Is an LoD <300pg/ml better than 400pg/ml?30 Nov 2020 10:50
There was quite a bit of chatter about this over the w/e. Innova’s test has a detection limit of 400pg/ml and Avacta’s is <300pg/ml, the assumption was therefore that Avacta’s was 25% better.
This may or may not be the case, you cannot tell from these numbers. Innova’s test is for the nucleocapsid (N) protein, as far as we know Avacta’s is the S1 protein.
From what I can find, there are about 1000 N proteins and 100 S1 proteins per viron. So potentially you could get 10x more signal for 1 viron looking at the N protein, so the assay would be 10x more sensitive for a given viron concentration.
The N protein is approx. 46kd, the S1 protein is about 1/3 larger (based on amino acid chain length). Available standards look to have at least some of the modifications that happen in human cells, the S1 is highly glycosylated, the N protein is less modified. This will mean that the S1 will be even larger (note- it is not clear if the modifications are taken into account with the amounts quoted). So, 300pg/ml of S1 will contain fewer individual protein molecules than the same weight of N proteins (it why it is more common to use uMol than ug/ml etc as the former measure takes into account the molecular weight). In round numbers, if say the MW of S1 was twice that than N, then a 100pg/ml LoD for both, would mean that the S1 assay was twice as sensitive (because there would be half the number of S1 molecules there).
One more complication is that in solution the N protein forms dimers and the S1 trimers, they would aim to supress this with salts, detergents etc in the LFT’s but we do not know how well this works. The relevance of this would depend on where the Affimers or Mabs bind, as this may or may not be affected. Another aspect of this will be the protein binding affinities of the Affimer/Mabs to their target proteins, this will determine the levels of detergents/salts which can be used which could therefore impact on the sensitivity of the assay.
Finally you get the advantage of the smaller Affimers which should help reduce nonspecific binding, also their more reproducible production (note- that variability was flagged with Innova tests used in trials).
To summaries – comparing sensitivities by pg/ml for two different sized proteins which occur in different abundances is meaningless, until they do a direct comparison we will not which is better.
This is not a deramp just an attempt to clarify some false inferences that were being made.
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