Genetic risk scores enhance the diagnostic value of plasma biomarkers of brain amyloidosis.

Blood-based biomarkers offer strong potential to revolutionize diagnosis, trial enrolment and treatment monitoring in Alzheimer's disease (AD). However, further advances are needed before these biomarkers can achieve wider deployment beyond selective research studies and specialty memory clinics, including the development of frameworks for optimal interpretation of biomarker profiles. We hypothesized that integrating Alzheimer's disease genetic risk score (AD-GRS) data would enhance the diagnostic value of plasma AD biomarkers by better capturing extant disease heterogeneity. Analysing 962 individuals from a population-based sample, we observed that an AD-GRS was independently associated with amyloid PET levels (an early marker of AD pathophysiology) over and above APOE ε4 or plasma p-tau181, amyloid-ß42/40, glial fibrillary acidic protein or neurofilament light chain. Among individuals with a high or moderately high plasma p-tau181, integrating AD-GRS data significantly improved classification accuracy of amyloid PET positivity, including the finding that the combination of a high AD-GRS and high plasma p-tau181 outperformed p-tau181 alone in classifying amyloid PET positivity (88% versus 68%; P = 0.001). A machine learning approach incorporating plasma biomarkers, demographics and the AD-GRS was highly accurate in predicting amyloid PET levels (90% training set; 89% test set) and Shapley value analyses (an explainer method based in cooperative game theory) indicated that the AD-GRS and plasma biomarkers had differential importance in explaining amyloid deposition across individuals. Polygenic risk for AD dementia appears to account for a unique portion of disease heterogeneity, which could non-invasively enhance the interpretation of blood-based AD biomarker profiles in the population.

Predicting amyloid PET and tau PET stages with plasma biomarkers.

Staging the severity of Alzheimer's disease pathology using biomarkers is useful for therapeutic trials and clinical prognosis. Disease staging with amyloid and tau PET has face validity; however, this would be more practical with plasma biomarkers. Our objectives were, first, to examine approaches for staging amyloid and tau PET and, second, to examine prediction of amyloid and tau PET stages using plasma biomarkers. Participants (n = 1136) were enrolled in either the Mayo Clinic Study of Aging or the Alzheimer's Disease Research Center; had a concurrent amyloid PET, tau PET and blood draw; and met clinical criteria for cognitively unimpaired (n = 864), mild cognitive impairment (n = 148) or Alzheimer's clinical syndrome with dementia (n = 124). The latter two groups were combined into a cognitively impaired group (n = 272). We used multinomial regression models to estimate discrimination [concordance (C) statistics] among three amyloid PET stages (low, intermediate, high), four tau PET stages (Braak 0, 1-2, 3-4, 5-6) and a combined amyloid and tau PET stage (none/low versus intermediate/high severity) using plasma biomarkers as predictors separately within unimpaired and impaired individuals. Plasma analytes, p-tau181, Aß1-42 and Aß1-40 (analysed as the Aß42/Aß40 ratio), glial fibrillary acidic protein and neurofilament light chain were measured on the HD-X Simoa Quanterix platform. Plasma p-tau217 was also measured in a subset (n = 355) of cognitively unimpaired participants using the Lilly Meso Scale Discovery assay. Models with all Quanterix plasma analytes along with risk factors (age, sex and APOE) most often provided the best discrimination among amyloid PET stages (C = 0.78-0.82). Models with p-tau181 provided similar discrimination of tau PET stages to models with all four plasma analytes (C = 0.72-0.85 versus C = 0.73-0.86). Discriminating a PET proxy of intermediate/high from none/low Alzheimer's disease neuropathological change with all four Quanterix plasma analytes was excellent but not better than p-tau181 only (C = 0.88 versus 0.87 for unimpaired and C = 0.91 versus 0.90 for impaired). Lilly p-tau217 outperformed the Quanterix p-tau181 assay for discriminating high versus intermediate amyloid (C = 0.85 versus 0.74) but did not improve over a model with all Quanterix plasma analytes and risk factors (C = 0.85 versus 0.83). Plasma analytes along with risk factors can discriminate between amyloid and tau PET stages and between a PET surrogate for intermediate/high versus none/low neuropathological change with accuracy in the acceptable to excellent range. Combinations of plasma analytes are better than single analytes for many staging predictions with the exception that Quanterix p-tau181 alone usually performed equivalently to combinations of Quanterix analytes for tau PET discrimination.

