09 Aug 2024

“I can feel the ground move under the podium.”

No, not an earthquake. At this year’s Alzheimer Association International Conference, held July 27 to August 1 in Philadelphia, Stephen Salloway, Brown University, Providence, Rhode Island—along with many others—felt a slight sense of dizziness at the speed at which blood-based biomarkers for Alzheimer’s disease are racing forward. The newest data suggest that plasma p-tau217 knocks the socks off clinical assessment for amyloid pathology.

“The acceleration has been amazing,” said Gil Rabinovici, University of California, San Francisco. “Amyloid PET took 20 years to be approved. The first p-tau paper came out in 2020, and here we are talking about using it in primary care.”

At AAIC, data showed that several different tests perform well in head-to-head comparisons. A fragment of tau in the blood appeared exquisitely accurate at identifying people who have neurofibrillary tangles. Even a mail-in test for Alzheimer’s disease could be in the cards. Before long, a teeny drop of dried blood might be all it takes. Alzforum will summarize findings on each of these points in the days to come. For the first—just how robust are these tests—read on below.

Over the last four years, blood measures of tau phosphorylated on serine 217 have consistently distinguished amyloid-positive from amyloid negative people, at least in memory clinics and carefully recruited cohorts. The question on clinicians’ minds was whether it would work equally well in primary care. Based on several presentations in Philadelphia, the answer seems to be a resounding yes.

In a highly anticipated talk of the meeting, Oskar Hansson, University of Lund, Sweden, reported that mass spectrometry tests for %p-tau217 in the plasma, that is, the ratio of fragments of phosphorylated versus unphosphorylated at that amino acid, identified people with amyloid pathology with high accuracy in primary and secondary care settings in Sweden. Randall Bateman, Washington University, St. Louis, subsequently showed data from the Seabird trial, which evaluates blood markers in a representative sample of the St. Louis population. Here, too, plasma %p-tau217 identified amyloid-positive volunteers with specificities and sensitivities akin to those seen in memory center cohorts.

It’s not just the C2N test. Nicholas Ashton, Banner Alzheimer’s Institute, Phoenix, offered a snapshot of how the fully automated ADx Neuroscience Lumipulse assay for plasma p-tau217 performs in secondary care. This runs on a desktop machine that renders results on the same day, whereas the C2N test requires shipping of the sample to a mass spec lab for analysis. Measured onsite at four centers across three countries, it identified AD with accuracies upwards of 92 percent. “I think we are in desperate need of a fully automated version of this biomarker to meet the needs of the general population,” Ashton said.

Markers for Primary Care
As reported in JAMA on July 28, Hansson, first author Sebastian Palmqvist, also at Lund University, and colleagues evaluated how well C2N’s tests for plasma %p-tau217 alone, or in combination with the plasma Aβ42/40 ratio, i.e., their APS2 test (see Dec 2023 news), identified people with amyloid pathology or Alzheimer’s disease. From 1,213 volunteers, they tested samples in two ways—storing then analyzing samples in one large batch, as is typically done in cohort studies, or prospectively analyzing individual samples as they were collected over four years. The former tends to be more accurate since all samples are measured simultaneously, reducing variability due to handling, instrument accuracy, and other systematic errors. The latter, however, is how routine clinical care works, hence useful tests must perform reliably in this setting, too.

In this study, both types of analysis seemed to work equally well. “I was surprised to see that there was really no drop in performance in the prospective analysis,” Hansson told Alzforum. “The assays are very robust.”

How accurate were these tests? Among 515 primary care subjects in Sweden who had cognitive symptoms, 276 were deemed to have amyloid pathology based on a positive CSF Aβ42/40 test or an amyloid PET scan. Batch analysis of 307 of 515 plasma samples with the APS2 and the %p-tau217 tests correctly identified AD pathology in 91 and 86 percent of cases, respectively, when a single cutoff point was used (image above). As in a previous analysis by this group, the cutoff was chosen to ensure a specificity of 90 percent (Barthélemy et al., 2024). In this study, that equated to an APS2 score of 36 and a %p-tau217 ratio of 3.26. Based on those numbers, positive and negative predictive values rang in at 91 and 85 percent, respectively, with the APS2 tests performing slightly better on most metrics. Importantly, in prospective analyses of 208 of the 515 samples, the two tests worked just as well as in the batch analysis.

