Suppose a facility manager opens a lab report and finds a single line: Legionella pneumophila detected, 4 CFU/mL, one of eight samples. Nothing on the page says whether that is a crisis, a nuisance, or noise. The obvious next question is whether the building is safe, and the report will not answer it.
That is not a failure of the laboratory. Legionella results are not designed to answer that question, and understanding what they do answer is most of the job.
Start With What the Number Is Not
The CDC is direct about this. Legionella test results on their own do not measure health risk and are not predictive of disease. There is no known safe level or type of Legionella, and the presence of any Legionella should trigger response activities.
That combination frustrates people who want a threshold. It exists because the relationship between a concentration in a pipe and an infection in a person runs through too many variables: whether that water gets aerosolized, who breathes it, how healthy they are, and what the system does between your sample date and next month.
So a Legionella result is a performance indicator for your water management program. It tells you how well your controls are working. It does not certify a building.
Culture and PCR Measure Different Things
Most reports come from one of two methods, and mixing up their units is the most common reading error.
Culture grows the organism on a plate and counts what appears, reported in colony forming units per volume, usually CFU/mL. It detects viable bacteria and all Legionella species, has a limit of detection around 10 CFU/mL, and yields an isolate the lab can characterize further, which matters if a case investigation ever follows. Results typically take 7 to 14 days. Culture is also subject to the skill, experience, and procedural rigor of the particular laboratory, which is a real source of variation: in a CDC proficiency study of laboratories culturing Legionella from environmental samples, participants underestimated concentrations by an average of more than a log compared to reference values. Culture is generally preferred for evaluating growth trends over time.
Nucleic acid amplification, usually qPCR, detects Legionella-specific DNA or RNA. It returns results in roughly 2 to 48 hours, which is its main advantage. It may not differentiate between live and dead bacteria, and results are typically reported in genomic units, or GU, which are not directly equivalent to CFU. PCR is useful for negative screening and is often preferred for checking whether a remediation worked.
Two practical consequences. A GU number cannot be compared to a CFU threshold, so do not read a PCR result against a culture-based action level. And a high PCR number shortly after a disinfection may be detecting the DNA of bacteria your treatment already killed.
The Bands the CDC Actually Publishes
CDC frames results as performance indicators rather than pass or fail, with response levels tied to concentration and to change over time. In broad terms:
- Growth appears well controlled below 1 CFU/mL for potable water and below 10 CFU/mL for cooling towers
- At or above those figures, conditions may allow growth, and response activities are indicated
- A 10-fold to 100-fold increase indicates poorly controlled growth
- More than a 100-fold increase indicates uncontrolled growth
- Because of how fast they grow and spread Legionella, any detection of viable Legionella in a hot tub should prompt a response
Note what carries the weight there. Two of the four bands are about change rather than absolute concentration. A result of 3 CFU/mL means something very different in a system that read 0.2 last quarter than in one that read 4. This is the argument for routine testing on a schedule rather than testing only when someone is worried: a single number has no trend to sit in.
The other thing to read is what was found, not just how much. Legionella pneumophila serogroup 1 causes most disease, and reports that identify species and serogroup give you more to work with than a bare count. Sampling should generally cover all Legionella species, since similar conditions support multiple species.
Why a Negative Result Can Still Be Wrong
A clean report is good news only if the sampling behind it was sound. Several things produce a false sense of security:
A sampling plan that does not represent the system. CDC guidance is to perform an environmental assessment first, identifying where growth and spread are most likely based on sediment and biofilm, temperature, water age, and disinfectant residual, and then build a plan that represents the entire building water system. Eight samples from convenient fixtures on two floors is not that. The far end of a hot water loop, the low-use wing, the dead leg nobody has flushed, and the aerosol-generating equipment are the points that carry information.
Sampling too soon after treatment. After remediation, CDC advises waiting at least 48 hours after the system returns to normal operating conditions before testing to confirm effectiveness. Sample during or immediately after treatment and you are measuring the treatment, not the system.
The viable but non-culturable problem. Culture counts organisms that grow on a plate. Legionella can persist in a state where it does not culture but can still be infectious, which is part of why a negative culture is not proof of absence.
Handling. Sample handling, transport, and lab processing all affect results.
What Makes the Sampling Plan Defensible
- An environmental assessment done before any sample is pulled, so locations are chosen for risk rather than convenience
- Sample points that represent the whole system, including far points, low-use outlets, and every aerosol-generating device
- Consistent points sampled the same way each round, because trend is the most informative part of the data
- Adequate volume, with 250 mL typically sufficient for routine testing, and swabs or larger volumes where more information is needed
- A laboratory accredited to a recognized standard such as ISO/IEC 17025, willing to retain isolates for further characterization
- Method, units, detection limit, sample location, and date on every report
That last point sounds bureaucratic until a result turns into a conversation with a health department, an insurer, or a plaintiff’s attorney. A report that cannot say where the sample came from and what method produced the number is very hard to defend.
When Results Are Not Well Controlled
The sequence CDC recommends is worth following in order, because the instinct to jump straight to disinfection skips the steps that prevent a repeat:
- Review for potential errors in sample collection, handling, and testing
- Examine the equipment to confirm the system is working as intended
- Review records to confirm the water management program was implemented as designed
- Review assumptions about operating conditions, including the incoming water
- Re-evaluate the program, including chemical treatment, cleaning, maintenance, and any hazardous conditions
- Adjust the program to address deficiencies
- Consider remedial treatment only after the preceding steps
- Re-test at least 48 hours after the system returns to normal operation
There is a reason remediation sits at step seven. Following a successful remediation, Legionella will likely recolonize the system if the underlying conditions that supported growth are not addressed. Shock treatment without a control change buys months, not a solution.
In facilities serving people at increased risk, immediate measures that limit exposure to aerosols can be appropriate while the rest of that work proceeds.
What to Do
- Check the units before you interpret anything. CFU/mL and GU are not the same measurement and cannot be read against the same thresholds.
- Put the result next to your last one. Change over time carries more meaning than a single value.
- Read the species and serogroup, not only the count.
- If the result is unexpected, audit the sampling before you audit the building. Collection error is the first thing CDC tells you to rule out.
- If the result is clean, confirm the plan was capable of finding a problem. Ask where the samples came from and why those points.
For background on how water management programs are structured and what CMS requires of healthcare facilities, see our post on Legionella in healthcare facilities. For the system-level view of where risk concentrates, see Cooling Towers, Hot Water Systems, and Legionella.
If you have a Legionella report you are not sure how to interpret, or you want a sampling plan built from an assessment of your building rather than a list of convenient taps, the EnviroPro 360 team works with facilities across the CSRA. Reach out here and we will walk through it with you.
FAQ
What is a safe level of Legionella in water? There is no known safe level. CDC treats results as performance indicators for a water management program rather than a pass or fail threshold, with potable water below 1 CFU/mL and cooling towers below 10 CFU/mL indicating growth appears well controlled.
Is Legionella culture or PCR better? They answer different questions. Culture detects viable bacteria, takes 7 to 14 days, and is better for tracking trends. PCR returns results in hours, may not distinguish live from dead bacteria, and is useful for negative screening and for checking remediation.
Does a negative Legionella test mean my building is safe? No. A negative result reflects the samples that were taken. If the sampling plan did not represent the whole system, or samples were collected too soon after treatment, a negative result can be misleading.
