. various foodstuffs like bakery products, dairy products, vegetables, cereals and convenience foods or animal food.

The most commonly used method is the turbidometric method in test tubes and this is described very exactly in Pharmacopoeias. Other methods are also acceptable as well. Referring to British Pharmacology Appendix XIV: "The methods of biological assay described in the Pharmacopoeia have been found satisfactory, but will not necessarily be the best methods for use in all circumstances. In most instances they may be replaced by other methods if it can be shown that such methods are at least equally accurate and precise and provide a measurement of the same active priniples".

The principle of the method is that the level of a certain vitamin or amino acid is determined by a microbiological assay relying on the growth of micro-organism that has a strict requirement for vitamin or amino acid. A growth medium lacking the vitamin or amino acid but containing all other nutrients necessary for growth which the micro-organism attains after a specified incubation period is related to the level of vitamin or amino acid in the sample. The amount in a sample can be quantified by comparing the test result to a standard curve produced by performing the assay with known standard concentrations of the vitamins or amino acid. The advantage of the microbiological method over the chemical (e.g. HPLC) method is that the microbiological method gives the amount of active vitamin while the chemical method gives the sum of active and non-active vitamin.

The growth of a micro-organism in a liquid medium results in the liquid becoming turbid. In the conventional end point method the turbidity is measured once after a fixed time of incubation resulting in one absorbance value for each test tube. The optimum time point is not always the one used in the test and this may lead for example to decreased concentration range for standards. Furthermore, conventional turbidometric methods are time and reagent consuming and need skilled technical assistance.

Bioscreen offers a new way to perform bioassays using a conventional biological principle. In Bioscreen the changes in turbidity are measured kinetically from each cuvette. This enables the use of numerous parameters which describe the changes in the growht kinetics of the micro-organism e.g. maximum absorbance time, fixed absorbance level, duration of lag time, and generation time all of which can greatly expand the reliability of the quantification of vitamins or amino acids. As an automated instrument it is easy to operate, fast and reagent consumption is minimal. High capacity and microculture conditions enable the user to examine and quantify with the same inoculum either single growth limiting factors or combinations of these.