You all might have come across the term CT VALUE (cycle threshold) present in RT-PCR reports in COVID Reports.
Let us understand what is this CT-VALUE and does this co-relate with disease spreading and disease severity!
We will understand the following points related to CT value-

  1. brief working of RT-PCR test
  2. relation of CT VALUE with viral load
  3. relation of CT VALUE with disease severity


RT-PCR (reverse transcripts polymerase chain reaction) test is done for diagnois of multiple diseases such as COVID-19, Dengue and other viral diseases and even some Cancers.
In RT-PCR, the test sample undergoes multiple cycles to detect the virus. With each cycle, virus is detected.
CT VALUE (CYCLE THRESHOLD) indicates the number of cycles run for a sample till the virus was detected in that sample.
When more virus is present in a test sample, less cycles are required for detection.
And when less virus is present, more cycles are required. 
Hence, the samples with less virus, will have a HIGHER CT VALUE, and the sample with more virus will have a LOWER CT VALUE.


One of the COVID testing methods includes RT-PCR testing. Sample taken from nose or mouth of the patient is run into the RT-PCR machine. The end results are shown in the form of CT VALUE. this ranges till 30 or 35. As discussed earlier, it indicates the virus particles present in the sample.
In the research studies conducted, it was found that CT VALUE co-related strongly with the cultivable virus present in the samples of COVID-19.
During the disease course of COVID-19, symptoms usually peak on the 3rd or 4th day of getting the virus and then the symptoms severity decrease as time passes by (in most cases). This is in co-relation with the virus present in the throat of an individual.
During the course of disease, highest number of virus are present in throat swab samples around the time when the patient had symptoms.
CT VALUE was found to be lowest during that same time.
Also keep in mind, more virus in the throat sample means that patient can spread more virus in its surrounding.
As time progressed, the viral load in throat decreased.
The probability of cultivating virus was higher in week 1 (76%) than week 2 (20%).
Also, probability of cultivating virus dropped to 6%, 10 days after onset of symptoms.
As a result as days pass by there is an increase in CT VALUE in RT-PCR REPORTS.
The probability of cultivating virus dropped to 8% when CT VALUE was more then 35.
What is CT VALUE in RT-PCR?  Understanding the COVID RT-PCR report.


Although, CT-VLAUE has strong relations with the virus present in the sample, weather it will co-relate with disease can not be said with surity.
This is because the disease severity depends highly on many factors such as-
  • the immunity status,
  • age
  • co-morbidities of an individual
  • nutritional status
In a study conducted by Jesse FAJNZYLBER, found that higher viral load is associated with lower lymphocyte (WBC) count in blood and increased marker of imflamation such as CRP and IL-6
In another study conducted by – I.F.N. Hung and team, found an interesting co-relation that stool viral loads was associated with diarhea, hepatic dysfunction. and urine viral load was also associated with diarrhea.
In a study to understand the relation between CT-VALUES and COVID-19 more than 300 sample subjects were included. It was found that
the average CT VALUE in week 2 was 30.65
after 2 weeks CT VALUE was 31.60.
The above study, included asymptomatic subjects, and patients with mild, moderate and even severe disease severity
And there was not much difference in the CT-VALUE findings of these subjects. Hence it followed the same fashion irrespective of the disease severity.
Hence Ct VALUE may or may not relate with disease severity of COVID-19.

That’s all for this topic !!

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  1. Duration of infectiousness and correlation with RT-PCR cycle threshold values in cases of COVID-19 by Anika Singanayagam and others on , England, January to May 2020.
  2. Pitfalls in interpretation of CT-values of RT-PCR in children with acute respiratory tract infections by Jérôme O.Wishaupta and Nico G.Hartwiggh
  3. Duration of infectiousness and correlation with RT-PCR cycle threshold values in cases of COVID-19, England, January to May 2020 separator commenting unavailable by Anika Singanayagam1,2 and others
  4. CDC (Centre for disease control)
  5. SARS-CoV-2 viral load is associated with increased disease severity and mortality by Jesse FAJNZYLBER, and others in The Massachusetts Consortium for Pathogen Readiness, Nature Communications volume 11.
  6. Viral Loads in Clinical Specimens and SARS Manifestations by I.F.N. Hung and others
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