Optimizing cutpoints for clinical interpretation of brain amyloid status using plasma p-tau217 immunoassays.

INTRODUCTION: We aimed to evaluate clinical interpretation cutpoints for two plasma phosphorylated tau (p-tau)217 assays (ALZpath and Lumipulse) as predictors of amyloid status for implementation in clinical practice. METHODS: Clinical performance of plasma p-tau217 against amyloid positron emission tomography status was evaluated in participants with mild cognitive impairment or mild dementia (n = 427). RESULTS: Using a one-cutpoint approach (negative/positive), neither assay achieved ≥ 90% in both sensitivity and specificity. A two-cutpoint approach yielding 92% sensitivity and 96% specificity provided the desired balance of false positives and false negatives, while categorizing 20% and 39% of results as indeterminate for the Lumipulse and ALZpath assays, respectively. DISCUSSION: This study provides a systematic framework for selection of assay-specific cutpoints for clinical use of plasma p-tau217 for determination of amyloid status. Our findings suggest that a two-cutpoint approach may have advantages in optimizing diagnostic accuracy while minimizing potential harm from false positive results. HIGHLIGHTS: Phosphorylated tau (p-tau)217 cutpoints for detection of amyloid pathology were established. A two-cutpoint approach exhibited the best performance for clinical laboratory use. p-tau217 assays differed in the percentage of results categorized as intermediate.

Modeling the temporal evolution of plasma p-tau in relation to amyloid beta and tau PET.

INTRODUCTION: The timing of plasma biomarker changes is not well understood. The goal of this study was to evaluate the temporal co-evolution of plasma and positron emission tomography (PET) Alzheimer's disease (AD) biomarkers. METHODS: We included 1408 Mayo Clinic Study of Aging and Alzheimer's Disease Research Center participants. An accelerated failure time (AFT) model was fit with amyloid beta (Aß) PET, tau PET, plasma p-tau217, p-tau181, and glial fibrillary acidic protein (GFAP) as endpoints. RESULTS: Individual timing of plasma p-tau progression was strongly associated with Aß PET and GFAP progression. In the population, GFAP became abnormal first, then Aß PET, plasma p-tau, and tau PET temporal meta-regions of interest when applying cut points based on young, cognitively unimpaired participants. DISCUSSION: Plasma p-tau is a stronger indicator of a temporally linked response to elevated brain Aß than of tau pathology. While Aß deposition and a rise in GFAP are upstream events associated with tau phosphorylation, the temporal link between p-tau and Aß PET was the strongest. HIGHLIGHTS: Plasma p-tau progression was more strongly associated with Aß than tau PET. Progression on plasma p-tau was associated with Aß PET and GFAP progression. P-tau181 and p-tau217 become abnormal after Aß PET and before tau PET. GFAP became abnormal first, before plasma p-tau and Aß PET.

Vascular risk, gait, behavioral, and plasma indicators of VCID.

