Thanks for very useful information.prasannasimha wrote:Incidentally rapid antigen tests(Lateral Flow tests0 may be negative but RTPCR can be positive. It depends on viral load and test sensitivity. So one need not be surprised that RAT was negative one day and next day it was positive or RAT Negative RTPCR positive9I was RAT negative but RTPCR positive)
To add: The quality and sensitivity of RAT tests varies by manufactures but in general and especially for the better (more reputable) ones - see the following:
(For example Abbot's Bionax - which we have been using routinely for several months)
- The false positive rate is *extremely* low - close to zero. (Specificity > 99%). So if the test is done correctly, a positive result implies it is really positive.
- The sensitivity for better RAT's. is also *very* high. More importantly it measures 'contagiousness'. BinaxNOW's sensitivity was 98% ( Nasal swab; ....(Gold standard=PCR w/ Ct<30, ie, infectious) (See < this from recent UCSF actual data >
- This means - If you have symptoms (of a few days after the suspected exposure) the RAT is extremely useful - It is quick, takes 15 minutes and can be done by you at home.
(I have talked about this here in BRF. Many ,(especially doctors in India) put this sensitivity around 70%. But good quality RAT's, and process done correctly (Nose swab etc) will achieve >90 % sensitivity - if you are contagious. (This >90 sensitivity is for good quality RATS - don't know what is available in different part of the world. As said before, quality varies a lot, some have been found to be very un-reliable).
Our (extended) family has used this for last many months - hundreds of test results (along with PCR tests for verification) - In *all* cases there were zero false positives or false negatives. What is more - RAT *gave* the result a day *before* PCR - as PCR result - though taken at the same time - came out later. To be clear - PCR tests are more accurate - especially when viral load is low - but the results take longer time.
RAT tests take only 15 minutes while PCR turn-around in our case is about a day).
One more take away - Nasal swab for RAT (as recommended) is much better than Throat swab (which was found only about 50-60% reliable). I know in some circles - they are using both but it really doesn't improve the result -- apart from gross factor).
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Another helpful hint for those who are in USA. If you have access to computer with a camera and sealed/approved kit and you are registered etc - you can perform the test at home (for you or any other person) and can get an official vaccine certificate - all through internet within half an hour. (For International travel where PCR test is required, this might not be helpful but for domestic travel - or other places where RAT test is acceptable this is quite convenient).