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I'm curious how sensitive the earliest retrospective diagnoses for SARS-CoV-2 (in China) might be. According to the paper which put the earliest case to December 1, 2019 that retrospective diagnosis was done via PCR on plasma samples that were retained from patients.

How often is SARS-CoV-2 recovered from plasma samples though, for patients otherwise diagnosed, e.g. through the more common nasopharyngeal samples?

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  • You ask “recovered,” and I wonder if you mean that technically? Viremia occurs, but it’s not common in SC2, and recovering virus out of serum samples is even harder. It’s also worth noting that it’s more common in people seeking medical treatment (rather than the majority of lightly symptomatic), and most of the data I’ve seen is only of local proportions of admitted pts, not of generally infected (which I suspect is quite low).
    – Atl LED
    Commented Sep 17, 2021 at 3:44

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Well, it's hard for me to say how closely this matches the Chinese testing methods, but there is one study from Nigeria that compared the methods:

Diagnosis of coronavirus disease 2019 (COVID-19) is currently established with a polymerase chain reaction (PCR) test by means of oropharyngeal-, nasopharyngeal-, anal-swabs, sputum and blood plasma. However, oral and nasal swabs are more commonly used. This study, therefore, assessed sensitivity and specificity of plasma as a diagnostic in comparison with a combination of oral and nasal swab samples, and the implications for blood transfusion. Oropharyngeal (OP) and nasopharyngeal (NP) swab samples were obtained from 125 individuals suspected to have COVID-19 and stored in viral transport medium (VTM) tubes. [...]

Average age of study participants was 41 years, with 74 (59.2%) being male. Out of the 125 individuals tested for COVID-19, 75 (60%) were positive by OP/NP swab. However, only 6 (4.8%) had a positive plasma result for COVID-19 with median Ct value of 32.4. Sensitivity and specificity of RT-PCR SARS-CoV-2 test using plasma was 8% and 100% respectively. There was no false positive recorded, but 69 (55.2%) false negatives were obtained by plasma. SARS-CoV-2 viral RNA was detected, albeit low (4.8%) in plasma. Plasma is likely not a suitable biological sample to diagnose acute SARS-CoV-2 infection.

So it does look like a lot of cases could have been missed by relying only on plasma testing.

There's also a study from China that albeit just tested blood not plasma in comparison.

The positive detection rate in nasopharyngeal swab was the highest (54.05%), followed by anal swab (24.32%), and the positive detection rate in saliva, blood, and urine was 16.22%, 10.81%, and 5.41%, respectively. However, some patients with negative nasopharyngeal swabs had other specimens tested positive.

The ratio here is only like 5:1 instead of 12:1 as in the Nigerian study, but still there's a significant difference.

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    Looking at the protocol of the RNA extraction kit the Chinese authors used, it seems they almost certainly centrifuged the blood and used the plasma.
    – Ian Campbell ♦
    Commented Sep 17, 2021 at 1:35

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