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Simultaneously detects both total coliforms and Escherichia coli in water, or fecal coliforms in wastewater, giving you results in 18 hours.

  • Read afternoon samples the next morning—before the next day’s samples arrive.
  • Lift boil water alerts in 18 hours.
  • Provide results in record time for real estate, new well, and new construction samples.
  • Read Friday samples early Saturday, leaving the rest of the weekend free.
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  • Ease of use simplifies training.
  • Unit-dosed packaging eliminates media preparation.
  • No repeat testing due to clogged filters or heterotrophic interference.
  • Quality Control (QC) procedure can be done in 15 minutes.
  • Under 1 minute hands-on time.
  • Detects coliforms and E. coli simultaneously in 18 hours or less.
  • No confirmations needed.
  • No glassware cleaning or colony counting.

  • Identifies E. coli specifically, eliminating unnecessary public notification due to non-target organisms.
  • Suppresses up to 2 million heterotrophs per 100 mL.
  • Eliminates the subjective interpretation found in traditional methods.
  • Detects a single viable coliform or E. coli per sample.
  • Minimizes evening and weekend work.
  • Up to 15-month shelf life at room temperature.


How it works

The Colilert-18 test uses a proprietary Defined Substrate Technology (DST) nutrient indicators ONPG and MUG to detect coliforms and E. coli. Coliforms use their β-galactosidase enzyme to metabolize ONPG and change it from colorless to yellow.

E. coli use β-glucuronidase to metabolize MUG and create fluorescence. Since most non-coliforms do not have these enzymes, they are unable to grow and interfere. The few non-coliforms that do have these enzymes are selectively suppressed by the Colilert-18 test’s specifically formulated matrix.

This approach is different from traditional media, which provide a nutrient-rich environment that supports the growth of both target organisms and nontargets. When nontargets grow and mimic target organisms, false positives occur. Growth of nontargets can also suppress target organisms and give false negatives in traditional media. To suppress nontargets, traditional media often include high levels of salts, detergents, or other selective agents that may inadvertently suppress target organisms and give further false negatives.

Only IDEXX reagents have been validated and are approved for use with Quanti-Tray and Quanti-Tray/2000.

How to Use

Learn how to use the Colilert-18 test

File Icon View/Print the Colilert-18 test Procedure

Learn how to use Colilert-18
Learn how to use the Colilert-18 test

Step-by-step guide to using the Colilert-18 test


Step 1

Add reagent to sample and incubate at 35°C ± 0.5°C for 18 hours.

Step 2

Read results:

  • Colorless = negative
  • Yellow = total coliforms
  • Yellow/fluorescent = E. coli



Step 1

Add reagent to sample.

Step 2

Pour into Quanti-Tray (counts from 1–200) or Quanti-Tray/2000 (counts from 1–2,419)

Step 3

Seal in Quanti-Tray Sealer and place in 35°C ± 0.5°C incubator for 18 hours (in other countries, the temperature requirement may be different per regulatory requirements).

Step 4: Quanti-Tray

—Read results:

  • Yellow wells = total coliforms
  • Yellow/fluorescent wells = E. coli
  • Count positive wells and refer to MPN table

 Step 4: Quanti-Tray/2000

—Read results:

  • Yellow wells = total coliforms
  • Yellow/fluorescent wells = E. coli
  • Count positive wells and refer to MPN table

Q & A

Frequently asked questions and answers

Is the Colilert-18 test an ISO method?
Yes. The International Organization for Standardization (ISO) has withdrawn ISO 9308-2:1990, which follows the multiple-tube (most probable number) method, and replaced it with Colilert-18/Quanti-Tray, the new, worldwide ISO standard 9308-2:2012 for detecting total coliforms and E. coli in water.

Learn more about ISO

Is the Colilert-18 test approved by the U.S. EPA?
Yes. The Colilert-18 test is U.S. Environmental Protection Agency (EPA) approved as a method for total coliform and E. coli detection in drinking water, E. coli and fecal coliform detection in wastewater and for E. coli detection in recreational water (including fresh and marine), source water, surface water and ground water.

What is the detection limit of the Colilert-18 test?
The Colilert-18 test detects coliforms and E. coli at 1 organism/100 mL.

What is the shelf life of the Colilert-18 test?
The Colilert-18 test shelf life is up to 15 months from date of manufacture.

What color and consistency should the Colilert-18 powder be?
The powder should be off-white in color, dry, and free flowing. A slight tinge of color may be observed when the Colilert-18 test is added to the water sample, but this in no way interferes with the “readability” of the test. Any concerns about the color or integrity of the powder should be directed to IDEXX Technical Service.

Do the Colilert-18 test samples need to be prewarmed?
Yes, presence/absence (P/A) samples need to be prewarmed; however, Quanti-Tray or Quanti-Tray/2000 samples do not. For P/A, if the sample is not already at 33°C–38°C, place the sample in a 35°C waterbath for 20 minutes or a 44.5°C waterbath for 7–10 minutes. This prewarming time is part of (not in addition to) the 18-hour incubation period.

Can the Colilert-18 test be read after 18 hours?
Yes, if an inoculated Colilert-18 test sample is inadvertently incubated beyond 18 hours, the result is stable for 4 additional hours (22 hours maximum incubation). Heterotrophic organisms are suppressed during the incubation. After 22 hours, a negative test is valid, but any yellow color greater than or equal to the comparator is not valid and should be repeated or verified.

Can I quantify my Colilert-18 test results?
Yes, the Colilert-18 test can be run with Quanti-Tray, Quanti-Tray/2000, or any multiple-tube format.

Can I use the Colilert-18 test to detect fecal coliforms?
Yes, please refer to the Colilert-18 test product insert for the fecal coliform procedure.

What are the disposal requirements for a Colilert-18 test sample?
Check with your local, state, and/or federal authorities for proper disposal of bacteriological biohazard materials at your facility.

What are the recommended storage conditions for the Colilert-18 test?
2°C–25°C away from light.

Test & Accessories

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Resources & Product Information

IDEXX Water has reference materials and approval documents to support the many products in our water portfolio. Find the document(s) you need by selecting the link below.

>>  Search the Reference & Regulatory documents tool

Product Information

Colilert-18 Test (20-test pack)
Catalog Number: WP020I-18

Colilert-18 Test (100-test pack)
Catalog Number: WP100I-18

Colilert-18 Test (200-test pack)
Catalog Number: WP200I-18

Snap pack and Quanti-Tray bundles are available.

IDEXX Water Customer Support

Corporate Headquarters
IDEXX Laboratories, Inc.
One IDEXX Drive
Westbrook, Maine 04092 USA

Tel: 1-800-321-0207
Fax: 1-207-556-4630

Find a representative near you.

ISO Certifications

File Icon  ISO 9001:2008

File Icon  ISO 17025:2005

File Icon  ISO 14001:2004