Understanding CT value and repeating PCR test, is necessary?
Currently, while treating patients for COVID-19 at Danphe Care, My team and I get asked one question almost all the time, "Doctor, my report says my CT value is 'this number'. What does it signify? Well to understand CT value let’s understand the concept of it.
CT value in your swab sample report refers to Cycle Threshold. Now as you all know, the Threshold implies the upper limit to the given test. Consider a simple example; if you cross the threshold of filling a glass with water, the water spills out. Now, with context, the RT-PCR (Reverse Transcriptase Polymerase Chain Reaction) has a threshold that is coded in the machine from the collection and analysis of data to the specific microbe. It does a significant amount of multiplication of the microbial genome (DNA or RNA); understandable as cycles and if that number crosses the threshold of the machine, you get a positive result.
For COVID-19 how can you understand this concept?
So consider two patients, Patient A and Patient B. Let’s say Patient A has a viral load of 10 and Patient B has a viral load of 1 virus at the time of collection of the swab. Now consider that the threshold of the PCR is 100 for detection of a positive result for COVID. The PCR does 40 cycles in total for the detection of the virus and in each cycle, it tends to double the amount of viral RNA (the genetic material). So, for Patient A to Cross, the threshold and have a positive result (i.e. >100) it would take 3 cycles (i.e. in the 1st cycle it would be from 10 to 20, in the 2nd 20 to 40, and in the third 40 to 160). Now for Patient B, it would take 5 cycles (i.e. in the 1st 1 to 2, in the second cycle 2 to 4, in the third 4 to 16, then 16 to 64 in the fourth, and finally 64 to 128 in the 5th). So we can say that the patient with the lower viral count needs a higher number of cycles to get detected and that the patient with the higher viral count needs a lower number to get detected (i.e it gets detected earlier). Simply put, the CT value in your report, the closer it is to 40 the lower your viral load. That is the interpretation of the report.
What it DOES NOT say or PREDICT is if you would develop severe COVID. This can be only known via your symptom progression. Have a read at my previous blog on Home Isolation Monitoring in COVID-19 to know what all symptoms I am talking about. For instance, if the CT value of your swab test done today is 30, it implies that at the time of the swab collection you had a low viral load, however, the viral load can tend to increase as the days pass on and if you do develop an increase in your symptoms in the following days of isolation and were to do a PCR again, you would likely find the number to drop (i.e. your viral load has started to rise). However you need not panic, it is not only the viral load that increases, your immune cells increase as well! So the only way to understand your clinical state and the viral progression is to monitor your symptom progression. We have had patients who had CT values of 18/19 have mild symptoms throughout their isolation whereas those with a CT value of 28/29 were also found to develop COVID Pneumonia. So, you should not just relax on the value given in your swab test. Even if it were not given, i.e. if you had just been given a report stating you are COVID positive, that would be sufficient. The CT value guides us physicians to anticipate any probable course of the disease if it looked to see increasing via symptoms with supportive investigations. Your treatment will be changed promptly if your symptoms tend to increase. Say you have a CT value of 30 today. You start to develop a gradual increase in fever, not controlled by medications with fluctuation in oxygen level to maybe 91-92% in Room air, will we as Doctors keep our hands raised and not treat you because your CT value is 30? DEFINITELY NOT! You will be categorized immediately as Severe COVID and treatment changed promptly.
What the PCR machine also does not say is whether the virus you have within you is active or dead. Remember the PCR machine detects the genetic material of the virus, this can also be detected of a deadly virus! Here's an example I usually like to give to my patients: I'm sure you all just love crime thriller series, maybe you watch it on Netflix or Youtube or your TV. Imagine an episode where the detectives come across a crime scene with a dead victim. The team collects the sample of the area and sends it to the lab for identification. The lab then confirms the presence of 2 DNA! One of the victim and other of the killer, but what the lab does not say is whether the person whose DNA is detected are alive or dead or maybe in captivity. The state of the victim and the criminal can only be assessed by the detectives. Similarly, your PCR machine detects viral genetic makeup, which can be of active or dead virus. Now how do I know if the virus inside my body is Active or Dead? Well again, it comes down to your symptoms. From your day of symptom onset and diagnosis till your isolation period, viral activity is suggested by your ongoing symptoms.
When you get diagnosed for the first time via RT-PCR, regardless of your CT value you need to be in isolation for a minimum of 10 days. During this period, if your symptoms start to subside and you start to feel clinically better, with having a fever-free period for >72hours (3 days), the last dose of Anti-pyretic(fever medication) for >72 hours and if there was an indication for antibiotic use (due to underlying secondary infection) the last dose should have been >72 hours, then you can break your isolation. If you have ongoing fever for 8 days, you need to add 3 more days to your total isolation (i.e 11 days and break from 12th and so on). However, please be in contact with your physician during the isolation period so that there is no confusion regarding this matter. If you have been improving clinically the virus inside your body will be neutralized to a limit where it will not be able to infect others even if it leaves your body. This happens by the 9th day (Hence the 10 days isolation!). So you need to be monitoring yourself closely for these 10 days. Some patients may experience the return of fever after a fever-free period of >72 hours, if this is the case you need to check back with your doctor immediately. If the disease is seen to progress into severity it is also generally seen to do so in the second week i.e after 7 days, usually between 7-10 days. Therefore SYMPTOMS MONITORING IS VERY CRUCIAL.
Now since the virus gets neutralized by the 9th day and the rate of virus elimination is different from person to person, if you repeat your PCR it might again come positive! This time we say that the detected viral particles have been neutralized tracing back your day of symptom onset and first PCR report. Therefore it is not advised for any patient to repeat their PCR following 10 days of isolation and symptomatic improvement because it can come positive even till 90 days (has been shown by research published in The Lancet as well; https://doi.org/10.1016/S2666-5247(20)30172-5). Repeat PCR testing has been discouraged by the WHO and even NMC (Nepal Medical Council).
So, in conclusion, I would like to say to my readers to not develop a headache and waste their time stressing on CT values and to have to get a Negative printed in your PCR. When infected be in touch with a physician and be vigilant in monitoring your symptoms. Like Gandalf's battle with the Balrog of Morgoth, you say to the virus "YOU SHALL NOT PASS".
-Dr. Suyash Timalsina, Medical Coordinator, Danphe Care (@drsuyasht.wordpress.com)