Colilert 18


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.
  • Minimises 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 metabolise ONPG and change it from colourless to yellow.

E. coli use β-glucuronidase to metabolise 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



Step 1

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

Step 2

Read results:
Colourless = 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

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

Quanti-Tray/2000—Read results:

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

Frequently asked questions


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

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 and for E. coli detection in the following types of water:

  • Ambient waters (includes fresh, marine or estuarine surface water used for recreation, propagation of fish, shellfish, or wildlife; agriculture; industry; navigation; or as source water for drinking water facilities)
  • Surface water
  • Ground water
  • Wastewater: can also be used for fecal coliform detection upon approval by a facility’s EPA Region and State NPDES permitting office

The Colilert-18 test detects coliforms and E. coli at 1 organism/100 mL.

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

The powder should be off-white in colour, dry, and free flowing. A slight tinge of colour 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 colour or integrity of the powder should be directed to IDEXX Technical Service.

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.

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 colour greater than or equal to the comparator is not valid and should be repeated or verified.

Yes, the Colilert-18 test can be run with Quanti-Tray, Quanti-Tray/2000, or any multiple-tube format.

Yes, please refer to the Colilert-18 test product insert for the fecal coliform procedure.

Check with your local, state, and/or federal authorities for proper disposal of bacteriological biohazard materials at your facility.

Resources & Tools

Tests & Accessories

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Product Information

Colilert-18 Test (20-test pack)
Product Number: 98-08876-00
Catalog Number: WP020I-18

Colilert-18 Test (100-test pack)
Product Number: 98-27164-00
Catalog Number: WP100I-18

Colilert-18 Test (200-test pack)
Product Number: 98-08877-00
Catalog Number: WP200I-18

IDEXX-QC Fecal Coliform Kit
Product Number: 98-29001-01
Catalog Number: WQC-FC

Snap pack and Quanti-Tray combination packs are also available.



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.

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Water Customer Support

IDEXX Laboratories Canada Corp.
1345 Denison Street
Markham, Ontario
L3R 5V2


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

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

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