Is a Positive Truly a Positive?
Today, a new article came out in 𝗡𝗬𝗧 on whether the 𝗖𝗼𝘃𝗶𝗱-𝟭𝟵 𝗣𝗖𝗥 𝘁𝗲𝘀𝘁 𝗶𝘀 𝘁𝗼𝗼 𝘀𝗲𝗻𝘀𝗶𝘁𝗶𝘃𝗲 𝗮𝗻𝗱 𝗶𝗳 𝘆𝗼𝘂𝗿 𝗽𝗼𝘀𝗶𝘁𝗶𝘃𝗲 𝘁𝗲𝘀𝘁 𝗶𝘀 𝗶𝗻𝗱𝗲𝗲𝗱 𝗮 𝗽𝗼𝘀𝗶𝘁𝗶𝘃𝗲 𝗼𝗿 𝗻𝗼𝘁.
𝗣𝗖𝗥 𝘁𝗲𝘀𝘁𝘀 𝗮𝗺𝗽𝗹𝗶𝗳𝗶𝗲𝘀 𝗴𝗲𝗻𝗲𝘁𝗶𝗰 𝗺𝗮𝘁𝗲𝗿𝗶𝗮𝗹 𝗼𝗳 𝘁𝗵𝗲 𝘃𝗶𝗿𝘂𝘀. What this means is that imagine you are using your camera to take a close-up picture of something and you use the zoom function to take a picture with higher magnification. In PCR tests, the amplification is done to make the signal of the virus be brighter so it can be detected. This amplification is done in cycles. If you need more cycles to amplify, it means your viral load in the sample was low. If less amplification cycles are required, then it means there was a higher load of virus in the sample. 𝗧𝗵𝗲 𝗻𝘂𝗺𝗯𝗲𝗿 𝗼𝗳 𝗮𝗺𝗽𝗹𝗶𝗳𝗶𝗰𝗮𝘁𝗶𝗼𝗻 𝗰𝘆𝗰𝗹𝗲𝘀 𝗿𝗲𝗾𝘂𝗶𝗿𝗲𝗱 𝘁𝗼 𝗳𝗶𝗻𝗱 𝘁𝗵𝗲 𝘃𝗶𝗿𝘂𝘀 𝗶𝘀 𝗸𝗻𝗼𝘄𝗻 𝗮𝘀 𝗰𝘆𝗰𝗹𝗲 𝘁𝗵𝗿𝗲𝘀𝗵𝗼𝗹𝗱, 𝗼𝗿 𝗰𝗼𝗺𝗺𝗼𝗻𝗹𝘆 𝗿𝗲𝗳𝗲𝗿𝗿𝗲𝗱 𝗶𝗻 𝘀𝗵𝗼𝗿𝘁 𝗮𝘀 𝗖𝗧.
There is debate on what should the threshold for cut-off should be. Currently it is set at 40 cycles. Some believe it should be lower, between 30 and 40. 𝗜𝗺𝗽𝗼𝗿𝘁𝗮𝗻𝘁𝗹𝘆, 𝘁𝗵𝗲𝗿𝗲 𝗲𝘅𝗶𝘀𝘁𝘀 𝗻𝗼 𝗰𝗹𝗲𝗮𝗿 𝗰𝗼𝗻𝘀𝗲𝗻𝘀𝘂𝘀.
We already know that 𝗣𝗖𝗥 𝘁𝗲𝘀𝘁𝘀 𝗱𝗼𝗻’𝘁 𝘁𝗲𝗹𝗹 𝘂𝘀 𝘄𝗵𝗲𝘁𝗵𝗲𝗿 𝘁𝗵𝗲 𝘃𝗶𝗿𝘂𝘀 𝗶𝘀 𝗰𝘂𝗿𝗿𝗲𝗻𝘁𝗹𝘆 𝗶𝗻𝗳𝗲𝗰𝘁𝗶𝗼𝘂𝘀 𝗼𝗿 𝗶𝗳 𝗶𝘁 𝘀𝗶𝗺𝗽𝗹𝘆 𝗹𝗲𝗳𝘁𝗼𝘃𝗲𝗿 𝘃𝗶𝗿𝗮𝗹 𝗱𝗲𝗯𝗿𝗶𝘀. This is a known flaw of the test. Theoretically, it could be picking up virus from a Covid-19 infection in the recent past vs being currently infectious. 𝗪𝗲 𝗵𝗮𝘃𝗲 𝘀𝗲𝗲𝗻 𝗳𝗿𝗼𝗺 𝗱𝗮𝘁𝗮 𝘁𝗵𝗮𝘁 𝘄𝗶𝘁𝗵 𝗺𝗶𝗹𝗱 𝘁𝗼 𝗺𝗼𝗱𝗲𝗿𝗮𝘁𝗲 𝗱𝗶𝘀𝗲𝗮𝘀𝗲, 𝗴𝗲𝗻𝗲𝗿𝗮𝗹𝗹𝘆 𝗽𝗲𝗼𝗽𝗹𝗲 𝗮𝗿𝗲 𝗻𝗼𝘁 𝗶𝗻𝗳𝗲𝗰𝘁𝗶𝗼𝘂𝘀 𝗮𝗳𝘁𝗲𝗿 𝟭𝟬 𝗱𝗮𝘆𝘀 𝗼𝗳 𝘀𝘆𝗺𝗽𝘁𝗼𝗺 𝗼𝗻𝘀𝗲𝘁 (𝟮𝟬 𝗱𝗮𝘆𝘀 𝗳𝗼𝗿 𝘀𝗲𝘃𝗲𝗿𝗲 𝗱𝗶𝘀𝗲𝗮𝘀𝗲).
