Assessing risk of health care-acquired Legionnaires' disease from environmental sampling: the limits of using a strict percent positivity approach.
BACKGROUND
Elevated percent positivity (≥30%) of Legionella in hospital domestic water systems has been suggested as a metric for assessing the risk of health care-acquired Legionnaires' disease (LD).


METHODS
We examined the validity of this metric by analyzing data from peer-reviewed studies containing reports of Legionella prevalence in hospital water (ie, percent positivity) and temporally matched reports of patients with health care-acquired LD.


RESULTS
Our literature review identified 31 peer-reviewed publications reporting matched data. We abstracted a total of 206 data points, representing 119 hospitals, from these articles. We determined that the proposed 30% positivity metric has 59% sensitivity and 74% specificity (ie, a 41% false-negative rate and a 26% false-positive rate). These notable error rates could have significant implications, given that we identified 16 peer-reviewed articles and 6 government guidance documents that referenced the 30% positivity metric as a risk assessment tool.


CONCLUSIONS
Environmental sampling of hospital water distribution systems for Legionella can be an important component of risk management for LD. However, the possible consequence of using a percent positivity metric with low sensitivity and specificity is that many hospitals might fail to mitigate when a true risk is present, or might unnecessarily allocate limited resources to deal with a negligible risk.
Building Science & Forensics, Environmental Health & Engineering, Inc., Needham, MA, USA. jallen@eheinc.com
Joseph G
JG
Allen
Theodore A
TA
Myatt
David L
DL
Macintosh
Jerry F
JF
Ludwig
Taeko
T
Minegishi
James H
JH
Stewart
Bryan F
BF
Connors
Michael P
MP
Grant
John F
JF
McCarthy
D003428:Cross Infection / Q000453:epidemiology*; D006801:Humans; D007875:Legionella / Q000302:isolation & purification*; D007877:Legionnaires' Disease / Q000453:epidemiology*; D012308:Risk Management / Q000379:methods*; D012680:Sensitivity and Specificity; D014871:Water Microbiology