Finding out you have a positive Legionella sample result can feel alarming. But not every positive result means crisis.

What it does mean is that you need to respond correctly, proportionately, and in line with UK guidance under ACoP L8 and HSG274.

Here’s what a positive result really tells you — and what you should do next.

First: Don’t Panic — Understand the Result

A Legionella laboratory report will usually include:

• Where the sample was taken
• The date and time
• The system conditions at the time (temperature, biocide residual, etc.)
• The species identified
• The concentration reported as cfu/l (colony forming units per litre)

The number matters. But the context matters just as much.

Before taking action, ask:

• Was this routine monitoring or reactive sampling?
• Is it a single positive result?
• Is it confined to one outlet?
• Or are multiple areas of the system affected?

A low reading from one infrequently used tap is very different from repeated failures across several parts of a building.

What Do the Numbers Mean?

UK guidance uses action levels to determine the appropriate response.

Under 100 cfu/l
Generally considered satisfactory.
Review your control measures, but no major intervention is usually required.

100–1000 cfu/l
This is a warning sign.
Investigate your control regime. Check temperatures, flushing, biocide levels and monitoring records. Resampling is normally appropriate.

Over 1000 cfu/l
Corrective action is typically required.
Review your risk assessment immediately. Disinfection may be necessary. Resample after remedial action.

Over 10,000 cfu/l
This suggests significant colonisation.
Consider removing affected outlets or systems from service. Disinfect urgently and escalate to senior management and the dutyholder.

The key point is this: action should be proportionate to the result.

Why Positive Results Happen

In most cases, positive Legionella samples can be traced back to one or more of the following:

• Poor temperature control
• Infrequently used outlets
• Stagnation or dead legs
• Incomplete flushing regimes
• Inadequate biocide dosing
• Biofilm build-up
• A written scheme that exists on paper but isn’t followed consistently

Legionella rarely appears without a reason. There is almost always a control weakness somewhere in the system.

The Most Important Step: Review Your Controls

Under ACoP L8, the dutyholder must ensure that control measures are effective.

Before jumping straight to shock disinfection, check:

• Is stored hot water reaching 60°C?
• Are hot outlets achieving at least 50°C within one minute (55°C in healthcare settings)?
• Is cold water below 20°C?
• Are TMVs inspected and serviced?
• Are infrequently used outlets flushed weekly?
• Are monitoring records accurate and up to date?

A positive result is often a signal that something operational needs tightening up.

When Is Disinfection Necessary?

If counts exceed 1000 cfu/l — particularly across multiple outlets — system disinfection is often required.

However, disinfection alone is not a long-term solution.

If the root cause is not identified, bacteria will return.

We regularly see situations where a system is disinfected, resampled clear, and then three months later the same issue reappears. Without addressing stagnation, turnover, temperature control or system design issues, the cycle continues.

An effective response should include:

Reviewing the risk assessment
Checking the schematic diagram
Inspecting tanks and calorifiers
Auditing maintenance compliance
Identifying stagnation points
Verifying the written control scheme is being implemented properly

Repeated Positives: A Bigger Issue

If resamples continue to fail, the problem is unlikely to be minor.

Repeated positive Legionella results may indicate:

Established biofilm within pipework
Oversized or poorly designed storage systems
Poor turnover
Inadequate management oversight
A breakdown in monitoring procedures

At this stage, senior management involvement is essential. Legionella control is a legal responsibility under the Health & Safety at Work Act and COSHH Regulations. It cannot sit solely with maintenance teams.

Keep Clear Records – Every action taken must be documented:

• What was found
• What corrective measures were implemented
• Who authorised the works
• When resampling took place
• What the follow-up results showed
• Clear records demonstrate compliance and protect your organisation if ever challenged.

Need Support Interpreting a Positive Legionella Result?

If you’ve received a positive Legionella sample result and aren’t sure what level of response is required, it’s better to speak to someone competent early rather than react blindly.

Whether you need help:

Interpreting laboratory results
Deciding if disinfection is necessary
Reviewing your risk assessment
Updating your written scheme
Managing repeat positives
Or strengthening ongoing control measures

We can help.

Our team supports dutyholders, facilities managers and estates teams across the UK to ensure water systems remain compliant, controlled and defensible.

Call us on 01454 417 920
or
Email [email protected]

A positive result doesn’t have to become a bigger problem — but it does require the right response.