What to Do When You Receive a Positive Legionella Result
At Qlearwater, one of the most common questions we hear is:
“We just got a positive Legionella sample — what do we do now?”
Legionella Bacteria
A positive result doesn’t always mean there’s an immediate danger, but it does mean it’s time to respond strategically. The goal is to take informed, data-driven actions — not reactive ones. Let’s walk through what to do next.
Step 1: Review the Laboratory Results
Start by identifying the species and serogroup that tested positive.
Was it Legionella pneumophila serogroup 1 (Lp1)?
This is an important distinction — over 90% of Legionnaires’ disease cases are attributed to Lp1.
If the result was not Lp1, the overall health risk is generally lower, and your response may be adjusted accordingly. You should sample more frequently and monitor your control measures as the system may be susceptible to other strains including Lp1 if your control measures are out of range. However, it is not as acute of a risk.
Step 2: Evaluate the Sampling Location
Next, consider where the sample was taken:
Was it from a high-risk area, like a patient or resident shower?
Was it a rarely used outlet, such as a slop sink or utility faucet?
Was it taken from the hot water or cold water system?
This context helps determine the likelihood of exposure and guides how aggressive your response should be.
Step 3: Assess the Extent of Positivity
If multiple locations — typically more than 30% of sampled points — return positive results, this suggests system-wide colonization. In this case, the facility is considered at risk and broader disinfection and monitoring steps are necessary.
If only one or two isolated samples are positive, more investigation may be needed before determining the level of concern. Sometimes additional data helps clarify whether it’s an isolated finding or part of a developing trend.
Step 4: Determine Risk and Action Level
Once the species and sample location have been reviewed, it’s time to decide the appropriate response level:
| Result Type | Location Type | Recommended Response |
|---|---|---|
| Non-Lp1 species | Any | Lower response, continue monitoring |
| Lp1 – low-risk area (e.g., mop sink) | Not frequently used by residents | Clean, flush, and resample |
| Lp1 – high-risk area (e.g., patient shower) | Frequent human exposure | Take corrective action immediately |
Step 5: Take Corrective Actions Before Retesting
Before conducting retesting, several physical and operational steps should be taken to reduce potential bacterial presence:
Flush the affected line for at least 15 minutes (remove aerator first to maximize velocity).
Replace the supply line.
Clean or disinfect the showerhead or aerator using an approved method.
Install an ASTM F838–certified point-of-use bacterial retention filter as a temporary control.
If subsequent testing remains positive, expand the investigation to nearby outlets or zones to determine whether the issue is localized or systemic.
Step 6: Strengthen Your Water Management Program
The best long-term defense against confusion and reactionary decisions is a comprehensive Water Management Plan (WMP).
A WMP serves as your facility’s roadmap — outlining responsibilities, monitoring schedules, response steps, and documentation procedures for any water safety event.
Your WMP team should include:
The Maintenance Director or Building Engineer
A Legionella Specialist (preferably ASSE 12080 certified)
An Infection Preventionist
Facility Stakeholders and Administrators
This multidisciplinary approach ensures a balanced and well-informed response, as well as clear communication among all responsible parties.
Step 7: Document, Review, and Communicate
Every action — including flushing, cleaning, replacement, and retesting — should be thoroughly documented. This record not only supports compliance but also helps your team identify trends and measure the success of corrective actions.
Communication among departments (maintenance, nursing, environmental services, infection control) ensures the facility acts consistently and confidently.
Final Thoughts
A positive Legionella sample doesn’t mean your system has failed — it means your monitoring program is working. Detecting the bacteria early gives your facility the chance to act before health risks arise.
By responding calmly, following a structured process, and maintaining a strong water management program, your team can handle positive results with confidence and clarity.
At Qlearwater, our specialists help facilities interpret results, assess risk, and take the right corrective actions while maintaining compliance with NJ S2188 and CMS guidelines based ASHRAE 188 standards.
Article written by Avi Klein
ASSE 12080 Certified Legionella Water Safety & Water Management Specialist