Legionella quantitations in a floor drain and sink?

We have found Legionella in a sink and floor drain of a building on campus, some of the results have come back as “present, but below quantifiable levels”, and others in the 500 cells/swipe area. I really don’t know what these numbers mean. One of the occupants has been diagnosed with a “Legionella-like disease”, who believes that when the plumbers worked on the system the previous week “caused her disease”. This doesn’t make any sense to me, however. Does the 500 number sound high, normal, etc.? Does this mode of infection make any sense?

Given that the mode of transmission of Legionella bacteria is either via exposure to intense aerosol or aspiration (after drinking contaminated water or after direct instillation into the upper airway), it is not common to culture the drain of a sink or a floor drain. A more appropriate location is the sink faucet or the water in the hot water distribution system. We already have an answer on the web site to your question regarding culture results and quantity vs. extent of colonization on the website. Quantitation is not a useful parameter to gauge risk.

Regarding the individual with the “Legionella-like disease”, our first question is whether a diagnostic test for Legionnaires’ disease was performed (Legionella urine antigen test or respiratory culture) and whether this test was positive. If a diagnosis of Legionnaires’ disease cannot be confirmed, then you need not perform any environmental investigation.