COVID-19 (caused by SARS-CoV-2 infection) has been a notifiable disease
since 30th
of January 2020. As a notifiable disease, health
practitioners and laboratories are required to notify Medical Officers of
Health of cases of SARS-CoV-2 infection. It should be noted that this test
detects nucleic acid of SARS-CoV-2 and clinical correlation is required to make
decisions regarding virus viability and infectivity. Non-viable nucleic acid
may persist at low levels following recovery from acute infection.
The
COVID-19 pandemic continues to evolve. Consequently testing and management
guidelines require regular review to make sure they remain fit-for-purpose. ADHB
guidelines will continue to be updated with the most recent and relevant information;
make sure you are following the most up to date guide on the Hippo page (ADHB
intranet).
All respiratory virus PCR assays
undergo a number of amplification cycles to detect the viral nucleic acid
target of interest. Several (but not all) of the machines that test for
SARS-CoV-2 provide a cycle threshold (Ct) which indicates how many cycles it
has taken to detect the virus.
In general, the lower the Ct, the
higher the amount of viral DNA/RNA present in the specimen. Whilst the Ct value
is an indirect measure of viral load, it is influenced by a number of factors
including when in the illness the sample was taken, the quality of the specimen
and the site sampled. Positive results with low viral load (high Ct) can be
seen in the early or late stages of infection and this differs between
individuals, therefore the clinical significance of positive results with high
Ct values are difficult to interpret in the absence of clinical history and
context.
Auckland Hospital
laboratory (LabPLUS) VIM department applies a comment to samples where a low amount
of virus has been detected:
"
Low level SARS-CoV-2
detection (Ct over 30). This requires interpretation in clinical context. In
isolation, low level SARS-CoV-2 detection may reflect a low viral load present
in either very early infection
or in late infection (e.g. delayed presentation
or historic) or inadequate sampling. If acute infection is suspected consider
repeating testing after 24 hours if this is likely to change patient
management."
In absence of the above comment on
a
positive
SARS-CoV-2 result/report, it can be assumed that the Ct is 30
or less.
Where no comment has been
attached,
there
are
select patient groups
where the raw Ct may be useful, such as:
-
Immunosuppressed
patients such as those with cancer, auto-immune disorders or on
immunosuppressive therapy.
-
Patients who have undergone a
stem cell or solid organ transplant
-
Patients in
intensive care units with severe disease who may have prolonged shedding of
virus particles
These
select patients will require specialised input from Clinical
Microbiology/Infection Prevention and Control or Infectious Diseases Services
to interpret the raw Ct value and in some cases set parameters specific to the
individual patient.
For
monitoring progress with resolution of viral shedding, and hence a reduction in
infectiousness, a routine SARS-CoV-2 PCR (not a rapid PCR) can be performed
weekly.
In
general, a CT value greater than 30 on two sequential tests (48 hours apart) in
an asymptomatic, or minimally symptomatic, individual indicates low infectivity
and the patient is considered low risk for transmission to other patients and
healthcare workers.
If you
are taking care of a patient in the aforementioned select patient groups, and
require a Ct value then email your request for a raw Ct value to
virology@adhb.govt.nz
including the
patient
details (name and NHI), date the sample was taken and the clinical indication
for your request
.
Ct values are NOT provided
afterhours or on weekends or as a Rapid COVID-19 Test.
Further information about the
current COVID-19 response can be found at the NZ Ministry of Health website
See also RPAN Respiratory panel PCR
See also SARS-CoV-2 Rapid Test