What Should I Do If My Building Tests Positive for Legionella?

What does a positive Legionella test actually mean?

Legionella was detected at some level. A signal to investigate, not a reason to panic.

The organism was detected in one or more water samples. The CDC has stated this does not necessarily mean people will get sick, as Legionella is present in roughly 50 percent of large building water systems. What matters is location, concentration, positivity rate, and species of Legionella.

What concentration of Legionella requires action?

No known safe level of Legionella. Any detection requires action. Higher concentrations and positivity require faster action.

The CDC has stated there is no known safe level of Legionella, so testing alone cannot predict who will get sick. Any detection warrants a review, with urgency scaled to concentration, positivity rate, species, and location in the building.

What is distal positivity rate and why does it matter?

The share of distal locations testing positive. Above 30 percent, risk rises sharply.

If you sample ten distal locations and three are positive, your rate is 30 percent. Peer-reviewed studies show the risk of Legionnaires’ disease rises significantly at 30 percent or above: below that, targeted fixture action with an engineering review; above it, a broader response.

Distal Positivity Rate and Recommended Engineering Response

Should we take affected fixtures out of service?

No. Taking fixtures out of service can make the problem worse.

Taking a shower or sink out of service lets water sit stagnant, which promotes bacterial growth. The General Services Administration and ASHRAE Standard 188 both recommend keeping fixtures in use with appropriate water management, not removing them. The right response is point of use filtration on affected fixtures, not shutting them off.

What is the first thing we should do if my building tests positive for Legionella?

An engineering investigation and root cause analysis, which costs between $4,900 and $12,000 per building.

A positive Legionella test is a data point; the engineering investigation tells you what to do about it. An ASSE 12080 certified mechanical engineer reviews your data and systems and produces a written root cause report showing whether you need full remediation, targeted fixture action, filtration, system adjustments, or a combination.

Should we install point of use filters if we test positive for Legionella?

Yes, on high-risk fixtures like showers, especially for at-risk populations.

For immunocompromised, elderly, or vulnerable populations, point of use filtration is one of the fastest protective measures available. FDA-cleared healthcare-grade filters at 0.2 micron remove Legionella pneumophila, Pseudomonas aeruginosa, and Nontuberculous Mycobacteria at the fixture. Shower head filters run $99 to $299, sink and faucet filters $99 to $249, and inline filters $186 to $489, with volume pricing over 100 units. To purchase point of use fitlers contact us (insert contact page link).

Do we need to notify anyone if our building tests positive for Legionella?

Not required for routine testing without cases, but check your jurisdiction.

Buildings that detect Legionella during routine validation testing are generally not required to report it. Reporting applies during an active case investigation from a clinical Legionnaires’ diagnosis. Some jurisdictions and facility types carry more: healthcare under CMS, NYC cooling tower registration, and certain state programs.

Does this require a full remediation?

Often no. The engineering investigation tells you whether remediation is warranted.

The investigation may show that targeted corrective measures, system adjustments, or point of use filtration will address the risk far cheaper than a full Legionella remediation. At Legionella Control Systems, we do not sell unnecessary remediations. If one is warranted, it runs $25,000 to $100,000 per building, with monochloramine and copper-silver ionization completing it in three to four weeks without evacuation.

What if the Legionella test was a pre-flush sample versus a post-flush sample?

Pre-flush reflects water at rest near the fixture; post-flush reflects the loop. Both are useful.

A pre-flush or first-draw sample, taken without running the water, reflects what is in the line at rest, including biofilm shedding. A post-flush sample, taken after 30 to 60 seconds, reflects what is moving through the system. Pre-flush counts often run higher. Both need expert interpretation.

What about Legionella pneumophila Serogroup 1 versus other species?

Legionella pneumophila Serogroup 1 is the highest-pathogen variant and raises urgency.

Legionella pneumophila is the most clinically significant of more than 60 species, and Serogroup 1 causes the large majority of confirmed Legionnaires’ disease cases in the United States. A positive for pneumophila Sg1 warrants more urgency than other species. Speciation and serotyping are included on our Legionella test kit packages for your risk.