Among 698 volunteers from secondary care sites, the tests identified the 344 amyloid-positive cases just as well as they had in primary care, with PPVs and NPVs hovering around 90 percent. Again, it was a dead heat between the batch and prospective analysis (image at right). 

Using a two-cutoff approach improved accuracy. Hansson and co-author Suzanne Schindler at Washington University proposed doing this to improve assay performance. Values above the upper cutoff would indicate with a high degree of certainty that a person has AD pathology, values below the lower cutoff would rule out AD with equal certainty, while anyone who fell in the middle “gray” area would need further testing via PET or CSF (see Nov 2023 news). In the study shown in Philadelphia, Palmqvist and colleagues set the lower and upper limits for the APS2 at 31 and 62, respectively, and 3.93 and 5.18 for %p-tau217 ratio. This improved the PPVs to around 95 percent or higher in the batch analysis, and only slightly less in the prospective analysis (image below).

Two Beats One. Using upper and lower cutoffs made the APS2 and %p-tau217 tests more accurate in primary and secondary care cohorts (top). Fewer people fell into the intermediate zone using the %p-tau217 test (bottom). [Courtesy of Palmqvist et al., 2024.]

One caveat to this approach is that if many people land in the gray zone, that would necessitate so many follow-up tests as to negate the advantage of a blood test. At least in these Swedish cohorts, this did not happen. With the two APS2 cutoffs, 11 to 15 percent fell into the intermediate category; for the %p-tau217 cutoffs, 4 to 8 percent did (image above).

Douglas Galasko, University of California, San Diego, thinks the assays show great promise in both primary and secondary care settings. He cautioned that the two cohorts seemed quite alike. “Characteristics of patients who had clinical screening/assessment and then biomarker evaluation were remarkably similar in primary care and in the BioFINDER secondary care cohort, including age, sex, and APOE e4 genotype. The last of these may have been an important aspect that contributed to the high diagnostic accuracy/predictive value in the primary care cohorts,” he wrote to Alzforum (comment below).

Are these assays good enough for real-world applications? Hansson believes some further testing is required, especially in more diverse populations and in other countries. University of Pennsylvania’s David Wolk agreed. “I wonder how much we really know about the PPV and NPV across much wider clinical practice,” he said during a session on translating biomarker into practice. “It is very much based on the prevalence of the disease, and there are likely big differences in prevalence and the ability to classify with these blood-based biomarkers.”

On that note, Palmqvist and colleagues found that in people with subjective cognitive decline, these tests predict amyloid pathology less well, with PPVs around 75 percent and 83 percent for the one- and two-cutoff approaches, respectively. This might be a disadvantage in identifying people with asymptomatic AD to include in clinical trials. Negative predictive values, however, still clocked in at above 90 percent, suggesting that these tests might be most helpful at ruling out AD in people whose symptoms are very subtle.

As for people with mild cognitive impairment or dementia—still the largest group of people presenting at clinics today—the tests beat clinical assessment hands-down. In the primary care setting, physicians correctly diagnosed AD pathology in only 63 percent of cases. In secondary care, namely the memory clinics at Skåne University Hospital and Ångelholm Hospital, both in Sweden, specialists only got it right 73 percent of the time.   

“This study makes the case convincingly that highly sensitive blood measures of Alzheimer’s disease can be integrated into the clinical decision-making process, including in the primary care setting,” wrote Salloway, Christopher Rowe, Florey Institute of Neuroscience and Mental Health, Melbourne, Australia, and Jeffrey Burns, University of Kansas Medical Center, Lawrence, in an editorial in JAMA. “Accurate and early diagnosis of Alzheimer disease is increasingly important because of the new era of monoclonal antibodies targeting amyloid reduction in the brain.” This means that more people will get diagnosed with early Alzheimer’s in primary care, from where they will get referred to secondary care for treatment and side effect monitoring, adding pressure to these systems, Rowe told Alzforum. “We need to train care navigators and staff to handle this,” Salloway said.