INTRODUCTION: Cost-effective screening tools for vascular contributions to cognitive impairment and dementia (VCID) has significant implications. We evaluated non-imaging indicators of VCID using magnetic resonance imaging (MRI)-measured white matter (WM) damage and hypothesized that these indicators differ based on age. METHODS: In 745 participants from the Mayo Clinic Study of Aging (≥50 years of age) with serial WM assessments from diffusion MRI and fluid-attenuated inversion recovery (FLAIR)-MRI, we examined associations between baseline non-imaging indicators (demographics, vascular risk factors [VRFs], gait, behavioral, plasma glial fibrillary acidic protein [GFAP], and plasma neurofilament light chain [NfL]) and WM damage across three age tertiles. RESULTS: VRFs and gait were associated with diffusion changes even in low age strata. All measures (VRFs, gait, behavioral, plasma GFAP, plasma NfL) were associated with white matter hyperintensities (WMHs) but mainly in intermediate and high age strata. DISCUSSION: Non-imaging indicators of VCID were related to WM damage and may aid in screening participants and assessing outcomes for VCID. HIGHLIGHTS: Non-imaging indicators of VCID can aid in prediction of MRI-measured WM damage but their importance differed by age. Vascular risk and gait measures were associated with early VCID changes measured using diffusion MRI. Plasma markers explained variability in WMH across age strata. Most non-imaging measures explained variability in WMH and vascular WM scores in intermediate and older age groups. The framework developed here can be used to evaluate new non-imaging VCID indicators proposed in the future.

Comparison of plasma biomarkers and amyloid PET for predicting memory decline in cognitively unimpaired individuals.

BACKGROUND: We compared the ability of several plasma biomarkers versus amyloid positron emission tomography (PET) to predict rates of memory decline among cognitively unimpaired individuals. METHODS: We studied 645 Mayo Clinic Study of Aging participants. Predictor variables were age, sex, education, apolipoprotein E (APOE) ε4 genotype, amyloid PET, and plasma amyloid beta (Aß)42/40, phosphorylated tau (p-tau)181, neurofilament light (NfL), glial fibrillary acidic protein (GFAP), and p-tau217. The outcome was a change in a memory composite measure. RESULTS: All plasma biomarkers, except NfL, were associated with mean memory decline in models with individual biomarkers. However, amyloid PET and plasma p-tau217, along with age, were key variables independently associated with mean memory decline in models combining all predictors. Confidence intervals were narrow for estimates of population mean prediction, but person-level prediction intervals were wide. DISCUSSION: Plasma p-tau217 and amyloid PET provide useful information about predicting rates of future cognitive decline in cognitively unimpaired individuals at the population mean level, but not at the individual person level.

Plasma biomarkers of Alzheimer's disease in the continuum of dementia with Lewy bodies.

INTRODUCTION: Patients with dementia with Lewy bodies (DLB) may have Alzheimers disease (AD) pathology that can be detected by plasma biomarkers. Our objective was to evaluate plasma biomarkers of AD and their association with positron emission tomography (PET) biomarkers of amyloid and tau deposition in the continuum of DLB, starting from prodromal stages of the disease. METHODS: The cohort included patients with isolated rapid eye movement (REM) sleep behavior disorder (iRBD), mild cognitive impairment with Lewy bodies (MCI-LB), or DLB, with a concurrent blood draw and PET scans. RESULTS: Abnormal levels of plasma glial fibrillary acidic protein (GFAP) were found at the prodromal stage of MCI-LB in association with increased amyloid PET. Abnormal levels of plasma phosphorylated tau (p-tau)-181 and neurofilament light (NfL) were found at the DLB stage. Plasma p-tau-181 showed the highest accuracy in detecting abnormal amyloid and tau PET in patients with DLB. DISCUSSION: The range of AD co-pathology can be detected with plasma biomarkers in the DLB continuum, particularly with plasma p-tau-181 and GFAP.

Identification, Measurement, and Assessment of the Clinical Utility of Human Pancreatic Polypeptide by Liquid Chromatography-Tandem Mass Spectrometry.

