While we have seen some field tests that give you an 'absence/presence' for E. coli, they are bulky and expensive, and often single use. They also don't give us a numerical value, making them irrelevant for natural water systems (where a count of 200 would still be safe for swimming). Absence/presence would give too many false alarms. All of this to say- you need a lab to test for E. coli!