How to Read Your Lab Report

When the lab sends back your water test results, it can look confusing. Here is what it all means in plain English.

Common Words on Lab Reports

CFU = Colony Forming Units. This is how bacteria are counted. One CFU means one bacteria (or group of bacteria) that grew into a visible spot on the lab plate.

CFU/L = CFU per litre. Used for Legionella testing.

CFU/ml = CFU per millilitre. Used for TVC testing.

CFU/100ml = CFU per 100 millilitres. Used for E. coli and Pseudomonas testing.

ND or Not Detected = The bacteria was not found. This is good.

Presumptive = The lab thinks it might be the bacteria but is not 100% sure yet. They will do more tests to confirm.

Confirmed = The lab is sure what the bacteria is.

Serogroup = Used for Legionella. Tells you which type it is. Serogroup 1 is the one that causes most Legionnaires' disease.

TVC 22°C = Bacteria grown at room temperature. These are usually from the environment (water, soil).

TVC 37°C = Bacteria grown at body temperature. These are usually from people or animals and are more concerning.

How to Interpret Your Results

Step 1: Check the Details

Make sure the sample details are correct - date, time, location, and tap name. If these are wrong, the results might not be for your sample.

Step 2: Look at Each Test

Go through each test result one at a time. Compare the number to the action levels (see the Results Interpretation tables in the Micro Sampling section).

Step 3: Check the Colour

• Green (good): The system is under control. Keep doing what you're doing.
• Yellow (caution): Something needs checking. Investigate and re-test.
• Red (bad): Take action now. Clean, disinfect, and re-test.

Step 4: Write It Down

Record the results in your site log. Note:

• Date of sampling
• Date of results
• Each test result
• Pass or fail
• What action was taken
• When re-testing is due

What to Do with Bad Results

1. Do not panic. One bad result does not always mean a big problem.
2. Check if the sample was taken properly. Was it contaminated? Was the chain of custody complete?
3. Re-sample if in doubt.
4. If the bad result is confirmed, follow the action plan in the 'What to Do When Results Are Bad' section.
5. Tell the responsible person.
6. Keep records of everything you do.

When to Get Help

Contact a water treatment specialist if:

• Results are repeatedly bad after cleaning
• You are not sure what the results mean
• You need help investigating the source
• The system is complex or has been modified
• You are in a healthcare setting and results are bad