Human pancreatic polypeptide (HPP) is a 36 amino acid peptide hormone that plays a role in the bidirectional communication between the digestive system and the brain. HPP measurements are used to assess vagal nerve function following sham feeding and to detect gastroenteropancreatic-neuroendocrine tumors. These tests have historically been conducted by radioimmunoassays, but liquid chromatography-tandem mass spectrometry (LC-MS/MS) has several advantages such as improved specificity and elimination of radioactive molecules. Here, we present our LC-MS/MS method. Initially, samples were immunopurified and subjected to LC-high resolution accurate mass tandem mass spectrometry (HRAM-MS/MS) to identify circulating forms of the peptide in human plasma. We identified 23 forms of HPP, including several glycosylated forms. The most abundant peptides then were used for targeted LC-MS/MS measurements. LC-MS/MS performance for precision, accuracy, linearity, recovery, limit of detection, and carryover met our acceptance criteria based on CLIA regulations. Additionally, we observed the expected physiological rise in HPP in response to sham feeding. Our results indicate that HPP measurement by LC-MS/MS produces clinically equivalent results to our established immunoassay when several peptides are monitored, making it a suitable replacement. The measurement of peptide fragments, including modified species, might have additional clinical value.

The effect of burosumab on intact and C-terminal FGF23 measurements.

OBJECTIVE: To investigate the effect of CRYSVITA® (burosumab-twza) on FGF23 measurements in an intact and a C-terminal immunoassay. METHODS: An intact serum FGF23 (MedFrontier) and a C-terminal plasma FGF23 assay (Immutopics) were used. Serum/plasma pools were spiked to span the burosumab therapeutic range (1.4-11.3 µg/ml) and FGF23 recovery was assessed. Patient serum and plasma samples obtained pre and post-burosumab treatment were evaluated on both assays and compared with corresponding phosphorus measurements RESULTS: Spiking burosumab (1.4-11.3 µg/ml) into sample pools resulted in a dose-dependent negative analytical interference on intact FGF23 measurements and no significant interference for C-terminal FGF23 measurements. However, more than a 500-fold median increase (post- vs. pre-burosumab administration) in in vivo FGF23 concentrations were observed by both assays. CONCLUSIONS: Therapeutic concentrations of burosumab result in a negative analytical interference of the intact, but not the C-terminal FGF23 immunoassay. Despite this in vitro analytical interference in the intact assay, relatively large elevations of both intact FGF23 and C-terminal FGF23 measurements were observed in vivo following burosumab administration. Following burosumab administration, FGF23 measurements must be interpreted within the clinical context of the patient and other relevant biomarker results. SUMMARY: This article describes a negative analytical interference by burosumab in an intact FGF23 immunoassay. The recovery of C-terminal FGF23 is not significantly affected by the presence of burosumab. In vivo, both assays demonstrate extreme FGF23 elevations in the presence of the drug. Furthermore, the measurement of FGF23 blocked by burosumab is not clinically useful regarding hypophosphataemia.

Plasma biomarkers for prediction of Alzheimer's disease neuropathologic change.

While plasma biomarkers for Alzheimer's disease (AD) are increasingly being evaluated for clinical diagnosis and prognosis, few population-based autopsy studies have evaluated their utility in the context of predicting neuropathological changes. Our goal was to investigate the utility of clinically available plasma markers in predicting Braak staging, neuritic plaque score, Thal phase, and overall AD neuropathological change (ADNC).We utilized a population-based prospective study of 350 participants with autopsy and antemortem plasma biomarker testing using clinically available antibody assay (Quanterix) consisting of Aß42/40 ratio, p-tau181, GFAP, and NfL. We utilized a variable selection procedure in cross-validated (CV) logistic regression models to identify the best set of plasma predictors along with demographic variables, and a subset of neuropsychological tests comprising the Mayo Clinic Preclinical Alzheimer Cognitive Composite (Mayo-PACC). ADNC was best predicted with plasma GFAP, NfL, p-tau181 biomarkers along with APOE ε4 carrier status and Mayo-PACC cognitive score (CV AUC = 0.798). Braak staging was best predicted using plasma GFAP, p-tau181, and cognitive scores (CV AUC = 0.774). Neuritic plaque score was best predicted using plasma Aß42/40 ratio, p-tau181, GFAP, and NfL biomarkers (CV AUC = 0.770). Thal phase was best predicted using GFAP, NfL, p-tau181, APOE ε4 carrier status and Mayo-PACC cognitive score (CV AUC = 0.754). We found that GFAP and p-tau provided non-overlapping information on both neuritic plaque and Braak stage scores whereas Aß42/40 and NfL were mainly useful for prediction of neuritic plaque scores. Separating participants by cognitive status improved predictive performance, particularly when plasma biomarkers were included. Plasma biomarkers can differentially inform about overall ADNC pathology, Braak staging, and neuritic plaque score when combined with demographics and cognitive variables and have significant utility for earlier detection of AD.

