Thread by @petermbenglish

A problem with privatisation of testing.

Now that we are routinely testing lots of people - including care home staff and residents, and NHS staff - we are picking up positive results in asymptomatic people.

1/17

Where these are genuine cases, this is good.

The problem is many are being detected in people who had tested positive previously.

And we don't know how to interpret them.

This helps:
cebm.net/covid-19/infec…
2/17

tldr: each time you run a "cycle" you double the amount of RNA in the sample. The number of times you do it is called the "cycle threshold" "Ct) ; the more often you have to do this before you can detect it, the less RNA there must have been in the original sample.
3/17

So the Ct value is a way of quantifying the amount of RNA. If there was very little RNA in the sample you'll have to double the amount more often - giving a higher Ct value. If there was a lot of RNA, you'll get a low Ct value.
4/17

If there is current infection, there is likely to be a lot of RNA in the sample (and a low Ct value). If somebody has left-over RNA from a previous infection, there'll be less of it, and a high CT value.
5/17

These aren't "false positives" in the sense of being positive tests in the absence of viral RNA (there is RNA in the swab); but they are "false positives" in the sense that they do not show current infection or infectiousness.
6/17

So… Now that we are seeing all of these people who had tested positive previously, we need to know if they are currently infected (and infectious), or simply showing residual RNA from a previous infection.

7/17

It is *extremely* disruptive otherwise. If we treat these positive cases as new infections, care homes have to be closed all the residents have to self-isolate for 14 days; key health and social care workers have to self-isolate for 10 days, and their contacts for 14.
8/17

Which is where I come to the private "lighthouse" labs.

Some of them don't record the Ct. They simply are unable to give use the information, because (they claim) they weren't commissioned to record these data.
9/17

One of the private lighthouse labs (in Cambridge) apparently does record the Ct - but it won't routinely share it. So the PHE teams have no way of assessing whether these are true cases or prolonged RNA from previous infection - no risk at all.
10/17

(Yes, it is PHE doing this work, not local authority public health - LA public health is doing a brilliant job, but not this).
11/17

The lighthouse labs are commercial for-profit organisations. When we discovered swabs from a care home had gone to a lighthouse lab when they should have gone to a PHE one, they could easily have run the swabs at minimal cost - & absorbed the cost, or sent us a small bill.
12/17

Instead, they just said "not covered by our contract - come and collect them if you want, otherwise we'll bin them".

Which caused huge delays and expense.
13/17

It will be the same with Ct values.

"Not in our contract. We can do it if you like. It will cost [now, how much can we sting the suckers for, we know that Hancock will do anything to improve testing numbers, and the NHS and social care are desperate not to lose staff]."
14/17

That's if they decide it's commercially worthwhile at all. If they've set up a system that's making them rich, and it would be tricky and costly to start sharing Ct values, they might decide not to bother. They don't care if key workers are put off work unnecessarily.
15/17

I'm not blaming the labs for this. The private sector is not a charity. They are doing what they were commissioned to do (and no more) - because they are answerable to their shareholders, to make a profit.
16/17

Now, tell me again.

Who commissioned the laboratories?

How much attention did they give to what would be needed (other than simply "ramping up testing" for political rather than public health reasons)?
17/17

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