WATER HYGIENE AND SAFETY
How to address positive Legionella test results
Matt Morse and Nick Barsby of the Legionella Control Association (LCA) take a detailed look at some of the key steps to take, and the pitfalls to avoid, in the event of positive Legionella test results.
The LCA requires its members to have in place management procedures, and appropriately competent staff, that give them the capability of responding appropriately to positive Legionella results. Positive Legionella results following sampling are one area where it’s possible to misinterpret the meaning and end up with what could at best be inappropriate expense, and, at worst, a dangerous situation and risk to life.
Sample planning
Interpretation of results starts with a suitable sample plan. If inappropriate samples are taken, results will be difficult to interpret meaningfully. Samples can be taken pre- or post-flush, and with or without disinfection of the outlet. The selection of location, type of sample, and method of analysis chosen, form part of the sample plan.
For populations with elevated susceptibility, such as in some healthcare settings, or for evaporative cooling systems, there is some specific guidance on what to take and where. In many other situations there is no specific guidance that indicates that samples are required, and the need must be driven by risk assessment.
Eliminating or reducing the risk Where risk assessment identifies a risk of Legionella growth within a water system, and the need to monitor using Legionella samples, the first priority should be to consider measures to eliminate or reduce the risk before designing monitoring measures. HSG274 Part 2, paragraphs 2.120 to 2.125, give useful advice on where and why to sample. Where it is impossible or not reasonably practicable to reduce the risk, and the risk assessment recommends monitoring for the presence of Legionella by sampling, there are a number of factors to consider in a sample plan. The points below give some further considerations for hot and cold water system sample plans:
How often should samples be taken? n Legionella is relatively slow-growing, but may develop to hazardous levels within days or weeks in ideal conditions.
A Legionella positive result on a lab plate.
n The HSE guidance in HSG274 Part 2 details weekly samples for Legionella where controls, such as temperature or biocide, are not being consistently achieved.
Where should samples be taken? n Samples should be checking areas where Legionella is likely to grow – deadlegs, underused outlets, areas where controls have failed, or where risk is identified for some other reason.
How should samples be taken? n For systems otherwise under control, this method should generally be pre- flush without disinfection, to examine the risk that exists to the users of the system from the outlets.
n In some cases, it would also be appropriate to sample post-flush, with disinfection of the outlet to examine how the system is performing as a whole.
n There is limited value in sampling post- flush downstream of mixing valves, or sampling post-flush, without disinfecting the outlet, although sometimes this is useful in investigation work.
How should samples be analysed? n There are many options available, and all have advantages and disadvantages.
n Culture, MPN, PCR, IMS, ICA, and rapid protein ID etc. will all give either a count or a yes/no.
n For the purposes of the laboratory section below, we are assuming that culture is to be used.
n The option selected must be appropriate for the question being asked by the sample plan.
The laboratory process Positive Legionella results occur in around 10-20 per cent of laboratory water samples that are analysed using the ISO 11731 culture method. The method
September 2021 Health Estate Journal 49
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88