How fast can we get the Legionella testing done?

7 business days for CDC Elite Lab culture, 24 to 48 hours for qPCR, and 25 minutes for onsite tests.

Our culture test kit packages are $999 for the 4-pack and $1,499 for the 10-pack, both with CDC Elite Lab culture testing and engineering review; culture results return in 7 business days. Quantitative PCR (qPCR) returns results in 24 to 48 hours, and our 25-minute onsite test detects Legionella pneumophila serogroups 1 through 15. qPCR and onsite testing are quoted by facility.

What happens if we ignore this?

A second linked Legionnaires case creates an outbreak, and it gets significantly worse.

Legionella does not resolve on its own; if the conditions are not addressed, colonization grows, and the chance of a clinical case rises. Once two confirmed Legionnaires’ disease cases are linked to your facility within 12 months, it becomes an outbreak by CDC definition, and the cost and legal exposure escalate.

How do we keep this from happening again?

Address the root cause, then maintain control through a documented water management plan.

The investigation tells you why Legionella is growing; corrective measures address the cause. Long-term control requires a documented water management plan aligned with ASHRAE Standard 188. Plan development runs $4,000 to $10,000 plus annual software. A functioning program rarely gets caught off guard, because routine testing catches issues early.

Does Legionella Control Systems do anything beyond responding to positive tests?

Yes. We are a full-service Legionella and waterborne pathogen control firm.

We work with facilities at every stage of Legionella risk: routine testing, engineering investigations, risk assessments, water management plan development, secondary disinfection design and installation, point of use filtration, remediation, and ongoing engineering support. A positive test is often when a facility starts taking Legionella seriously.

Do you have questions about your positive Legionella test?

Call Legionella Control Systems at 888-416-8626, or get in touch with our team through our contact page.

FAQ: Positive Legionella Test Response

What does a positive Legionella test mean?

Legionella was detected in your water. It does not necessarily mean people will get sick; the response depends on concentration, positivity rate, and species.

Should we take the affected fixtures out of service?

No, that creates stagnation and can make it worse. The GSA and ASHRAE recommend keeping fixtures in use; deploy point of use (POU) filtration instead.

What is the first thing we should do after a positive test?

An engineering site investigation and root cause analysis, $4,900 to $12,000 per building, which tells you what the positive means and what action is required.

Does a positive Legionella test require a full remediation?

Often no. Targeted corrective measures, system adjustments, or point of use filtration may address the risk. We recommend full remediation only when the data calls for it.

What does a full remediation cost if it is required?

$25,000 to $100,000 per building. Monochloramine and copper-silver ionization do not require building evacuation; shock treatments do.

What is distal positivity rate?

The percentage of distal sampling locations that came back positive. Above 30 percent, the risk of Legionnaires’ disease increases significantly.

Are we required to report a positive test?

Generally not for routine testing without cases; it becomes a factor during an active case investigation. Healthcare under CMS and NYC cooling towers carry more.

How much do point of use Legionella filters cost?

$99 to $489 per unit: shower head filters $99 to $299, sink and faucet $99 to $249, and inline $186 to $489. Volume pricing over 100 units.

How fast can we get Legionella test results?

Culture testing takes about 7 business days, qPCR 24 to 48 hours, and onsite 25 minutes. Kit packages (culture test) are $999 (4-pack) or $1,499 (10-pack).

What does Legionella pneumophila Serogroup 1 mean?

The species and serogroup behind 80 to 90 percent of confirmed Legionnaires’ cases in the United States. A pneumophila Sg1 positive warrants more urgency.

How much does a water management plan cost?

$4,000 to $10,000, plus annual software charges depending on the level of service selected.

Which facilities does Legionella Control Systems serve?

Hospitals, long-term care facilities, hotels, jails, prisons, condo associations, large commercial buildings, food and beverage, and manufacturing across the United States.

Contact Us

We can help you interpret a positive Legionella test, schedule an engineering investigation, arrange Legionella testing, supply point of use filtration, design Legionella remediation if warranted, and build a water management plan to prevent recurrence. Contact the Legionella experts at [email protected] or 888-416-8626.