Pre-symptoms virus tests prove nothing, MD says
Online and in the street, many people are saying that everyone should be tested for Covid-19 infection and it should be free, but there’s a caveat to consider: If the test isn’t done correctly, a “negative” might mean only “no infection found”, as opposed to “not infected”.
It could even be a false negative, masking the dire truth.
In a talk this week, Dr Opas Karnkawinpong, head of the Health Ministry’s Department of Medical Sciences, said there are two ways to detect Covid-19 infection:
• A real-time reverse transcription polymerase chain reaction (RT-PCR) test, which looks for the virus in the cells of mucus from the throat or tissue from behind the nasal cavity. If infection might be present in the lungs, sputum from the lungs is examined. Care must be taken to prevent environmental contamination, so the test has to be done in a proper laboratory. It takes 2.5-3 hours to get a result and costs Bt2,500.
• The “rapid test”, which assesses immunity after the immune system has begun fighting the virus. it takes 15-30 minutes. Imported test kits cost Bt500, but the department is developing its own, which will cost Bt200.
Opas said laboratory testing is beneficial in determining whether there is infection, in helping monitor, prevent and control an infection, and in supplying epidemiological data to guide measures to control the diseases and gauge the efficacy of vaccines.
During the Covid-19 incubation period of about 14 days, before any symptoms become apparent, a respiratory test can determine if there is infection, but it’s a tricky process and the virus might easily escape detection, Opas said.
If the test result is negative, the examiner can only conclude “virus not found”, but that doesn’t mean “not infected”. Infection only becomes easily detected after symptoms appear.
“If you get sick, you have to go to the lab again,” he said. “If you have the symptoms, infection will be detected 100 per cent, and this is when the testing becomes useful in diagnosing and controlling the disease.”
Midway through the incubation period, 5-7 days after infection, the immune system should be at work and a blood sample will show the state of immunity, Opas said. A positive result indicates an immune response, but it remains unclear what stage the infection has reached, so any such result “is likely to be useless”. He nevertheless recommends undergoing a blood test if there is suspicion of infection.
“Leave it to medical personnel or health officials to interpret every result. Don’t try to check yourself or interpret the results yourself. That requires lab study. Not everyone can immediately know whether they’re infected or not,” he said. “The Health Ministry conducts free respiratory-infection diagnoses and blood tests to monitor disease control.”
The Department of Medical Sciences has certified 40 laboratories to test for Covid-19. They have the combined capacity to examine 4,000-5,000 samples per day, but currently are receiving only about 500 samples a day.
The ministry will soon have 100 labs in the system, increasing the potential capacity to 10,000 samples per day.
