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Your Guide to Interpreting Colitag™ Water Test System Results
April 07, 2026
Colitag is an EPA-approved method for detecting total coliforms and E. coli in drinking water, delivering compliant results in as little as 16 hours with Presence/Absence (P/A) and Most Probable Number (MPN) formats. Colitag offers water testing labs a high-quality, cost-effective, and reliable choice.
This article is intended to help lab managers, plant supervisors, and quality assurance professionals accurately assess water test results, so you can protect your end users. Bookmark this article or share it with your team to simplify day-to-day lab work.
Water Testing Made Simple Using the Presence/Absence (P/A) Method
Step 1: Add Colitag media to 100 mL water sample.
Step 2: Incubate from 16 up to 48 hours at 35° +/- 0.5°C. If reading results from 16-24 hours, the sample should be pre-warmed in a water bath at 35°C for 20 minutes before placing the sample in an incubator.
Step 3: Visually check the sample for yellow color. If the sample is equal or more yellow than the Comparator, it is positive for coliforms.
Step 4: Visually check fluorescence using a UV lamp (365 nm). If the sample fluoresces, it is positive for E. coli.
Quantitative Coliform & E. coli Results Using the MPN Method
Colitag is now paired with the MPNTray, delivering more insight from every sample with dependable quantitative results using the Most Probable Number (MPN) method.
- Sterile, ready-to-use multi-well tray intended for a 100 mL water sample
- Enables enumeration of microorganisms, including total coliforms and E. coli
- Compatible with sealers intended for 97-well trays
- Streamlines microbiology testing for efficiency and consistency
MPN Method
Step 1: Add Colitag media to 100 mL water sample.
Step 2: Pour sample mixture into the MPNTray.
Step 3: Seal the tray and incubate from 16 hours up to 48 hours at 35° ± 0.5°C
Step 4: Visually check wells for yellow color (coliforms) and fluorescence using a UV light (E. coli). Count the positive wells and use the MPN table to determine results.
Interpreting Your Water Quality Results
After incubation, compare each sample to the appropriate controls or Colitag Comparator:
- Yellow color darker than a blank or negative control, or equal or darker than Comparator = Positive result for coliforms
- Yellow + fluorescence greater than blank or non-E. coli coliform control, or equal or greater than Comparator = Positive for E. coli
- No color change = Negative result
If color is unclear, gently invert the sample 2–4 times, and incubate for an additional 2–4 hours; weak positives will intensify. For complete information on result interpretation, consult the Colitag Product Instructions.
Watch our step-by-step Colitag method training video to ensure reliable detection of E. coli and total coliforms in water samples or visit our Colitag page for more information.
Recommended Products
Accurate interpretation of water test results is essential for meeting regulatory requirements and maintaining confidence in drinking water quality. Colitag™ provides an EPA-approved, reliable solution for detecting total coliforms and E. coli, offering both Presence/Absence and MPN water testing to support efficient, informed decision-making. For laboratories seeking an alternative to traditional coliform testing methods, Colitag delivers the speed, consistency, and performance needed for modern water monitoring programs.