HTM 04-01 and Pseudomonas aeruginosa: Water Safety in Augmented Care

Most water safety guidance is written in the abstract. HTM 04-01 Part C is not. It exists because of something that happened.

Between late 2011 and early 2012, neonatal units in Northern Ireland experienced a serious outbreak of Pseudomonas aeruginosa infection among critically vulnerable babies. The subsequent investigation traced the organism to the water supply, and specifically to the taps and outlets at the point of use rather than to the incoming mains or the storage tanks.

That finding changed how healthcare estates think about water. Legionella control had been about temperature and about the system as a whole. Pseudomonas control turned out to be about the last few centimetres of pipework, the tap itself, and the people using it.

What HTM 04-01 covers

HTM 04-01 is Safe water in healthcare premises, issued for the NHS estate. It is structured in parts:

Part A deals with design, installation and commissioning. New build and refurbishment.

Part B deals with operational management. The day to day running of an existing system.

Part C deals with Pseudomonas aeruginosa specifically, and specifically in augmented care settings.

It is worth being precise about the organism here. Part C is about Pseudomonas aeruginosa specifically, not “Pseudomonas” as a genus. Lab reports often report a positive result as “Pseudomonas spp.”, meaning a Pseudomonas species has been found without yet confirming which one. Most Pseudomonas species living in water systems are common environmental organisms with little clinical significance in this context. P. aeruginosa is the one that matters, because it is the species capable of causing serious infection in immunocompromised or line-dependent patients. A “Pseudomonas spp.” result should prompt speciation before it is treated as equivalent to a confirmed P. aeruginosa result, not trigger the same response by default.

Part C is the addendum most people mean when they talk about “the pseudomonas guidance”, and it is the one that introduced requirements that had no real equivalent in general legionella practice. It did not start life as Part C. It was originally published as a standalone addendum to HTM 04-01, and was only folded into the main series once the approach it set out had bedded in. The name changed, the substance did not: it is still the document that tells a Trust what “augmented care” costs it in practical terms. If your own water hygiene work is mostly on the legionella side, our guide to HSG282 and spa pool legionella control covers that more familiar ground.

What “augmented care” means

This is the pivotal definition, because everything in Part C hangs off it.

Augmented care covers settings where patients are at materially increased risk from waterborne organisms that would not trouble a healthy adult. Broadly it means units where patients are severely immunosuppressed, where they have indwelling lines or devices, or where the skin barrier is significantly broken.

In practice that typically includes intensive care, neonatal and special care baby units, transplant and haemato-oncology units, burns units, and renal units.

The important discipline is that this is a decision made by the Water Safety Group and written into the Water Safety Plan, not something assumed informally at estates or ward level. Two hospitals with the same specialties may draw the line in different places for good clinical reasons. What is not acceptable is never having drawn it at all, because then nobody knows which outlets fall under the enhanced regime.

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Why self-definition matters more than it should

That local decision carries more weight than the wording suggests, because of how a lot of NHS estate is actually owned and run. A significant share of hospital buildings sit under Private Finance Initiative contracts, where the physical building and the hard facilities management, including the water system, belongs to a Special Purpose Vehicle rather than to the Trust. The Trust, and the Water Safety Group that sits within it, is in effect a tenant on that system.

The augmented care designation itself is usually made by the Water Safety Group, working through the Water Safety Plan and drawing on clinical and infection prevention judgement about which wards genuinely carry that risk. But the obligation to deliver the water safety regime that decision requires usually falls on the SPV or its FM contractor, working to a specification written into a contract signed years, sometimes decades, earlier. When the Water Safety Group designates a new unit as augmented care, or widens the boundary of an existing one, it can raise the standard the estate has to meet on a system it does not itself operate, under a contract that did not necessarily anticipate that footprint when it was signed.

This is not a theoretical problem. Services move constantly inside a hospital, and augmented care boundaries move with them. Where the estate is directly managed, that is a scheduling headache for the Water Safety Group. Where it sits behind a PFI landlord and tenant relationship, it is a contractual one as well, and it is worth understanding which side of that relationship you are on, and who actually sits on the Water Safety Group, before assuming responsibility, or blame, sits where you think it does.

Why Pseudomonas behaves differently to Legionella

If you have spent your career on legionella, some of Part C will feel counter-intuitive.

It is not primarily a temperature problem. Pseudomonas aeruginosa is comfortable across a wide temperature range. Running your hot water at 60 degrees does not solve it the way it substantially helps with legionella.

It lives at the outlet. Pseudomonas forms biofilm on the wetted surfaces closest to the point of use. Flow straighteners, aerators, spray heads, the internal components of mixer taps and thermostatic mixing valves. It colonises the very components that were fitted to improve the user experience or to prevent scalding.

The route of infection is different. Legionella is inhaled as an aerosol. Pseudomonas typically transfers by contact, via hands, via equipment rinsed under a tap, via water splashing into a sterile field. It does not need to become airborne to cause harm.