Intact Fibroblast Growth Factor 23 Concentrations in Hypophosphatemic Disorders.

OBJECTIVE: Evaluation of circulating fibroblast growth factor 23 (FGF23) concentrations plays a key role in the differential diagnosis of patients presenting with hypophosphatemia. FGF23 concentrations obtained by different immunoassays are not comparable and subsequently, differences in the clinical performance of the assays might arise. In this study, we evaluated the clinical performance of the Medfrontier FGF23 Intact immunoassay (MedFrontier, Minaris Medical Co, Ltd, Tokyo, Japan) in clinically relevant hypophosphatemic conditions. METHODS: Intact FGF23 (iFGF23) was measured in serum samples from 61 patients with FGF23-dependent hypophosphatemia (42-tumor induced osteomalacia [TIO] and 19-X-linked hypophosphatemia [XLH]); 8 patients with FGF23-independent hypophosphatemia (6-Fanconi Syndrome and 2-Vitamin D dependent rickets); 10 normophosphatemic patients; 15 chronic kidney disease (CKD) stage-2/3 and 20 CKD stage-4/5 patients; and a healthy control population. Disease-specific differences in measured iFGF23 concentrations and FGF23 concentration association with phosphate concentrations were reported. RESULTS: iFGF23 concentrations were significantly elevated in 90% and 84% of TIO and XLH hypophosphatemia patients as compared to healthy controls (both TIO and XLH, P = .0001). There was no significant correlation between iFGF23 and phosphate concentrations (P = .74 and P = .86) for TIO and XLH, respectively. Patients with CKD showed a significant increase in serum iFGF23 as the estimated glomerular filtration rate decreased (ρ = -0.79, P ≤ 0.0001). CONCLUSIONS: This study evaluated the clinical performance of the MedFrontier iFGF23 assay in a large cohort of XLH and TIO Caucasian and Asian patients. The clinical sensitivity of this iFGF23 assay is appropriate for clinical use.

An Update on Laboratory-Based Diagnostic Biomarkers for Multiple Sclerosis and Beyond.

BACKGROUND: Multiple sclerosis (MS) is an immune-mediated central nervous system (CNS) inflammatory demyelinating disease in which analysis of clinical presentation, imaging studies, and laboratory tests aid in diagnosis. CONTENT: This review discusses laboratory tests ordered to rule out and rule in MS, such as the traditional measurement of cerebrospinal fluid (CSF) IgG index and oligoclonal bands. Biomarkers discovered in the past 2 decades, such as aquaporin-4 (AQP4) antibodies and myelin oligodendrocyte glycoprotein (MOG) antibodies, have been incorporated into clinical practice in the diagnosis of disorders referred to as MS mimics. The importance of test selection, assay methodology, optimal sample for testing, and diagnostic utility of these biomarkers is reviewed. Other laboratory testing that can aid in the differentiation between MS and these biomarker-defined CNS demyelinating diseases is described. There is a focus on emerging biomarkers such as the use of kappa immunoglobulin free light chain concentration in CSF and kappa CSF index measurement as an alternative to oligoclonal bands which has a potential for an improvement in laboratory workflows. Finally, the role of biomarkers of disease activity and prognosis are discussed, including neurofilament light chain, glial fibrillary acidic protein, and myelin basic protein. Future perspectives with improved laboratory testing tools and discovery of additional biomarkers are provided. SUMMARY: Laboratory testing for demyelinating disorders using CSF and serum are routine practices that can benefit from an update, as novel biomarker-defined entities have reduced the potential for MS misdiagnosis, and CSF/serum biomarkers reinstated in the diagnostic criteria of MS.

