RE: ON THE RAPID ANTIGEN TEST AT KOTOKA AIRPORT: THE CASE OF FALSE POSITIVES.

DR. OKOE BOYE 

MEMBER, GHANA'S COVID-19 TASK FORCE 

JULY 31,2021.

A PCR test is usually considered as the gold standard for identifying the SARS COV 2 VIRUS( COVID VIRUS). It is so because it actually isolates the genetic material of the virus and magnifies it to indicate not just the presence of the virus but with the capacity to produce the genomic sequence or map of the virus as well. All these features notwithstanding, a number of factors can result in a false negative PCR test. False negative meaning you get a negative result when actually you should have had a positive (as in person is carrying the virus). 

1. If the one taking the sample goes for a throat swab instead of a nasal( pharyngeal) swab, the presence of the virus could be compromised by throat content. Such a sample can end up giving a negative result when really the person is positive. 

2. The sample collected after being treated must be subjected to the process of genetic material extraction; here too the quality of skill and limitedness of finesse of technician can result in a false negative test. 

3. Very low viral load can result in sparse genetic material isolation and consequently result in a negative test. 

IT IS IMPORTANT TO NOTE THAT IN ADDITION TO THESE LIMITING FACTORS, THE SCIENTIFIC LITERATURE REMINDS US THAT IN EARLY STAGES OF COVID INFECTION WHEN THE VIRAL LOAD IS LOW, FALSE NEGATIVE RATE IN PCR TESTING CAN BE AS HIGH AS 100%.It is for these reasons that some jurisdictions apply the use of the rapid antigen tests. 

It is also IMPORTANT to state that NOT ALL ANTIGEN TESTS COME WITH THE SAME TECHNOLOGY AND FOR THAT MATTER SENSITIVITY. some are simple lateral flow tests like the one involving use of cartridges which people carry in their bags. Such tests depend on a chemical reaction between a component in the virus and a substance in reagent used. These simple lateral flow test have sensitivities that can be as low as 20%. Other good brands can have average 65 to 70%. This means when you have 100 cases of COVID-19, they can pick about 20 out of the 100 or at best 70 of the cases. It is the simplicity in the technology of these cartridge, pregnancy kit like, rapid tests that account for their cheap price and low sensitivities. 

The antigen test at the KOTOKA INTERNATIONAL AIRPORT WAS BENCHMARKED WITH PCR TESTING FROM NOGUCHI. When 100 samples of positive cases from Noguchi were put in the KIA antigen testing ,the sensitivity was 99% with both giving essentially same results. This wasn't surprising because the antigen test at Ghana's airport is not a simple lateral flow test, it is actually A FLUORESCENT IMMUNOASSEY TECHNOLOGY BASED TEST with sensitivity and specificity in the 90s. The technology uses a software that is backed up and upgraded periodically to pick up emerging variants and trends. The technology and set up is periodically subjected to quality control by Ghana's FDA to ensure that the high sensitivity and specificity are sustained at all times. 

The various variants including Delta that have been isolated from travelers at KIA were picked up by the testing at our airport. Ghana's protocol allows for positive samples to be sent to Noguchi for sequencing and by extension confirmation. However, travelers who test positive must isolate compulsory as these sequencing are done. The antigen technology at the airport was chosen over PCR because it gives you reliable results as we get in the PCR but over a shorter time, about 15 to 30 minutes. Our protocol doesn't allow for a second opinion instantly at the airport from same testing or PCR because that will open the floodgates for challenges that can complicate the arrangement for isolation, increase testing time, create self imposed credibility issues with our setup as well as trigger subjective application of our protocols instead of laid down objective non discriminatory protocols. 

No testing regime in the world is 100% or absolute in its results. Infact the same sample from two different PCR labs can come out with different results. A negative or positive result only speak to probability of not carrying the virus or having it in your system. We don't have to fight over results, neither do we have to change protocols because someone tested positive. What we need to do is to ensure that Quality Control Checks from Accredited State Organizations like Ghana's FDA and Ghana Standards Authority are done regularly through the use of benchmarking with PCR testing and other relevant quality appraisal tools.

Over the past 12 months , the testing regime at KIA has recorded positives that reflect the waves that have been sweeping the globe. It has served us well, a claim that speaks to its reliability and not perfection. By the way, the title of the article is misleading. The fact that a travellor claims they doubt a test result doesn't make the result a false positive. You only say so categorically when same sample or another from the same travellor comes out positive from credible diagnostic tools. Some of the cases picked up at KIA ended up being sick requiring hospitalization and some ICU care. If you are in a haste, you mind classify these travellors as false positives when infact they were future candidates for ICU care. Scientists don't speak in categorical terms nor rule out sovereign interventions. Scientists ask questions and admit they could be wrong in their speculations.