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

  • Read afternoon samples the next morning—before the next day’s samples arrive.
  • Lift boil water notices in 18 hours.
  • Provide results in record time.
  • Read Friday samples early Saturday, leaving the rest of the weekend free.
  • The Colilert-18/Quanti-Tray method is the worldwide ISO standard for detecting total coliforms and E. coli (ISO 9308-2:2012)
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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.
Operational Value

With Colilert-18, you will have information to detect Coliforms and E. coli more quickly and accurately so that you can act sooner to prevent the spread of contamination.

  • 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.
Quality Assured
  • The Quality Control Certificate for Colilert-18 is ISO 11133:2004 compliant.
  • The IDEXX Quality Control Laboratory is ISO 17025:2005 accredited.
  • The Colilert-18/Quanti-Tray method is the worldwide ISO standard for detecting total coliforms and E. coli (ISO 9308-2:2012)


How it works

The Colilert-18 test uses 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

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:

  • 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 incubator at 35°C ± 0.5°C 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.K. Drinking Water Inspectorate (DWI)?
Yes. The Colilert-18 test is DWI 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:Drinking Water, Source Water, Sewage and Sludge.

Is Colilert-18 listed in the Standing Committee of Analysts Blue Books?
Yes, Colilert-18 is specified in the Microbiology of Drinking Water (MoDW) 2016, the Microbiology of Recreational Water (MoREW)
2016 and the Microbiology of Sewage Sludge (MoSS) 2003.

Is Colilert-18 included in the European Drinking Water Directive (EUDWD)?
Yes, Colilert-18, being the ISO standard EN ISO 9308-2, is one of two recognised methods in Part A, Microbiological Parameters for which methods of analysis are specified.

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 colour and consistency should the Colilert-18 powder be?
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.

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 colour 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 faecal coliforms?
Yes, please refer to the Colilert-18 test product insert for the faecal coliform procedure.

What are the disposal requirements for a Colilert-18 test sample?
Check with your authority 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.
Product Information

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

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

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

Snap pack and Quanti-Tray bundles are available.

IDEXX Water Customer Support

UK Headquarters

IDEXX Technologies Ltd Units 1B & 1C, Newmarket Business Park, Studlands Park Avenue, Newmarket, Suffolk, CB87ER

Tel: 01638 676800

ISO Certifications

File Icon  ISO 9001:2008

File Icon  ISO 17025:2005

File Icon  ISO 14001:2004