129
WaterManagement Teamsmust decide the validation criteria that are
relevant and practical for their facilities. Testing for
Legionella
and other
potentially pathogenic bacteria is useful for validation that hazard control
has been effective. But the limitations of themethodsmust be considered.
The accuracy and precision of these methods have been thoroughly
discussed (McCoy2012).
Table8.Interpretingresultsfrom
Legionella
PolymeraseChainReaction
(PCR) and
Legionella
tests of building water samples that were cultured
immediately after sampling (“TimeZeroCulture”).
PCR
Time Zero
Culture
Interpretation
OfResults
Occurrence
ofResults (ref.McCoy2012)
Negative Negative Themost definitive evidence that
the sample contains no detectable
Legionella
Overall, about 9 in10 negative PCR results
were also culture-negative. Therefore, PCR
resultswere a useful negative screen
especially for potablewater samples
(negative predictive valuewas >90%).
Positive
Negative
The sample contains non-viable
“dead”or injured (viablebut are
not culturable, VBNC
Legionella
) that if not
continuously controlledmay
result in the appearance of viable
potentiallydangerous
Legionella
in the buildingwater system.
Results indicate that hazard
control is inactivating a
significant quantityof
Legionella
Overall, about 40% of samples had killed
or injured
Legionella
present in the sample
that were not viablewhen the culturewas
inoculated in the field immediately after
removal from the buildingwater system at
“time zero”.
This categoryof results canbe veryuseful
information for water treatment
professionals.
Negative
Positive
PCR result not valid. PCR
reactionswere inhibited.
Results not interpretable.
InvalidPCR results occurred in about 1 of
10 potablewater samples and in about 3 of
10 coolingwater samples. Overall, about
3.6% of PCR resultswere invalid.
Table8
