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| With increasing number of coronavirus cases, demand for testing has increased. Under these conditions, it is important to know what are the available options. |
At present, coronavirus cases are rising across the world. This deadly virus has taken lives of people worldwide, shattered the economy, caused mental health issues and still doesn't seem to stop. However, researchers are working out on potential vaccines and let us hope we get one soon.
With growing number of cases, demand of test kits have also increased. As a result, different countries have developed different test procedures as per need. Among these, two types of tests are used widely - PCR and RDT.
Both these tests have their own roles. And, it is important that we know them and why and how they are used.
RDT detects antibodies or antigens present in the serum. PCR detects RNA or DNA of the virus. As PCR detects the viral RNA or DNA, it can confirm the presence of a virus in the early stages of infection. Due to this PCR allows infected ones to get isolated and treated early. So, PCR is considered as the gold standard for coronavirus testing.
SAMPLE
In RDT, the sample is blood, serum or plasma. In PCR, the sample is liquid or solid. For example, in respiratory infections, the sample is nasal swab or pharyngeal swab.
PROCESS
In RDT, blood, serum or plasma is taken and presence or absence of antibody against COVID-19 is checked. However, after a person has been infected, it usually takes 4-7 days for the antibodies to develop. So, it doesn't detect the virus specifically, if the testing is done in early days of infection.
Even the antibody formation varies from person to person and depends on age, nutrition, immunosuppressive diseases. So, if these people are infected, but antibody hasn't been formed, then the test becomes invalid.
In PCR, the swab collected from nose or pharynx has only a small amount of RNA, and is not enough for testing. Hence, it is converted to a two-strand DNA through a process known as reverse transcriptase. Then, doctors identify a specific area in the DNA and make copies of it for the PCR process.
RESULT
In RDT, results are obtained within 15 min to 2 hr. In PCR, results are obtained within 1 hr to 48 hrs.
COST
RDT is comparatively less expensive than PCR. PCR uses large machines which are quite expensive.
PORTABLE
RDT is also more portable and can be carried easily from one place to another. However, PCR requires large machines and are difficult to be shifted.
SENSITIVITY & SPECIFICITY
Sensitivity of a test refers to the proportion of positive samples, identified as positive by the test. For example, let us suppose, we are doing PCR test for COVID-19 and we have collected 100 throat swab samples. Among these, let us suppose all the 100 cases are infected and have coronavirus.
But, after doing PCR test, the test result shows 90 samples have coronavirus and 10 samples do not have coronavirus. It means, though the real result is that all the 100 samples have coronavirus, only 90 samples are correctly identified. So, there is some defect in the PCR test which could not identify the 10 cases. Here, we say, sensitivity of the test is 90%.
Specificity of a test refers to the proportion of negative samples, identified as negative by the test. For example, let us suppose, we take 100 throat swab samples from a community. And, all 100 cases are coronavirus negative.
However, after doing PCR test, only 90 samples are correctly identified as negative, and the rest 10 samples though negative, are incorrectly identified as positive. These 10 cases are false positive cases. More is the false positive cases, specificity of the test will be less and vice versa.
At present, different countries are using PCR and RDT test kits manufactured by different companies. So, there is some variation in sensitivity and specificity depending upon its place of manufacture. However, generally, sensitivity and specificity is above 90% in most of them.
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