The research, published in Annals of Clinical and Translational Neurology, evaluated two FDA-cleared capillary blood collection devices, TAPII and Tasso+, for measuring four key Alzheimer’s plasma biomarkers: the amyloid-beta 42/40 ratio, phosphorylated tau 217 (pTau217), neurofilament light chain (NfL) and glial fibrillary acidic protein (GFAP). These analytes tell complementary stories about the disease. The Aβ42/40 ratio reflects amyloid deposition, pTau217 is currently the strongest single plasma marker for discriminating autopsy-confirmed Alzheimer’s disease from other neurodegenerative conditions, NfL signals neuroaxonal injury, and GFAP marks astrocyte activation associated with the onset of amyloid pathology. The team, led by researchers at the Johns Hopkins Center for CSF Disorders, compared capillary plasma measurements to simultaneously collected venous plasma and cerebrospinal fluid from the same individuals.

The cohort comprised 192 patients examined between October 2021 and April 2024, with a mean age of 61 years and a clinically diverse mix of diagnoses. The group included 25 patients with Alzheimer’s disease, 42 with idiopathic normal pressure hydrocephalus, 28 with vascular dementia, 61 with other neurological conditions such as dementia with Lewy bodies, frontotemporal dementia, Parkinson’s disease and idiopathic intracranial hypertension, and 36 cognitively normal participants split into younger and older normal groups. Each patient who could provide samples underwent venipuncture on one arm and capillary collection on the other using either TAPII or Tasso+, both containing EDTA as an anticoagulant. Twenty patients provided samples with both devices. Cerebrospinal fluid was obtained by diagnostic large-volume lumbar puncture under sterile conditions.

The devices themselves are compact, single-use tools designed to collect small amounts of capillary whole blood painlessly from capillaries in the skin. TAPII, approved in 2019, uses an array of 16 microneedles, each 500 micrometers in length, and gentle vacuum to draw blood. Tasso+, cleared in 2022, uses a single lancet 5 millimeters long. Both are described by their manufacturers as suitable for use by a caregiver or the patient themselves, if able, from the comfort of home. In this study, Tasso+ collected a mean volume of 294.1 microliters and TAPII 266.94 microliters, with Tasso+ succeeding in collecting more than 50 microliters of whole blood in 90.3 percent of attempts and TAPII in 86 percent, differences that were not statistically significant. Both devices yielded sufficient blood for multi-biomarker immunoassays in more than 85 percent of collections.

All biomarker levels were measured in singlicate on a Simoa HD-X analyzer using commercially available kits, with inter-assay coefficients of variation below 12 percent for all analytes. The results for three of the four biomarkers were strikingly positive. Capillary concentrations of NfL, GFAP and Aβ42/40 from both devices correlated strongly with their venous plasma counterparts. TAPII-derived NfL showed a correlation coefficient of 0.86 with venous NfL, GFAP 0.84 and Aβ42/40 0.74, all highly significant. Tasso+ performed similarly, with NfL at 0.91, GFAP at 0.80 and Aβ42/40 at 0.75. Pairwise comparisons found no significant differences between TAPII, Tasso+ and venous measurements for these three analytes, and Bland-Altman analysis revealed minimal bias and tight agreement for Aβ42/40 across the range of values, with wider limits of agreement for NfL and particularly GFAP, which showed substantial variability and multiple outliers for both devices.

Then came the surprise. pTau217, the biomarker that has consistently outperformed other phosphorylated tau isoforms in distinguishing Alzheimer’s disease from other dementias and in predicting PET positivity, failed to replicate between capillary and venous samples. Tasso+ capillary pTau217 correlated only weakly with venous pTau217 (r = 0.23, p = 0.04), and TAPII pTau217 did not correlate significantly at all (p = 0.12). Bland-Altman analysis demonstrated a consistent positive bias for both devices, with mean biases of approximately 7 picograms per milliliter for TAPII and 3 picograms per milliliter for Tasso+, and this overestimation became more pronounced at higher pTau217 concentrations. Passing-Bablok regression confirmed substantial proportional bias, with slope estimates markedly deviating from one for both devices. The practical consequence was dramatic: over 95 percent of capillary pTau217 concentrations across all patient groups, even cognitively normal controls, exceeded the preliminary cutoff of 0.42 picograms per milliliter used in established literature to indicate significant brain amyloid pathology. In venous plasma, only the Alzheimer’s disease group and the mixed neurodegenerative Other group exceeded that threshold on average. The study reports a nearly 30-fold difference between TAPII and venous measurements, a 10-fold difference between Tasso+ and venous, and a 3-fold difference between the two capillary devices themselves.

The authors propose several hypotheses for this pTau217 anomaly. Phosphorylated tau species, including pTau217, as well as alpha-synuclein aggregates, have been shown to exist in peripheral nerve terminals of the skin, raising the plausible hypothesis that capillary collection devices may release peripheral pTau217 into the whole blood sample collected, thereby inflating the overall concentration. Matrix effects unique to the pTau217 assay may also be involved, since all the other analytes were run on a different assay kit. Differences in the dimensions and number of needles utilized by the two devices may explain variation in pTau217 concentration as well as hemolysis, which was significantly higher in capillary samples (TAPII 2.08 out of 4, Tasso+ 1.37 out of 4) compared with venous. Notably, hemolysis did not significantly alter the strong relationship between capillary and venous measurements of Aβ42/40, NfL and GFAP, suggesting the pTau217 problem is specific rather than a general quality issue with capillary specimens.