The Seabird data Bateman presented, and the secondary care data Ashton described, support this. Bateman and colleagues designed Seabird to test how well plasma %p-tau217 and the plasma Aβ42/40 ratio reflect amyloid pathology in a cohort that was representative of the greater St. Louis area. The trial, which enrolled 1,122 people aged 60 and older, largely met its goal for recruiting across diverse demographics. Bateman reported tight correlation between plasma %p-tau217 and amyloid PET SUVR values, which was minimally affected by race, sex, and comorbidities. This was true, as well, for chronic kidney disease, which can skew the level of p-tau217 in the blood (Aug 2022 conference news), The data suggest that using the p-tau217/217 ratio, rather than the absolute levels of these fragments, can largely account for effects of comorbidities, as has been seen by others (Janelidze et al., 2023). 

Clifford Jack, Mayo Clinic, Rochester, Minnesota, referring to intense discussion about the effects of race on plasma markers, was intrigued that it didn’t seem to affect the correlation between the %p-tau217 and amyloid. Bateman said the trial enrolled enough white and black Americans to detect an effect. “Race did not affect ratios, but it can affect levels,” he said. “We would not need different cutoffs among races for ratios, but we might need it for absolute values,” he said.

The secondary care study Ashton reported tested how well absolute levels of p-tau217, measured with the fully automated ADx Lumipulse immunoassay, identify people with Alzheimer’s. It recruited more than 1,500 participants from four sites in Sweden, Italy, and Spain, and plasma was analyzed in Gothenburg, Brescia, and Barcelona. Using a two-cutoff approach, accuracies reached 92 to 94 percent, and PPVs as high as 96 percent. “This meets the criteria for clinical implementation,” said Ashton.

Among the participants, 12 to 17 percent fell into the intermediate zone between the cutoffs. This is higher than in Hansson’s study using C2N’s mass spectrometry assay. Ashton thinks the cohorts explain some of difference, acknowledging that comorbidities, including chronic kidney disease, can affect fully automated tests for absolute p-tau levels. “We need to understand this quickly,” he said.

One option might be to account for a person’s glomerular filtration rate, a commonly used lab test. After all, chronic kidney disease is not difficult to diagnose. In her talk, Alicia Algeciras-Schimnich, from the Mayo Clinic in Rochester, showed how plasma p-tau217 levels rise as filtration rates fall in amyloid-negative people. Half of those with an eGFR of 35 mL/min/1.73m², which indicates moderate to severe kidney disease, had p-tau17 levels above the cutoff for positivity. “Knowing the eGFR could help avoid false positives and signal the need for an alternate test,” she said.—Tom Fagan

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References

News Citations

1. Two New p-Tau217 Blood Tests Join a Crowded Field 1 Dec 2023
2. Plasma p-Tau-217 Assays Work Well, But No Home Run for Diagnosis 30 Nov 2023
3. Blood Tests: Charting the Path to Primary Care 23 Aug 2022

Paper Citations

1. Barthélemy NR, Salvadó G, Schindler SE, He Y, Janelidze S, Collij LE, Saef B, Henson RL, Chen CD, Gordon BA, Li Y, La Joie R, Benzinger TL, Morris JC, Mattsson-Carlgren N, Palmqvist S, Ossenkoppele R, Rabinovici GD, Stomrud E, Bateman RJ, Hansson O. Highly accurate blood test for Alzheimer's disease is similar or superior to clinical cerebrospinal fluid tests. Nat Med. 2024 Apr;30(4):1085-1095. Epub 2024 Feb 21 PubMed.
2. Janelidze S, Barthélemy NR, He Y, Bateman RJ, Hansson O. Mitigating the Associations of Kidney Dysfunction With Blood Biomarkers of Alzheimer Disease by Using Phosphorylated Tau to Total Tau Ratios. JAMA Neurol. 2023 May 1;80(5):516-522. PubMed.

External Citations

1. trial

Further Reading

Papers

1. Palmqvist S, Tideman P, Mattsson-Carlgren N, Schindler SE, Smith R, Ossenkoppele R, Calling S, West T, Monane M, Verghese PB, Braunstein JB, Blennow K, Janelidze S, Stomrud E, Salvadó G, Hansson O. Blood Biomarkers to Detect Alzheimer Disease in Primary Care and Secondary Care. JAMA. 2024 Oct 15;332(15):1245-1257. PubMed.
2. Salloway S, Rowe C, Burns JM. Are Blood Tests for Alzheimer Disease Ready for Prime Time?. JAMA. 2024 Oct 15;332(15):1240-1241. PubMed.