In vitro bioassays are techniques that promise great steps forward in drinking water security. The technique is based on engineered cells that show a reaction if the sample they are exposed to contains toxins. A wide range of in vitro bioassays has been applied for water quality assessment for:

• Hazard identification and
• Testing of treatment efficiency of novel drinking water treatment methodologies.

These tests are mainly used as a diagnostic research and play an important role in the assessment of chemical  mixtures. However,  to  date,  regulations  and  water  quality  guidelines  focus  on  individual chemicals. Nevertheless, in  vitro  bioassays  for  human  health hazard  have  been  accepted  in  other regulatory frameworks, as food safety and chemical regulation. Examples from these international regulatory policies can act as successful examples for the roadmap to implementation for the acceptance of in vitro bioassays in drinking water quality regulation.

A crucial step on the road to implementation of in vitro bioassays is their validation to demonstrate the reliability and reproducibility of a test method. However, demonstrating the validity of a method is a complicated process. All steps in the formal validation are time consuming and costly. Formal validation is performed by special institutes such as the centres for Validation of Alternatives Methods (VAM). Harmonisation of a test method (such as achieving ISO-standards and OECD guidelines) is another dominant prerequisite for regulatory acceptance and facilitates a widespread use.

Regulatory requirements in food safety and chemical regulation have shown that three factors play an important role in the acceptance of in vitro bioassays for human health evaluation. These factors are: the necessity, the usefulness and the performance. First, the necessity could be determined by the regulatory requirements, such as the prohibited use of in vivo assays (or animal tests) in risk assessment of cosmetics. Second, the usefulness implies the benefits of in vitro bioassay. Third, the performance is determined by validation requirements such as the reliability and the reproducibility. Ideally, the assay results should reflect  the  potency  of  the  chemical  and  there  should  be  a  dose-response  relation  between  the concentration of the chemical and the effect measured in the assay.

Overall, in vitro bioassays in water quality assessment have proven their usefulness. Testing with in vitro bioassays has advantages over analytical techniques, such as the relatively low costs, the possibility of high-throughput methods, the derivation of mixture effects, and the preference over in vivo tests due to ethical reasons. However, several factors that hinder the implementation of in vitro bioassays in regulatory frameworks are:

1. The costly and time-consuming formal validation procedures
2. The need for a battery of in vitro tests to replace one in vivo test
3. The proven relevance to apply in vitro bioassays for drinking water

Besides the validation procedures, a further step to improve the regulatory acceptance is the derivation of human health based guideline values. This threshold can act as a filter mechanism where detailed evaluation is only performed in samples exceeding that trigger value.