Experts explain false positive and false negative results
Zulfiqar Abbasi, an Irish national who was in Karachi for a brief visit, had his COVID-19 test done on February 23, for his flight back home. The lab at the Liaquat National Hospital gave him a positive diagnosis. Confused because he said he had no symptoms, he got another test done the next day from the Dr Essa Laboratory. This time it was negative.
Unfortunately, the airline had cancelled his flight as soon as Liaquat National shared the positive report with it.
Abbasi believes he did not contract the virus. He also said that he was ready to get tested elsewhere.
“My parents are elderly and have underlying conditions, if I suspected I had the disease I would take precautions.”
Interestingly, his result falls in the category of what is known as high positive, explained Dr Israr Nasir, who is the head of the molecular pathology department and assistant professor at Liaquat National.
High positive means the person has active infection. Whereas results that are low positive are possible in patients recovering from the virus.
Dr Farhan Essa Abdullah, clinical microbiologist and CEO of Essa lab, understands the confusion around testing all too well. “No test is 100%,” he says. But the PCR test is the gold standard for a COVID-19 diagnosis. It has the lowest margin of error compared to other tests.
So, who or what is to blame when a COVID-19 test result is incorrect?
As it turns out, one crucial aspect of testing is the sampling. This is very critical, according to Dr Israr Nasir of LNH. The technique of the person taking samples can often determine the difference between a negative and positive result, he adds.
“If it is superficial, you can miss the virus.”
It is also possible that the sample in particular did not contain virus particles, even if the person was affected.
“We have a checklist where we follow the SOPs set by the Sindh Healthcare Commission,” said Dr Nasir, adding that their method was very thorough.
Sometimes even after recovery, positivity persists in sampling for a PCR test. This could be due to dead virus particles.
“My opinion is that it’s a grey area,” says Dr Essa. “If it’s not possible to rule out whether it’s the live virus or not, the person is better off taking precautionary measures and isolating.”
He adds that no matter how careful health personnel are, there’s always the possibility of random human error.
“If someone is positive they shouldn’t get aggressive,” Dr Nasir, advising that the person avoids getting multiple tests.
Testing kits, which are imported, are costly and resources are limited.
Often a patient comes to get their test done during the last stage of infection. This could result in a false negative result even when the person can still shed the virus to infect others.
In such cases, two negative PCR tests can determine if the person should stop isolating, said Dr Nasir.
Since the PCR test works by detecting certain genes of the coronavirus, it is possible to miss mutations.
So if someone is infected with a mutated version of the virus, they might get a false negative result.
People coming into the country from different places can bring in different variants, Dr Essa said. “Genetic variant testing isn’t widespread right now. Only a few specialised labs in Pakistan are doing it and usually only travellers’ samples are tested for mutations.”
Note: LNH does repeat testing free of cost in cases it feels a mistake has been made.