“Well in 30yrs of being a nurse, never seen anything quite like this. Pooling of tests. 5 different people's swabs sent off in one vial. So one positive will contaminate the other 4, giving 5 positives. What the hell is going on? Total madness #CovidUK #covidtest”
Posts on Facebook and Twitter have claimed that the existence of grouped swab tests mean that negative Covid-19 test samples are being contaminated with positive ones, and people are therefore being told they have tested positive when they haven’t.
This is not the case. Grouping swabs for testing is a standard method to save time and testing resources. It’s called sample pooling.
In general this involves different people’s samples being tested together—if the group tests positive, all the individuals can be tested again separately. If the group tests negative, there is no need to test each one individually. This could be useful in scenarios like student households, where a positive test for one person in a defined group would mean they all needed to self-isolate.
But this is not a major part of the UK’s testing strategy yet. At least two studies of sample pooling are being done in England with rapid PCR testing (which tests to see if the person currently has the SARS-CoV-2 virus, which causes Covid-19). One is a pilot study, and another is testing asymptomatic people. These studies will be reviewed by the government’s Technical Validation Group, which includes representatives from Public Health England and the Medicines and Healthcare products Regulatory Agency before being incorporated into the national testing strategy.
The purpose of pooled testing is to increase testing capacity at times when the prevalence of Covid-19 is low.
The Facebook post’s attached screenshot appears to be from a government website for registering Covid-19 swabs, and gives the options of “single swab tests” and “group swab tests” described as “More than one person’s swab is put into the same vial. You can put up to 5 swabs into one vial.”
There is no reason to believe this screenshot isn’t genuine, but pooled testing is not standard practise for Covid-19 PCR tests done in the UK.