Positive Legionella or high TVC result? Here's what happens next.
A high count doesn't mean panic, but it does mean action. We'll help you work through it calmly: review, disinfect where needed, resample and record everything.
Calm, quick and properly recorded.
HSG274 Part 1 sets action levels for both general bacteria (TVC) and Legionella in cooling water. Go over them and you're expected to do something: resample, review the control measures and risk assessment, and in some cases disinfect the system.
Call us as soon as you get the result. We'll look at the recent history (dosing, dip slides, chemistry, recent shutdowns), get an engineer out to check the system and agree the right response with you. That might be a dosing fix and a resample, or it might be an online disinfection and a full clean.
Every step gets recorded, including resample results, so your log book tells the whole story. If you'd like to read up first, our monitoring and sampling page explains how results are taken and tested.
Rapid review of the data
Recent dosing, chemistry and dip slide records checked to find the likely cause.
Site visit and system check
Dosing kit, bleed, biocide reserve and the tower itself inspected.
Online disinfection
Where the result calls for it, a controlled disinfection of the system.
Resampling
Follow-up samples to the UKAS-accredited lab to confirm control has been restored.
Risk assessment review
We'll check whether the risk assessment and written scheme need updating.
Full record of actions
What was found, what was done and the follow-up results, filed in the log book.
What HSG274 Part 1 says about high results
A simplified summary for cooling systems. Your written scheme should set out the exact responses for your site, and the guidance itself has the full wording.
| Result | Status | Typical response |
|---|---|---|
| TVC up to 104 cfu/ml | Under control | Carry on with the programme and routine monitoring. |
| TVC above 104, up to 105 cfu/ml | Review | Review the programme and dosing, and re-test to confirm the count. |
| TVC above 105 cfu/ml | Act now | Take corrective action straight away, review the controls and re-test. |
| Legionella above 100, up to 1,000 cfu/litre | Review | Resample. If the result repeats, or most samples are positive, review the controls and risk assessment and consider disinfection. |
| Legionella above 1,000 cfu/litre | Act now | Resample and immediately review the controls and risk assessment, with remedial action that may include disinfecting the system. |
cfu = colony forming units. This table is a plain-English summary, not a replacement for HSG274 Part 1 or your written scheme.
Positive result response: common questions
We've had a positive Legionella result. Do we have to shut the tower down?
Not automatically. It depends on the count, the history and the risk. Many results are dealt with by resampling, reviewing controls and disinfecting online. In some situations shutting down or isolating is the right call. We'll help you make that decision quickly and record why.
Do we need to tell anyone about a positive result?
A positive sample on its own isn't generally something you have to report, but you must act on it and record what you did. If there's a suspected case of Legionnaires' disease linked to your site, the local authority or UKHSA may get in touch and you'll need your records to hand.
How quickly can you resample?
As soon as practical after any corrective action, timed so the result tells you something useful. Samples go to a UKAS-accredited lab.
Why did our count go up when nothing changed?
Something usually did: a dosing pump losing prime, a biocide drum running out, warm weather, a period of low load, or debris building up. Finding the cause matters as much as fixing the result.
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Got a high result back? Call us now.
Tell us about your system and we'll come back with a clear, itemised quote. No hard sell, just a straight answer on what your site needs.