RATs: serial testing improves reliability

Repeating the process over days lifts sensitivity, especially for catching asymptomatic covid.

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The reliability of rapid antigen tests at detecting covid improves significantly when symptomatic individuals test twice and asymptomatic individuals test three times at two-day internals, a US study has found. 

Despite becoming commonplace during the pandemic as a speedy alternative to waiting in line for a PCR test, the reliability of individual RATs at detecting covid infections, especially in those not showing symptoms, has been suspect. 

“Reports on [RAT] performance among individuals testing while they are asymptomatic have been highly varied, ranging from sensitivities of 35.8% to 71% in cross-sectional screening evaluations. However, performance has typically been evaluated based on the single use of [RATs], and few studies have evaluated serial testing performance of [RATs] among asymptomatic individuals,” authors of a recent study published in Annals of Internal Medicine said. 

In this prospective cohort study, participants were instructed via an app to self-administer two separate nasal swabs – one for a RAT and one for a PCR test – and record any symptoms every 48 hours over a period of 15 days. All nasal swabs for PCR tests were posted to a laboratory for analysis. RAT tests were done at home and either self-reported or read by an automated reader.  

Of the over 5350 eligible participants enrolled, 154 participants tested positive for covid during the study. Eligible participants ranged from two to 90 years old and had no covid infections in the prior three months, no symptoms in the preceding fortnight and were covid negative at enrolment. 

The study found that although sensitivity of a one-off RAT test for a symptomatic participant was relatively high at 83%, this more than halved for asymptomatic individuals. 

However, RAT sensitivity improved for all participant when they tested more than once. 

Testing twice two days apart increased sensitivity to almost 95% for symptomatic individuals. But this increase only reached 55% for asymptomatic individuals, or 63% if positive PCR results with negative results in the days either side, known as singleton positives, were removed from analysis.  

But all was not lost. Sensitivity in asymptomatic individuals improved to almost 70% (or 80% with singleton positives removed) when they were tested a third time after an additional two days. 

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The researchers were quick to highlight the nuances of the results. 

“These results should be considered in the context of our study protocol, which indicated testing at 48-hour intervals; thus, these data cannot support conclusions about serial testing for time intervals shorter than 48 hours,” they said. 

Despite the potential limitations of self-administered nasal swabs for testing, the authors noted that “data have consistently shown substantial agreement between self-collected and clinician-collected anterior nasal swabs for [covid] testing”. 

The authors emphasised the need for clear guidelines on RAT testing based on the data to ensure confidence in the sensitivity of RATs and to reduce the spread of disease. 

“The public health implications of our findings are that people testing for [covid] should exercise caution despite an initial negative result on [a RAT] and favour mask wearing and avoiding crowded places if they suspect they may be infected or have been exposed.”  

“In addition, the rates of false-positive results in the study were low; therefore, any [RAT]–positive result should be considered positive without the need to retest.” 

Annals of Internal Medicine 2023, online 4 July