Responding To A Positive Legionella Sample

Responding To A Positive Legionella Sample

11th April 2018

Given the nature of Legionella, it’s almost a certainty that it will be present in any body of water. It’s usually nothing to worry about, providing the count is relatively small, but when a sample result comes back possible, it’s all too easy to panic.

Well take it from us, a positive sample isn’t the end of the world, but it is a great catalyst for improvement.

Why Take Samples

Sampling for Legionella isn’t a necessary requirement for any water safety programme and it’s not unheard of for organisations to implement schemes without regular sampling.

The main available guidance for Legionella sampling comes from the HSG274 Part 2:

  • If considered necessary by the risk assessment
  • From areas where the target control parameters are not met (ie where disinfectant levels are low or where temperatures are below 50 °C (55 °C in healthcare premises) for HWS or exceed 20 °C for cold water systems)
  • From areas subject to low usage, stagnation, excess storage capacity, dead legs, excessive heat loss, crossflow from the water system or other anomaly

We recommend samples be taken on a 6 monthly basis as a minimum, as it’s impossible to know how effective your control measures are if you’re not actively checking on a regular basis.

Interpreting The Results

Before we get to interpreting results, it is worth mentioning that all results should be taken with the context of the water system and the risk assessment. The same results found in a domestic system will have different impacts and consequences for an industrial system.

The style of the report generated is likely to differ slightly depending on the lab used for analysis, but the information provided will always cover the important elements.

Most Legionella tests will cover Legionella Pneumophilla serogroups and Legionella species and a positive result in some of these is less concerning than others.

Legionella Pneumophila serogroup 1 – This is the strain of bacteria most commonly associated with cases of Legionnaires’ Disease.

Legionella Pneumophila serogroup 2-14 – A little safer than serogroup 1, usually associated with non-fatal illnesses like Pontiac Fever.

Legionella Species – This primarily covers over 40 species of Legionellaceae that are commonly found in a wide spectrum of water, both artificial and natural. While not as big of a concern as the serogroups, their presence can indicate that conditions within the system could support the growth of the bacteria.

Results are usually shown as cfu/l which is the number of colony-forming units per litre of water. An exact count is often not provided or necessary as once a threshold has been reached, the exact numbers will make little difference.

Not detected/<100 – Not detected doesn’t necessarily mean not present or that there are no risks within the system.

>100 – <1000 – A low-level detection that is likely to require more context in order to effectively proceed. If multiple samples have been taken and the majority of them have returned positive, the system is most likely colonised albeit at a low level. Should the minority return positive, this will warrant a resample and the outcome of that will determine the best action to take.

>1000 – Should any sample return with a level this high, immediate action is required to prevent cases of Legionnaires’ Disease from arising. This includes taking the system/outlets out of use and carrying out a thorough clean and disinfection of the system.

By themselves, any of these can be a cause for some concern and action, but it’s the combination of the Legionella species and the amount found that will determine how to proceed.

The frequency of positive results is also something that will factor in, regular low levels of bacteria might not pose an immediate threat, but it clearly indicates that control measures might be failing or inappropriate and should be reviewed.

Corrective Actions

So a sample has returned positive and there’s panic in the air because people are at risk of contracting a potentially fatal disease, so what is there to do? Well, this will vary depending on the elements listed in the previous section.

The first step is to remain calm and take into consideration the contributing factors of the sample. Things like location, the frequency of use, the chance of exposure and previous sample results all influence the measures needed to correct the matter.

There are far too many situations to list a solution to them all, but we’ve listed a few measures, both preventative and reactive, that can be applied to most situations.

Continual low-level positives – While not an immediate concern, the constant results of a similar nature are enough to warrant a review and change of control measures.

Having some location information is also a big help in narrowing down the location of the contamination. If multiple samples have been taken from the same system, but different outlets, you’ll be able to narrow down where improvements or corrective actions are required. Should the results be similar across the whole system, the problem is most likely in the supply and a further investigation should be carried out and a disinfection considered.

Single low-level positive – A single positive isn’t something that can be completely ignored, especially it’s from an outlet that hasn’t been used recently. Legionella likes to cling to stagnant water and a tap that hasn’t been used in a few days might just be inviting enough to encourage the bacteria to grow.

Thoroughly flushing and resampling the outlet could be enough to get a clear result, but if the outlet is going to be unused for a while, a flushing regime will need to be implemented.

High-level reading – The immediate action is to take the affected outlets out of use and plan in corrective cleaning and disinfection, whether local to the outlet or across the whole system as necessary. A further sample should be taken following the completion of any corrective measures and the outlet can be put back into use following a second clear result.

Point of Use Filtration

Depending on the measures required to rectify the system, it could mean a portion of time with the bacteria within the system and limited or no access to affected outlets.

However, there is an alternative that is recommended by both HSG274 Part 2 and HTM 04-01 Part B – point of use filters.

“They should be used primarily as a temporary measure until a permanent safe engineering solution is developed, although long-term use of such filters may be needed in some healthcare situations. They may also be considered where high level of disinfection of water systems may dislodge biofilm.” – 2.117, HSG274 Part 2

“Point-of-use (POU) filtration should be considered and agreed by the WSG only as an interim safeguard where control measures have been ineffective, prior to and during engineering remedial works, during periods of plumbing refurbishments and maintenance works, and where additional protection is required for vulnerable patients.” – 7.45, HTM 04-01 Part B

We supply a range of point of use filters, including shower, tap and inline filters to suit any requirement. Check out our product page for more information.

A Legionella positive can be a difficult thing to handle and if not tackled appropriately, it can lead to some dire consequences. We heartily recommend speaking to your Legionella control provider or a water safety professional if you’re unsure on anything

By Simon Stone

Nant Ltd

The Development Centre,
Wolverhampton Science Park,
Glaisher Drive,
West Midlands
WV10 9RT


01902 636355