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Understanding the distribution of positive Legionella samples in healthcare-premise water systems: Using statistical analysis to determine a distribution for Legionella and to support sample size recommendations

Published online by Cambridge University Press:  08 October 2020

Dylan J. Nagy ,
David M. Dziewulski Open the ORCID record for David M. Dziewulski [Opens in a new window] ,
Neculai Codru  and
Ursula L. Lauper
Dylan J. Nagy
Affiliation:
New York State Department of Health, Bureau of Water Supply Protection, Albany, New York School of Public Health, University at Albany, SUNY, Rensselaer, New York
David M. Dziewulski*
Affiliation:
New York State Department of Health, Bureau of Water Supply Protection, Albany, New York School of Public Health, University at Albany, SUNY, Rensselaer, New York
Neculai Codru
Affiliation:
New York State Department of Health, Bureau of Water Supply Protection, Albany, New York
Ursula L. Lauper
Affiliation:
New York State Department of Health, Bureau of Water Supply Protection, Albany, New York
*
Author for correspondence: David M. Dziewulski, E-mail: [email protected]
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Abstract

Objective:

To significantly fit a statistical distribution to the proportion of positive Legionella samples in a series of water samples from multiple facility-premise water systems.

Design:

Statistical fit test.

Setting:

A hospital and associated long-term care facility (LTCF) in New York State, as well as temporal and culture data from a deidentified hospital site supplied by one of the vendor laboratories.

Methods:

Culture samples (n = 1,393) were segmented into 139 test cycles with roughly 10 samples in each. The proportion of positive samples was standardized to 25 total samples per test to give a distribution of discrete values. These values were analyzed for fit with the following discrete distributions: Poisson, negative binomial, geometric, and zero-inflated Poisson.

Results:

The zero-inflated Poisson distribution fitted to the copper–silver ionization (CSI)-treated and untreated test cycles indicates that 88% of the expected positive proportions should occur by the 30% cutoff (rounded up to 8 positive samples among 25 total samples), similar to the 93% expectation for just CSI-treated test cycles. The other treatment in these data (chlorine dioxide) was not effective in treating Legionella in the sampled buildings, and if there is an underlying distribution to these specific test cycles, it is not the zero-inflated Poisson distribution.

Conclusions:

In a well-maintained or well-treated premise water distribution system, ~30% or lower proportion of positive Legionella samples should occur. Anything above that cutoff is either very unlikely or not expected at all and indicates a problem in the water system.

Type
Original Article
Information
Infection Control & Hospital Epidemiology , Volume 42 , Issue 1 , January 2021 , pp. 63 - 68
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Copyright
© 2020 by The Society for Healthcare Epidemiology of America. All rights reserved.

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