CFU/mL as a guide to remedial action?

A technical bulletin from PathoCon Laboratories has a table with suggested Legionella Remedial Action criteria. They list various detectible limits of CFUs/mL as a guide for remedial action. For example, 109 CFUs may require cleaning and/or biocide treatment of equipment if indicated in potable water. Do you have any current information or references with action level based on CFU’s/mL.

It is logical that the overall burden of Legionella within the water distribution system should correlate roughly with the risk for contracting Legionnaires’ disease. While cfu/mL quantitation seems to be an intuitively plausible concept, in actuality, our studies showed that quantitation is highly variable. One reason is that Legionella resides within a biofilm coating in each pipe or distal fixture. When repeat samples are taken from the same site within a short period of time, the cfu/mL may vary widely. Also, water usage at that site and frequency of sampling from that site can also affect cfu/mL.

In short, no evidence exists to show that quantitation is useful for predicting risk, but considerable data exists to show it is an inherently unstable method. The recommendations from our website are evidence-based so we would not use this table as a guideline for remedial action. In contrast, the percent distal site positivity does correlate with occurrence of disease.