RT-PCR vs Rapid Test: Comparing two test methods

By Argie C. Aguja
Senior Features Writer, The Philippine STAR

Not all test kits to detect COVID19 are alike and each has its own advantages and disadvantages

As the worldwide number of infections reach more than five million cases, 1,899,675 recoveries and 328,172 deaths (as of May 21), the World Health Organization (WHO) continues to call for urgent action to test all suspected cases, noting that the ‘backbone’ of an effective coronavirus response is the ability to conduct large scale testing.

“We have a simple message got all countries: test, test, test,” said WHO director general Dr. Tedros Adhanom Ghebreyesus during a virtual press conference where he emphasized the crucial need to escalate testing, isolation and contact tracing efforts.

On January 10, the full genetic sequence of the SARS-COV-2 virus has been uploaded to the Global Initiative on Sharing All Influenza Data (GISAID) platform. In just a few short weeks, pharmaceuticals and research labs were able to develop diagnostic kits that can detect the coronavirus. But not all tests are alike, since each kit possesses different advantages and disadvantages.

In the Philippines, health professionals are using two different testing methods to screen suspected and probable COVID-19 cases: reverse transcriptase polymerase chain reaction (RT-PCR) tests also known as ‘swab tests’ and the rapid test kits (RTK). Here are some characteristics that sets each test apart.

RT-PCR: Slow and accurate

Majority of the tests being conducted today are based on the RT-PCR test method, considered as the ‘gold standard’ in testing. Specific and sensitive, the test accurately detects the presence of genetic material from the virus.

The RT-PCR test begins with a licensed healthcare worker or lab scientist extracting a specimen from a patient (with or without symptoms) through a throat or nasal swab. Samples are then transported in ice and delivered to testing labs for processing where the RNA virus is converted to DNA using reverse transcriptase enzymes. The presence of viral DNA can then be detected and interpreted by data either as positive or negative.

While accurate, RT-PCR tests are more labor intensive and processing samples in specialized laboratories can take anywhere from 24-48 hours (not counting the time it takes to deliver samples to the labs), making it more expensive.

Rapid test kits: fast but not fool-proof

Meanwhile, rapid test kits can produce near-instant results at the cost of accuracy. Instead of detecting the genetic materials from the virus, rapid tests rely on identifying the immune response of an individual.

A licensed healthcare worker will extract blood from a patient and the rapid test kits interpret the results in as little as 15 minutes. The test reads for two kinds of antibodies: the immunoglobulin M (IgM) or the early immune response, and the immunoglobulin G (IgG) or the delayed immune response. Because it relies on antibody detection rather than the presence of the virus, it is best used on symptomatic patients during the 5th to 21st day of showing symptoms.

Although speedy, the accuracy of rapid tests relies on the sensitivity of the kit itself. Thus, there’s a higher chance for errors — false positives and false negatives may be common. Another cause for concern is that antibodies only develop several weeks after the infection, so antibody-based tests like this might miss asymptomatic cases or those in the earliest stages of the disease. A confirmatory RT-PCR test is therefore administered for those turning positive results after a rapid test.