Gordon Stein, CFO of CleanTech Lithium, explains why CTL acquired the 23 Laguna Verde licenses. Watch the video here.
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https://www.bivda.org.uk/News-Events/Blog/ArticleID/270/A-short-FAQ-on-COVID-19-Testing
Thanks for pulling out that detail KK. Here’s hoping the 30 positive specimens are dribbling pure SARS-Cov-2.
Thanks to stanman for posting the FDA antigen approval process. It seems the LOD will be a characteristic of the antigen test itself. It appears that the lateral flow device should be tested with lower and lower viral load and the minimum LOD is when 95% of tests (minimum 19 of 20) return a positive result.
===== FDA antigen template section J part 1 =====
Serial dilutions of the characterized SARS-CoV-2 were then tested in [number of replicates] replicates. The lowest concentration at which all [number of replicates] replicates were positive was treated as the tentative LoD for each test. The LoD of each test was then confirmed by testing [number of replicates (at least 20 recommended)] with concentrations at the tentative limit of detection. The final LoD of each test was determined to be the lowest concentration resulting in positive detection of [number of positive replicates (at least 19 out of 20 replicates)]. [Include analysis of LoD results, indicating the final LoD for each test]
===== END =====
Also interesting is Clinical Evaluation (Section J, part 9) on how sensitivity is calculated. Basically the antigen test is compared to a PCR test and minimum 30 natural clinical specimens. This to me sounds like testing those infected and with symptoms, and not asymptomatic patients. To pass here, 80% sensitivity needed so not high a hurdle.
===== FDA antigen template section J part 9 =====
You should confirm the performance of your assay with a series of clinical specimens by testing a minimum of 30 positive specimens and 30 negative specimens in a randomized blinded fashion. We recommend only using a high sensitivity EUA RT-PCR test which uses a chemical lysis step followed by solid phase extraction of nucleic acid (e.g., silica bead extraction) as the comparator method. Specimens may be prospective or retrospectively collected. If you intend to seek a claim for saliva or oral fluid, you should test at least 30 positive specimens with paired PCR results from an NP swab.
...
Tests should demonstrate a minimum sensitivity of = 80% for all sample types submitted.
===== END =====
Not an expert but. LFD are on the whole yes or not - why they work for pregnancy.
Other devices can measure quantity - thinking mass spec. I read the number of cycles in PCR is an indicator of the original concentration.
Its doable (affirmers in biosensors are already in development - bind and fluoresce - amonuntbof light proportional to amount of binding).
The problem I think is none is measuring viral load in asymptomatic patients and at moment no-one is volunteering to be infected. I think data will come out soon from HCPs who will bevexposed.
Here’s a noddy example... https://www.wikihow.com/Calculate-Sensitivity,-Specificity,-Positive-Predictive-Value,-and-Negative-Predictive-Value
The part I’m wrangling with is the sampling. If you select 1000 in a hospital setting at varying points in their infection then our sensitivity might not look so great. If you sampled 1000 that had recently caught it with high viral loads in the throat i.e. when you’d want to use our test, there’s a fair chance we’d get 100%.
So you can have a range of viral loads and the virus also moves through the body as it progresses not forgetting the host might be asymptomatic. How on earth do you actually get to knowing the true sensitivity when validating? Genuine question to understand how are they going to come up with the magic number that will make or break the test. If anyone has a link to the methodology I’m keen to learn.
We’ve all read about problems with sampling for the gold standard of PCR, would our simple to use test therefore be comparable even at a lower sensitivity purely due to its ease of use?
As noted 80% is our starting point for the FDA, Quidel think they can get to 90% so the benchmarks have been set.
Agree but its a trade off. KK point that with so much spike protein present that the fact that Avacta reagents bind to this plus free floating is reassuring. Having read about 14 clinical papers on it today I believe that we won't spot the very beginning of infection but can detect people who are asymptomatic in the next phase. Thatvremains unproven. Truth is papers recognise more work is needed on viral loads during disease inpatients other than those in hospital.
PCR is a technique which amplifies amplifies the signal of nucleic acids (DNA or RNA). As such it is extremely sensitive and therefore the gold standard. The sensitivity of the Avacta's antigen will depend on the concentration of antigens in the sample and for the time being remains unknown as mentioned by Dr Alistair Smith in one of the recent interviews.
All these viral load numbers only measure the amount of whole virus particles correct? Note that each virus particle has on average 65 spike proteins, and probably multiples more of detached spikes in the upper respiratory area after infection that can be present in saliva. So while one virus particle has only so much RNA to be detected via PCR, there is a much higher factor when it comes to possible binding targets for Affimers. One would confidently assume then that sensitivity will not be a problem.
I just watched the Horizon programme on the beeb. They mentioned the high viral load during the first 5 days, this is also likely to be before symptoms appear. That sounds perfect for a home test where you’d want to catch the virus early and stop transmission. However, you’d be reliant on the population testing themselves when they don’t necessarily have symptoms. I wouldn’t be surprised if the first order was from the govt for full national testing to kick start track and trace.
Article in print.
Viral loads in saliva are comparable to those in NPS and ranged from 9.9 × 10 2 to 1.2 × 10 8 copies/mL during the first week of symptoms and decrease over time.
My take is that at peak and presymptomatic the viral load is high enough to be detected by a LFD. Affirmers have a binding dynamic that is proportional to concentration - they are reliable at nanomolar rather than picomolar concentrations. I'm guessing that this means they have a higher LoD than the current PCR. The downside of this is that someone with low viral presence may therefore be less infectious (I don't know). Suspect we will hear more soon but I was surprised that saliva seems more reliable than nasal but people keep sticking cotton buds up their noses. I'm hoping its just trumps doctors havent told him.