Water Services – Building Re-Occupation and Using Legionella Sampling & PCR Analysis.

What is the Issue?

Re-occupation of buildings following Covid-19 low occupancy or mothballing will need to involve validation Legionella sampling to demonstrate recommissioning measures have been effective.

The recommissioning process, involving cleansing flushing, system disinfection and validation sampling will potentially delay safe re-occupation of buildings.

How long will the validation sampling process take?

The standard for analysis of Legionella samples, involves a minimum 10 day laboratory incubation period. With the transport and administration time around the samples, this often means results are not available until 2 weeks after the sampling date.

High demand for laboratory services, when many buildings are attempting to safely re-occupy at the same time may cause the reporting times to increase further.

Is it possible to speed up the validation sampling process?

Yes and No.

A number of organisations, including the Legionella Control Association (LCA), have reminded the industry of the option that PCR analysis represents.

What is PCR analysis?

This is a rapid technique with results available to labs within 48 hours.

PCR is more specific than culture methods and will detect Viable but Not Culturable (VBNC) cells.

The analysis is significantly more expensive than standard Legionella culture.

Results are expressed in Genomic Units/Litre (GU/L), which cannot be directly compared against the HSE’s action thresholds, as these are based on Colony Forming Units/Litre (CFU/L).

If all PCR results are negative, then that will often be sufficient for validation purposes.

If there are any positive PCR results, then follow on actions can be difficult to justify until standard culture results are available – undermining the need to save time.

Therefore, carrying out standard Legionella culture, as well as PCR analysis, (i.e. double sampling each outlet) is recommended.


For any further advice, please contact:

Tom de Ronde

Principal Legionella Consultant / Water Team Leader

Quantum Compliance

M 07887 731912

E tom.deronde@qcompliance.co.uk