Why do we need to test for water quality in stool blood tests?

Contaminants. Contaminants in water can affect test results in both directions, i.e., false positives and false negatives. Laboratory accrediting agencies, rightly, insist on tight controls on the quality of water used in reagents.
Bacteria. You probably are referring to testing of bacteria often found in human intestines (mainly e. Coli) in drinking water samples. These tests are periodically performed by regulatory agencies to detect contamination of our water supply.

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