Dear Allyson van de Pol,

Thank you for your email on 2.7.2021.

Please find our reply as below:

Q:Could you please advise if the cycle/ cross over threshold  currently
used is the same whether the individual taking the test is Covid
vaccinated or not?
A: Currently, the result reporting algorithm are the same for samples
collected from vaccinated and unvaccinated subject(s).
Please be noticed Ct/Cp cutoff is not applicable to the SARS-CoV-2
molecular testings in PHLSB.

Yours sincerely,
(CHAN Che-wa)
for Director of Health

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I am out of the office until 15/07/2021.

For urgent matters, please contact Miss Denise WONG at 3107 2570 / &DH/SCO
[Per]3 or Miss Jackie LAM, CO(Per)4 at 3107 2571 / &DH/CO[Per]4.

Note: This is an automated response to your message "Internal review of
Freedom of Information request - Cycle/ Cross Over Threshold for PCR Tests"
sent on 14.07.2021 04:53:55 PM.

This is the only notification you will receive while this person is away.

Dear Allyson van de Pol,

Thank you for your email on 14.7.2021.

Please find our reply as below:

The in-house SARS-CoV-2 RT-PCR assays  of Public Health Laboratory Service
branch at the Centre for Health Protection, Dept of Health have 40 or 45
PCR cycles.
Currently, crossing point, instead of threshold cycle, is adopted by PHSLB
for the SARS-CoV-2 in-house assays (Please notice that the derivation of
crossing point is different from that of threshold cycle).
Any positive PCR signal appears before PCR cycle 40 or 45 is considered as
a preliminary positive PCR result. Currently, neither Ct nor Cp cutoff is
being adopted by PHLSB.
All preliminary positive PCR results were/will be further confirmed by
another PCR test targeting specific genes of SARS-CoV 2.

Yours sincerely,
(CHAN Che-wa)
for Director of Health

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Dear Allyson van de Pol,

Thank you for your email on 25.7.2021 for seeking additional information.

Please find our reply as below:

Q1: Please advise how the discovery of genetic material of SARS CoV2
rather than live viruses during the RT-PCR test, relates to clinical
illness?
A1: All laboratory results should be interpreted by the clinicians with
epidemiological and clinical correlations.  
Q2: What is the difference between crossing point and threshold cycle?
A2: The crossing point (Cp) of a PCR reaction as the point where the
reaction’s fluorescence reaches the maximum of the second derivative of
the amplification curve, which corresponds to the point where the
acceleration of the fluorescence signal is at its maximum. The threshold
cycle (Ct) is defined as the number of cycles required for the fluorescent
signal to cross the threshold level.
Q3: Which specific genes of SARS-CoV2 are subsequently targeted in the
follow up PCR test. How is this carried out?
A3: SARS-CoV-2 RdRP and Spike genes are tested by RT-PCR in PHLSB.

Q4: Given that the RT-PCR test is to be withdrawn on 31st December 2021,
what test will be used in its place?
A4: The requested information is not under PHLSB's purview as PHLSB is not
using the CDC 2019-nCoV RT-PCR assay.  

Yours sincerely,
(CHAN Che-wa)
for Director of Health

From:        Francis YM CHING/DH/HKSARG
To:        Raymond CW CHAN/DH/HKSARG@DH
Cc:        Sophy KK KWOK/DH/HKSARG@DH
Date:        26/07/2021 12:40
Subject:         *Restricted: Fw: Internal review of Freedom of
Information request - Cycle/ Cross Over Threshold for PCR Tests

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Dear Department of Health,

Please advise how the discovery of genetic material of SARS coV2 rather
than live viruses during the RT-PCR test, relates to clinical illness?

What is the difference between crossing point and threshold cycle?

Which specific genes of SARS-CoV2 are subsequently targeted in the follow
up PCR test. How is this carried out?

Given that the RT-PCR test is to be  withdrawn on 31st December 2021, what
test will be used in its place?

Yours faithfully,

Allyson van de Pol

show quoted sections

Dear Allyson van de Pol,

Thank you for your email on 29.7.2021. Please find our reply as below:

Pooling of samples has been used in Public Health Laboratory Services
Branch (PHLSB).

Yours sincerely,
(LUEY Kit-yee)
for Director of Health

From:        Allyson van de Pol <[email address]>
To:        FOI requests at Department of Health <[Department of Health request email]>
Date:        29/07/2021 18:50
Subject:        Freedom of Information request - RT-PCR Testing

--------------------------------------------------------------------------

Dear Department of Health,

Can you please advise whether pool testing is used when testing samples in
the RT-PCR test for Covid 19 now or at anytime over the last 18 months in
Hong Kong. Pool testing is when a number of samples are 'pooled together'
and any positive result is applied to all samples.

Yours faithfully,

Allyson van de Pol

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Dear Allyson van de Pol,

Your application for access to information dated 29.7.2021 and seeking
further information email dated 6.8.2021 were received.

Pooled sample testing is performed for selected samples only.  PHLSB does
not capture any statistics on percentage of pooled sample testing.

 Yours sincerely,
(LUEY Kit-yee)
for Director of Health

From:        Allyson van de Pol <[FOI #692 email]>
To:        FOI requests at Department of Health <[Department of Health request email]>
Date:        06/08/2021 13:39
Subject:        Re: *Restricted: Fw: Freedom of Information request -
RT-PCR Testing

--------------------------------------------------------------------------

Dear Department of Health,

Thank you very much for your reply.

Is Pool Testing used  for all samples (ie standard practice) or just
occasionally? If occasionally , can you please supply an approximate
percentage number how often pool testing is used?

Many thanks,

Yours faithfully,

Allyson van de Pol

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Dear Allyson van de Pol,

Thank you for your follow up question on 12/8/2021.

In PHLSB, sample pooling was performed on specimens for SARS-CoV-2
screening and surveillance purpose.

Yours sincerely,
(LUEY Kit-yee)
for Director of Health

From:        Allyson van de Pol <[FOI #692 email]>
To:        FOI requests at Department of Health <[Department of Health request email]>
Date:        12/08/2021 13:04
Subject:        Re: *Restricted: Fw: Freedom of Information request -
RT-PCR Testing

--------------------------------------------------------------------------

Dear Department of Health,

Many thanks for your reply.

May I then ask under what circumstances pool testing occurs? Under certain
circumstances or randomly? Why are certain selected samples chosen and not
others?

Many thanks

Yours faithfully,

Allyson van de Pol

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