• The Colilert test simultaneously detects or quantifies both total coliforms and Escherichia coli, with results in 24 hours.


Contact us


  • Approved for use in the United States and approved or accepted in more than 20 countries around the world. 
  • Millions of tests performed worldwide every year.

  • 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.

  • Less than 1 minute of hands-on time.
  • Detects coliforms and E. coli simultaneously in 24 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 12-month shelf life at room temperature.


The Colilert Test uses proprietary Defined Substrate Technology (DST) to simultaneously detect total coliforms and E. coli. Two nutrient-indicators, ONPG and MUG, are the major sources of carbon in Colilert and can be metabolised by the coliform enzyme β-galactosidase and the E. coli enzyme β-glucuronidase, respectively.

As coliforms grow in the Colilert Test, they use β-galactosidase 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 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 approved for use with Quanti-Tray and Quanti-Tray/2000.

How to use


Learn how to use the Colilert test



Step 1

Add reagent to sample and incubate at 35°C±0.5°C for 24 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 24 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


The Colilert comparator is a liquid colour and fluourescent reference. Its purpose is to assist in distinguishing a minimal positive from a negative test result.

At 24–28 hours, any yellow colour or fluorescence equal to or greater than the comparator is considered a positive test result.

The Colilert reagent powder should be white to off-white in colour; it should be dry and free-flowing. Any concerns about the colour or integrity of the reagent powder should be directed to IDEXX Technical Services.

Yes, if they are negative. If an inoculated Colilert sample is inadvertently incubated over 28 hours, lack of yellow colour is a valid negative test; a yellow colour after the 28 hours is not valid and should be repeated or verified.

Yes, if they are positive. An inoculated Colilert sample that is positive for coliforms and E. coli  before 24 hours is a valid positive test for both coliforms and E. coli. If the sample is positive before 24 hours for coliforms only, the sample is a valid positive test for coliforms. However, the sample should be incubated for a longer period (up to 24 hours, but not greater than 28 hours total time) to determine whether E. coli  are present.

The reagent system was designed to suppress non-coliform ONPG or MUG positive bacteria for at least 28 hours. If incubation extends past 28 hours, it is possible that such bacteria will overcome the Colilert test suppression system and react with ONPG or MUG. When incubation exceeds 28 hours, only negative results are valid. Positive results must be confirmed by classical methods.

You can quantify the Colilert test with Quanti-Tray, Quanti-Tray/2000 or any multiple-tube format.

Check with your local authority for proper disposal of bacteriological biohazard materials at your facility.

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

The shelf life of the Colilert test is up to 12 months from the date of manufacture for the snap pack format (100 mL). 

Resources & Tools

Product Information

Colilert Test (20-test pack)
Product Number: 98-12972-00
Catalogue Number: WP020I

Colilert Test (100-test pack)
Product Number: 98-27163-00
Catalogue Number: WP100I

Colilert Test (200-test pack)
Product Number: 98-12973-00
Catalogue Number: WP200I

10mL Colilert Test (100-test pack)
Product Number: 98-14770-00
Catalogue Number: W100I

10mL Colilert Test (200-test pack)
Product Number: 98-14771-00
Catalogue Number: W200I

IDEXX-QC Coliform and E. coli Kit
Product Number: 98-29000-01
Catalogue Number: WQC-TCEC

IDEXX-QC Faecal Coliform Kit
Product Number: 98-29001-01
Catalogue Number: WQC-FC

IDEXX-QC E. coli Kit
Product Number: 98-29007-01
Catalogue Number: WQC-EC

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.

Search the Reference & Regulatory Documents Library

Water Customer Support

Australia & New Zealand Head Office

IDEXX Laboratories Pty Ltd.
Unit 6, 38-46
South Street
Rydalmere, NSW 2116

Phone (AUS): 1300 44 33 99
Phone (NZ): 0800 838 522

Fax (AU): 1800 634 409
Fax: (NZ): 0800 448 443

Find a representative near you

ISO Certifications

Locations and ISO Certificates