Lowering the cycle threshold means that those with lower viral loads will not show up as positive. This could potentially help weed out those with leftover viral debris, vs those who are currently infectious.
𝙃𝙤𝙬𝙚𝙫𝙚𝙧, 𝙩𝙝𝙚𝙧𝙚 𝙖𝙧𝙚 𝙨𝙚𝙫𝙚𝙧𝙖𝙡 𝙞𝙢𝙥𝙤𝙧𝙩𝙖𝙣𝙩 𝙘𝙤𝙣𝙨𝙞𝙙𝙚𝙧𝙖𝙩𝙞𝙤𝙣𝙨:
– 𝗩𝗶𝗿𝗮𝗹 𝗹𝗼𝗮𝗱𝘀 𝗮𝘁 𝘁𝗵𝗲 𝘀𝘁𝗮𝗿𝘁 𝗼𝗳 𝗶𝗻𝗳𝗲𝗰𝘁𝗶𝗼𝗻 𝗮𝗿𝗲 𝘁𝘆𝗽𝗶𝗰𝗮𝗹𝗹𝘆 𝗹𝗼𝘄𝗲𝗿. If we have a lower cut-off threshold, we could potentially be missing those with a new infection. Some experts argue that a repeat rapid test should be done in these individuals within 24hrs; however, testing is still inaccessible in most of the US.
– 𝗚𝗶𝘃𝗲𝘀 𝗺𝗼𝗿𝗲 𝗳𝘂𝗲𝗹 𝘁𝗼 𝘁𝗵𝗲 ‘𝘄𝗲 𝗮𝗿𝗲 𝘁𝗲𝘀𝘁𝗶𝗻𝗴 𝘁𝗼𝗼 𝗺𝘂𝗰𝗵’ 𝗿𝗵𝗲𝘁𝗼𝗿𝗶𝗰 (as we’ve seen with new CDC testing guidelines). This will be used in arsenal to further reduce/limit testing — which is not what we need to be doing right now. While the article itself does not suggest this, and in fact, states the opposite — that more frequent and quick turnaround testing is require — this message will be lost and infact, misappropriated.
– Determining whether you are currently infectious vs previously infected does have important sequalae especially for those with 𝗶𝗻𝗳𝗹𝗲𝘅𝗶𝗯𝗹𝗲 𝘄𝗼𝗿𝗸 𝘀𝗰𝗵𝗲𝗱𝘂𝗹𝗲𝘀 𝗼𝗿 𝗹𝗶𝗺𝗶𝘁𝗲𝗱 𝘀𝗶𝗰𝗸 𝗹𝗲𝗮𝘃𝗲.
– This also stresses the urgency for the uptake of 𝗿𝗮𝗽𝗶𝗱 𝘁𝗲𝘀𝘁𝗶𝗻𝗴 𝗮𝗻𝗱 𝗮𝗻𝘁𝗶𝗴𝗲𝗻 𝘁𝗲𝘀𝘁𝗶𝗻𝗴.
***𝗨𝗹𝘁𝗶𝗺𝗮𝘁𝗲𝗹𝘆; 𝗵𝗼𝘄𝗲𝘃𝗲𝗿, 𝗯𝗲𝗶𝗻𝗴 𝗼𝘃𝗲𝗿-𝗰𝗮𝘂𝘁𝗶𝗼𝘂𝘀 𝘃𝘀 𝘂𝗻𝗱𝗲𝗿-𝗿𝗲𝗽𝗼𝗿𝘁𝗶𝗻𝗴 𝗶𝘀 𝗮𝗹𝘄𝗮𝘆𝘀 𝘁𝗵𝗲 𝗽𝗿𝗲𝗳𝗲𝗿𝗿𝗲𝗱 𝗼𝗽𝘁𝗶𝗼𝗻.***