When the researchers compared capillary and venous plasma to cerebrospinal fluid, venous NfL, GFAP and pTau217 all correlated significantly with CSF analytes (NfL r = 0.73, GFAP r = 0.59, pTau217 r = 0.33), consistent with previous CSF-plasma studies. Capillary NfL and GFAP also correlated modestly with CSF measures, whereas capillary Aβ42/40 and pTau217 did not correlate significantly with CSF. Age-adjusted logistic regression models constructed separately for each sample type found comparable discrimination of Alzheimer’s disease status across venous and capillary measurements for all four biomarkers, with no statistically significant differences in AUC. Venous pTau217 demonstrated the highest individual biomarker performance (AUC 0.88, 95 percent CI 0.82-0.94), while GFAP and Aβ42/40 showed similar diagnostic performance across capillary and venous sample types.

Lasso-regularized logistic regression, used to account for the small number of clinically diagnosed Alzheimer’s cases (n = 25) and reduce overfitting, identified age and plasma GFAP as the most consistent predictors of Alzheimer’s disease across all sample types. Models incorporating these two variables yielded AUC values of 0.75 for TAPII-derived capillary plasma, 0.87 for Tasso+-derived capillary plasma and 0.83 for venous plasma. Remarkably, the Tasso+ model, built from just age and GFAP, exceeded both the TAPII and venous models and fell close to AUC values generated by plasma Alzheimer’s biomarker testing in larger studies. The authors suggest that capillary GFAP, particularly in combination with age or APOE genotype, could serve as a scalable screening tool for Alzheimer’s disease in primary care or decentralized research settings. NfL, while elevated in Alzheimer’s disease, is also elevated in many other neurodegenerative conditions present in the cohort, explaining its weak discriminatory power between Alzheimer’s and non-Alzheimer’s participants. The reduced predictive utility of capillary Aβ42/40 concurs with previous studies citing the plasma amyloid ratio’s greatest diagnostic power in preclinical and mild cognitive impairment stages, whereas this cohort primarily comprised clinical Alzheimer’s dementia.

Pain scores, measured on a Numeric Rating Scale from 0 to 10, were dramatically lower for capillary collections than for traditional venipuncture. Tasso+ scored a mean of 0.2 out of 10, TAPII 0.73 out of 10, and venous collection 2.24 out of 10, with all pairwise comparisons highly significant. The authors note that lower pain associated with capillary collection may improve patient compliance for blood draws, a finding consistent with other studies using capillary collection devices. The study was conducted in a supervised clinical setting, and the authors caution that findings may differ in unsupervised or at-home collection environments. The cohort was also not randomized between devices, which may introduce bias but reflects real-world clinical conditions, and the number of clinically diagnosed Alzheimer’s cases was relatively small, limiting statistical power.

The overall conclusion is a nuanced one. The findings support the feasibility of capillary biomarker testing and the use of capillary collection devices in decentralized Alzheimer’s clinical trials and studies, with minimal loss of diagnostic signal for NfL, GFAP and Aβ42/40 compared with venous plasma. But a clear need exists for specific guidelines and assay optimization for capillary biomarkers, particularly pTau217, due to significant technical or biological differences between TAPII, Tasso+ and venous plasma samples. The authors envision a future in which patients or caregivers could provide a small capillary sample at home or in a clinic, and clinicians could triage them for confirmatory CSF or PET testing based on algorithmic risk. Before that vision becomes reality, capillary blood biomarker tests will need to be thoroughly evaluated as CSF and venous plasma assays have been over the past decade, potentially necessitating separate cutoffs and guidelines for different plasma sample types to account for the measurement gap that this study has now exposed.

Subject of Research: Validation of two capillary blood collection devices for measuring Alzheimer's disease plasma biomarkers against venous plasma and cerebrospinal fluid

Article Title: Alzheimer's Disease Biomarkers Measured by Capillary Blood Collection Versus Venous Plasma and Cerebrospinal Fluid

Article References: Ho, S., Shen, Z., Lewis, A., Patel, M., Zheng, A., Proscino, G., Ho, T., Wang, J., Albert, M., & Moghekar, A. (2026). Alzheimer's Disease Biomarkers Measured by Capillary Blood Collection Versus Venous Plasma and Cerebrospinal Fluid. Annals of Clinical and Translational Neurology, Article acn3.70524. https://doi.org/10.1002/acn3.70524

Image Credits: AI Generated

DOI: 10.1002/acn3.70524

Keywords: Alzheimer's disease, blood biomarkers, capillary blood collection, pTau217, GFAP, neurofilament light chain, amyloid beta 42/40, cerebrospinal fluid, Tasso+, TAPII, decentralized clinical trials, plasma biomarkers