PCR positive and culture negative water samples?

This is a general question about lab tests for Legionella, but I thought you may be able to help me. I know that the Pittsburgh VA is an authority on Legionella. We recently tested water samples from a camp that hosts retreats for both children and adults, after a Legionellosis case in our region occurred and the patient claimed he had visited the camp a few days before developing symptoms, We sent the samples to our state’s public health laboratory and they all came back culture-negative but positive by PCR. The public laboratory sent the results to us saying that this test is still experimental, but they could not explain what that means in terms of control measures. I get the impression that PCR detects Legionella DNA, but only a positive culture can define if the Legionella is viable enough to infect a person. Is this correct? Are repeat tests recommended?


The director of this camp is very concerned that any of the lab results came back positive and is thinking about repeating testing. He has looked into this and found a lab that will charge upwards of $1000 for each investigation. Could you also tell me an estimate of the cost of sending samples to the Pittsburgh VA Lab?


You are correct that the meaning of a positive PCR/negative culture is unclear. It is for this reason that culture remains the “gold standard” for investigation of cases. We would expect that the cultures would be positive if a water reservoir at the camp was the source of exposure – unless a disinfection procedure had been applied.

An individual that is active enough to go to a camp also would have had the opportunity to be exposed to Legionella in locations besides the camp. The camp director has done all that is necessary and follow-up cultures are not indicated. If they want to culture anyway, our lab’s information can be found under the Legionella Testing tab.