Sampling means something different. For legionella you are usually sampling to check whether the system is under control. For pseudomonas in augmented care you are sampling specific outlets to determine whether that outlet is safe for that patient group right now.

This is why an excellent legionella regime does not automatically satisfy Part C. They are targeting different organisms with different behaviour.

How much stricter the numbers actually are

It helps to see the scale of the difference rather than just the principle. Under the general legionella regime that applies to most water systems, HSG274 and ACOP L8, a result under 100 cfu/litre is treated as the system being under control. A result between 100 and 1,000 cfu/litre triggers review and resampling rather than immediate action. Only a result above 1,000 cfu/litre demands an immediate response.

Pseudomonas monitoring at augmented care outlets under Part C works on an entirely different scale. The target is not detected. Low counts, in the region of 1 to 10 cfu per 100ml, are typically enough to trigger retesting before and after flushing, and counts above that are commonly treated as an action level requiring the outlet to be taken out of service until the cause is resolved. Confirm the exact figures against your Trust’s own Water Safety Plan and the current edition of HTM 04-01 Part B, since local policies can set a tighter bar than the minimum, but the order of magnitude is the point. An augmented care outlet is judged against something close to zero tolerance, not against a few hundred cfu of headroom.

What Part C expects in practice

A Water Safety Group. A multidisciplinary group rather than an estates function working alone. It needs estates, infection prevention and control, microbiology and clinical representation, because the decisions are simultaneously engineering decisions and clinical ones.

A Water Safety Plan. A living document covering the whole system, from incoming supply to outlet, with augmented care areas identified explicitly.

Identification of augmented care areas and their outlets. Written down, agreed, and reviewed when services move, which in a hospital is constantly.

A sampling regime for those outlets, with defined actions on a positive result. The actions matter more than the sampling. A positive result with no agreed response is just information nobody used.

Reading a result in context, not in isolation. A positive sample at one outlet does not automatically mean the wider system is contaminated. Biofilm sitting inside the outlet fitting itself, or simply limescale and other deposit that has nothing to do with the supply, can shed organism into a sample and produce a positive result that is entirely local to that fitting. Treating each result in isolation risks two opposite mistakes: overreacting to a single localised fitting problem as if it were a systemic failure, or missing a genuine systemic issue because no individual result looked serious enough on its own. The discipline is to test outlets as part of a wider regime and look at the pattern across several points and over time, not any one result on its own, before deciding whether you are looking at a systemic issue or a local one.

Attention to outlet fittings. Whether complex fittings are appropriate in augmented care, how they are cleaned and maintained, and when they are replaced rather than cleaned.

Flushing regimes for infrequently used outlets, and genuine scrutiny of whether an outlet needs to exist at all. The most reliable control for a little used outlet in a critical area is often removal.

Consideration of point of use filters, understanding that these are a control measure with a defined service life, not a permanent fix, and that they carry their own management burden.

The design lesson worth carrying beyond the NHS

The uncomfortable finding from the Northern Ireland investigation, and from the work that followed it, is that several of the contributing factors were introduced by well intentioned design.

Thermostatic mixing valves are fitted to prevent scalding, a real and serious risk. But they create a permanently tepid mixed volume close to the outlet. Aerators and flow straighteners reduce water consumption and improve the feel of the flow. They also create surface area and turbulence at exactly the wrong point. Long tail pipes to basins create dead volumes.

None of these were mistakes in isolation. Together, in a setting full of patients with no immune reserve, they created a hazard.

That is a design principle that travels well beyond healthcare. Every fitting added to a water system for comfort, efficiency or safety needs to be assessed for what it does to the microbiology.

Who this applies to outside the NHS

HTM 04-01 is written for the NHS estate, but the principles are increasingly expected wherever vulnerable people and water meet. Private hospitals, hospices, care homes with nursing, dialysis units and specialist rehabilitation settings all face the same underlying risk, and the same regulatory expectation to manage it competently under general health and safety law.

If your setting cares for people who are immunosuppressed or who have breached skin, the honest question is not whether HTM 04-01 formally applies to you. It is whether you could explain to an inspector, or to a family, why you chose not to follow it.

Related reading

For water hygiene risk outside healthcare settings, see our guides to HSG282 and legionella in spa pools and hot tubs and legionella and landlord responsibilities. For the underlying legionella duty, see ACOP L8 and HSG274 explained, and our legionella risk assessment cost guide.

Getting the right competence

Water safety in a healthcare setting is a genuinely specialist field. It sits across estates engineering, microbiology and infection prevention, and the people who do it well tend to have come up through one of those and deliberately learned the others.

You can find legionella and water hygiene specialists on My Safety Consultant, including our featured consultants, where their qualifications, professional memberships and company accreditations are set out on the profile. Contact them directly. We take no commission and we never sell your enquiry on.

This guide is a plain English summary and is not a substitute for the current version of HTM 04-01 or for competent advice on your specific premises.