Thread by @jamewils

Delhi has not revealed if it is testing those who turned up negative using RAPID ANTIGEN TESTING as required by ICMR.

This will create a false sense of security in those who tested negative as they could be false negatives. 

nationalheraldindia.com/india/delhi-ic…

ICMR advisory asserts at least twice that suspected individuals who test -VE for COVID-19 by rapid antigen test should be definitely tested sequentially by RT-PCR to rule out infection, whereas a +VE test should be considered as a true +VE doesn't need reconfirmation by RT-PCR

Delhi released the data of how many antigen tests were done and out of that how many people got positive from those tests for June 18 & 19.

But thereafter, they have not released any separate data about Antigen tests, but all data as pooled and as a single data.

Here @ashlinpmathew explain in detail, about the sensitivity & specificity of the rapid antigen tests by the South Korean firm SD Biosensor.

Let me discuss more on the nuances beyond the points deliberated in it.

At present there are 4 type of tests are there for #COVID19

RT-PCR is still the gold standard as it detects the positive case with high degree of certainty. Here the sputum is collected and tested at designated labs. But this test is costly and take more than 12 hours to get results. But here you can tests around 120 samples at a stretch.

Then came Rapid Anti-body tests, here we look for the anti-bodies developed in blood of the infected people rather than looking for the virus as in the case of RT-PCR.

These serological tests are riddled with limitations and controversies & stopped due to its unreliability.

Then CBNAAT test, which is the Cartridge Based Nucleic Acid Amplification Test.

The test is a molecular TB test which detects the DNA in TB bacteria, uses a sputum sample & can give a result in less than 2 hours. 

This is now tweaked to find #COVID19 with WHO approval

There are two companies are there in the market. First is the Genexpert kits from American company Cepheid.

The TrueNat test has been developed by the Indian firm MolBio Diagnostics Pvt Ltd Goa. It’s development has been funded by Bigtec Labs, India.

This Genexpert test is known as CB-NAAT test but it is costlier than TrueNAAT developed by India.

More details here 👇

tbfacts.org/genexpert/

tbfacts.org/truenat/

These tests are now conduct on patients who needs immediate medical attention.

Major limitations of the TrueNAAT, though it is cheaper than its US sibling is that it can test one sample at a time & hence you can test 12 to 14 samples a day.

Hence it cannot be used for bulk testing or you need to employ more machines, which are costly affair.

Here is what ICMR says about all the above tests

Come back to our point of discussion here.

This stand-alone rapid point of care antigen detection assay which does not require a specialized machine and can be interpreted with a naked eye.

The test is a promising tool for quick diagnosis of SARS-CoV-2 in field settings.

The assay is known as
Standard Q COVID-19 Ag kit and has been developed by SD Biosensor with manufacturing unit at
Manesar, Gurugram.

The test has been found to have a very high specificity with moderate sensitivity - ICMR

This is company's page 👇

sdbiosensor.com/xe/product/7672

First look at the very limitations put out by the company themselves.

Also look at the sensitivity & specificity claimed by the company.

They claim 84.38% sensitivity after testing on 32 people at ideal conditions.

Claim 100% sensitivity after testing on 170 people.

"Specificity ranged from 99.3 to 100% at the 2 sites and sensitivity of the test [i.e. ability to detect true positives] ranged from 50.6% to 84% in two independent evaluations, depending upon the viral load of the patient." - ICMR

thehindu.com/news/national/…

Here is the advisory of ICMR on Rapid Antigen testing.

ICMR is totally silent here about how many samples they have taken to find the above sensitivity.

It is important to note that it is much lower than what company claimed.

icmr.gov.in/pdf/covid/stra…

recommendation no.(iii)

All labs/hospitals initiating testing through the rapid antigen PoC test need to ensure that all
symptomatic negative patients should be essentially referred to a real-time RT-PCR test for COVID-
19.

Here comes the nuances.

Company claimed 83.48% sensitivity and 100% specificity.

While ICMR put a range of 50.6% to 84% sensitivity and 99.3% to 100% specificity.

But as per them this was found at two sites viz. ICMR, Delhi & AIIMS, Delhi.

No details of samples. NOTE please

Real world use and its results are more important.

Because the less number of tests in the ideal conditions by company or a similar sample testing by ICMR gives us a skewed picture.

As Delhi employ these kits heavily, the very reliability of the tests in practical can be seen.

Delhi Govt should disclose to the public that every day how many antigen kits are used & how many positive cases are identified.

NOT ENOUGH.

THEY SHOULD DISCLOSE how many positives found out through subsequent RT-PCR test, which will reveal the real world FALSE NEGATIVES!

This is very important for other States, general public and as well as international community to verify the veracity of the claims of sensitivity claimed by the company.

Now they enjoy a monopoly on the kits. One big advantage of this kit is its quick & like the pregnancy kits

But if the antigen kits do not have the sensitivity as claimed, then they will be a costly burden on the exchequer.

Let me explain here.

Take Delhi data to find % positives strike

18th June - 7040 tests & 456 positive (6.5%)

19th June - 12680 tests & 951 positive (7.5%)

For my analysis, I assume 10% positive is detected through this.

Take 100 samples & the expenses for each kit @ Rs.450/-, I have to spend Rs.45,000/-

In case of RT-PCR for these 100 samples, assume that Govt has to spent Rs.2500/-, so the expense will be Rs.2,50,000/-

If I get 10% positive results, I save 2500-450 = Rs.2050 on each sample with antigen kit, that means Rs.20,500/- savings.

But as I have 90 negatives, I have to do RT-PCR on them, so I have to shed an additional 90x2500 = Rs. 2,25,000/-.

So my expense will be cost of antigen plus cost of subsequent RT-PCR, there for it will be Rs.45000 plus Rs.2,25,000 = Rs.2,70,000/-

Remember if I use RT-PCR alone, I need to spent only Rs.2,50,000/-

So I have to shed an excess 20,000/- here!

So if we found that the sensitivity is much low than what is claimed to by the company/ICMR, these tests strike rate will be much lower & it becomes not reliable & not economical.

If its sensitivity is good & then it can employed in containment zones & hot spots with high +VEs.

If you have employ these kits in zones you suspect 20% positive cases, it will become economical as it costs only Rs.2,45,000/- in our earlier example of 100 samples. So any place where you see more than 20% positivity, this saves money and time!

Hence data is more important!

We have already acquired 6 lakh rapid antigen tests, so we should know more data in the public domain about it efficiency before we push it indiscriminately.

Delhi Govt should disclose these data and it will help other State Governments as well as international community.

Before employing and adopting a new generation tests extensively, there should be more real world data should be disclosed at public domain to understand the trade-offs involved in such heavy deployment.

END.