Blood and plasma: research

How early can blood turn positive?

The preclinical window has been measured in a primate model rather than in people. Concha-Marambio and colleagues infected 3 macaques peripherally with macaque-adapted variant CJD prions and tested serial blood samples by protein misfolding cyclic amplification, using sarkosyl precipitation and a heparin-enhanced substrate, with a pseudo-passage step for viscous buffy coat and detection by western blot after proteinase K digestion.

The first blood collection, at 65 days post-inoculation, was already positive. Expressed against clinical onset, that was 759 days before onset in macaques 1 and 2 and 644 days before onset in macaque 3. On a blinded panel of 140 buffy coat samples, 72 from infected animals and 68 from 23 uninfected controls, 69 of 72 infected samples were positive and all 68 control samples negative. Counting each replicate separately, sensitivity was 95.8 percent (95 percent CI 88.3 to 99.1) and specificity 100 percent (95 percent CI 94.7 to 100); grouping replicates by sample, both were 100 percent, with a sample called positive when more than one replicate was positive. Buffy coat outperformed plasma at the preclinical stage: three PMCA rounds detected prions in all 3 preclinical buffy coat samples, while only 2 plasma samples were positive after four rounds.

The authors state two limitations that bear on any inference to human screening. The inoculum was brain homogenate from experimentally infected macaques, so equivalent detection in human samples cannot be assumed, and detection of PrPSc does not establish that the material would be infectious in an animal. They also report that one animal had lower blood PrPSc without a clear explanation, and that the three discordant replicates came from samples of poor physical quality.

Why does sporadic CJD resist blood assays?

Two lines of work converge on strain and substrate as the obstacle.

Nonaka and colleagues built an enhanced quaking-induced conversion assay using three human monoclonal antibodies and reported roughly 1000-fold higher sensitivity than standard RT-QuIC on brain material. Applied to blood from patients with sporadic human prion disease, it detected nothing. The abstract states six patients; the methods and tables describe three, all of the MM1 subtype, and the paper does not reconcile the two figures. Their conclusion is that methods other than RT-QuIC and eQuIC will be needed. A reader should note the sample size either way: a negative result in three to six patients bounds the assay’s sensitivity only loosely.

Belondrade and colleagues assessed PMCA of PrPTSE from 14 sporadic CJD brain samples in three substrates, two from transgenic mice expressing human PrP with either methionine or valine at codon 129, and one from bank voles. All five major clinicopathological subtypes seeded amplification, with strong seed and substrate dependence. Bank vole PrP substrate propagated every subtype while preserving the initial molecular PrPTSE type, whereas amplification in human PrP substrates shifted the molecular type during heterologous reactions, and valine 129 substrate was more permissive than methionine 129. The combination of amplification sensitivity and electrophoretic profile distinguished four sporadic strains, M1, M2, V1 and V2. The authors report reaching the sensitivity needed to detect VV2 sporadic prions in cerebrospinal fluid. They report no blood or plasma result for sporadic disease.

Taken together, a failure in sporadic blood is not evidence that seeds are absent. It is consistent with a substrate that is poorly matched to the seed, a shifting molecular product, and seed concentrations below a threshold that varies by subtype.

What might get past blood inhibitors?

Whole blood contains strong inhibitors of quaking-induced conversion. The nanoparticle-enhanced variant, Nano-QuIC, has been tested against this problem in a different proteinopathy: in spiked human plasma it doubled reaction speed and raised sensitivity about 100-fold, and in lysed whole blood it detected alpha-synuclein seeds down to 90 pg/mL where reactions without nanoparticles detected nothing. The patient comparison in that report was 4 people with Parkinson disease against 4 controls. This is a method result in alpha-synuclein, not a prion result, and it is listed here because the inhibitor problem it addresses is the same one that blocks prion detection in blood. For the equivalent work on cervid samples see chronic wasting disease and zoonotic risk.

What has the policy record done with this evidence?

Germany illustrates the direction of travel. The Arbeitskreis Blut statement S 27, on exclusion criteria for blood and plasma donation in relation to variant CJD, was adopted at the committee’s 95th meeting on 6 December 2024 and posted by the RKI on 3 February 2025. The criteria themselves are in the attached PDF, which was not read for this page, so the specific wording should be taken from that document.

The Paul-Ehrlich-Institut then published the revocation of its order of 4 April 2005, which had excluded donors who had undergone surgery or received transfusions in the United Kingdom or Northern Ireland after 1979 and had required a warning notice in product information. The revocation page carries a publication and update date of 28 May 2025. The page read states neither the effective date of the revocation nor its reasoning; both are in the linked decision document.

Deferral criteria set in 2000 and 2005 were built on modelled exposure, because no donor screening assay existed. That remains true for sporadic disease. For how this reaches patients and donors, see blood and plasma in plain terms, the regulations atlas and tissue and organs.