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

  • ISO Standard: The Colilert-18/Quanti-Tray method is a worldwide ISO standard for detecting total coliforms and E. coli in water (ISO 9308-2:2012).
  • 18-hour Confirmed Result: Samples incubated overnight are ready to read early the next day. Delivers significant workflow benefits and speed in the event of a contamination issue.
  • Sensitivity: Early coliform detection in the distribution system can help to reduce biofilm buildup and sporadic downstream contamination events.
  • Specificity: Significantly reduce false alarms created by inaccurate false positive results which give rise to uncertainty and unnecessary expense.
  • Lab efficiency: Our ISO 17025 accreditation and ISO 11133 compliance help simplify your Quality Control procedures.

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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
  • Faster results mean that you can act sooner to prevent the spread of contamination.
  • Extremely low false positives rates reduce the incidence of false alarms giving greater confidence to operations, reduced uncertainty and unnecessary expense. 
  • 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 11133:20004 compliant.
  • The IDEXX Quality Control Laboratory is ISO 17025:2005 accredited. 


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


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. Colilert-18 is the ISO Standard EN ISO 9308-2 and is one of two recognised methods in Part A, Microbiological Parameters for which methods of analysis are specified.

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 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. 

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
Catalogue Number: WP020I-18

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

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

IDEXX-QC Faecal Coliform Kit
Product Number: 98-29001-01
Catalogue 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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Newmarket Business Park
Studlands Park Avenue
Newmarket, Suffolk, CB87ER

Tel: +44 (0) 1638 676800

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