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Water Management Teams must 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 the methods must be considered.
The accuracy and precision of these methods have been thoroughly
discussed (McCoy 2012).
Table 8. Interpreting results from
Legionella
Polymerase ChainReaction
(PCR) and
Legionella
tests of building water samples that were cultured
immediately after sampling (“Time Zero Culture”).
PCR
Time Zero
Culture
Interpretation
Of Results
Occurrence
of Results (ref. McCoy 2012)
Negative Negative The most definitive evidence that
the sample contains no detectable
Legionella
Overall, about 9 in 10 negative PCR results
were also culture-negative. Therefore, PCR
results were a useful negative screen
especially for potable water samples
(negative predictive value was >90%).
Positive
Negative
The sample contains non-viable
“dead” or injured (viable but are
not culturable, VBNC
Legionella
) that if not
continuously controlled may
result in the appearance of viable
potentially dangerous
Legionella
in the building water system.
Results indicate that hazard
control is inactivating a
significant quantity of
Legionella
Overall, about 40% of samples had killed
or injured
Legionella
present in the sample
that were not viable when the culture was
inoculated in the field immediately after
removal from the building water system at
“time zero”.
This category of results can be very useful
information for water treatment
professionals.
Negative
Positive
PCR result not valid. PCR
reactions were inhibited.
Results not interpretable.
Invalid PCR results occurred in about 1 of
10 potable water samples and in about 3 of
10 cooling water samples. Overall, about
3.6% of PCR results were invalid.
Table 8