Comparison of intact protein and digested peptide techniques for high throughput proteotyping of ApoE.

INTRODUCTION: Apolipoprotein E (ApoE) genotyping has been shown to have diagnostic value in the evaluation of cardiovascular diseases and neurodegenerative disorders such as Alzheimer's disease. Although genetic testing is well established for this application, liquid chromatography-mass spectrometry (LC-MS) has the potential to provide a high throughput, low-cost alternative for ApoE evaluation. METHODS: Serum samples were analyzed by peptide, intact protein, and genomic techniques. For peptide analysis, samples were digested with trypsin followed by liquid chromatography-tandem mass spectrometry analysis (LC-MS/MS) using a high-throughput multichannel LC system coupled to a Sciex 7500 mass spectrometer. For intact protein analysis, ApoE was immuno-purified using a monoclonal antibody immobilized on magnetic beads followed by high-resolution LC-MS analysis using an Exploris 480. DNA was extracted and evaluated using Sanger sequencing as a reference method. RESULTS AND DISCUSSION: The peptide measurement method produced one discrepant result when compared to genomic sequencing (out of 38 sequenced samples), whereas the intact protein analysis followed by deconvolution resulted in two discrepant results and when the intact protein data was processed with chromatographic integration there were three discrepant results. Therefore, the intact protein method proved slightly less accurate, required longer analysis time, and is substantially more costly, while providing only a 30 min improvement in sample preparation time. CONCLUSIONS: With current MS technology clinical laboratories appear to be better served to utilize trypsin digest sample preparation and LC-MS/MS as opposed to high-resolution LC-MS intact protein analysis techniques for evaluation of ApoE proteotype. Peptide analysis methods are capable of producing accurate results with high throughput and minimal cost.

Leucine Zipper 4 Autoantibody: A Novel Germ Cell Tumor and Paraneoplastic Biomarker.

OBJECTIVE: This study was undertaken to describe a novel biomarker of germ cell tumor and associated paraneoplastic neurological syndrome (PNS). METHODS: Archival sera from patients with germ cell tumor-associated PNS were evaluated. We identified a common autoantigen in a human testicular cancer cell line (TCam-2) by Western blot and mass spectrometry. Its identity was confirmed by recombinant-protein Western blot, enzyme-linked immunosorbent assay (ELISA), and cell-based assay. Autoantibody specificity was confirmed by analyzing assorted control sera/cerebrospinal fluid. RESULTS: Leucine zipper 4 (LUZP4)-immunoglobulin G (IgG) was detected in 28 patients' sera, 26 of whom (93%) were men. The median age at neurological symptom onset was 45 years (range = 28-84). Median titer (ELISA) was 1:300 (1:50 to >1:6,400, normal value < 1:50). Coexistent kelchlike protein 11-IgG was identified in 18 cases (64%). The most common presenting phenotype was rhombencephalitis (17/28, 61%). Other presentations included limbic encephalitis (n = 5, 18%), seizures and/or encephalitis (n = 2, 7%), and motor neuronopathy/polyradiculopathy (n = 4, 14%). The most common malignancy among cancer-evaluated PNS patients was seminoma (21/27, 78%). Nine of the 21 seminomas detected by whole-body fluorodeoxyglucose positron emission tomography scan (43%) were extratesticular. Both female patients had ovarian teratoma. Regressed testicular germ cell tumors were found in 4 patients. Exposure of T-cell-dendritic-cell cocultures from chronic immunosuppression-naïve LUZP4-IgG-seropositive patients to recombinant LUZP4 protein evoked a marked increase in CD69 expression on both CD4+ and CD8+ T cells when compared to vehicle-exposed and healthy control cultures. INTERPRETATION: LUZP4-IgG represents a novel serological biomarker of PNS and has high predictive value for germ cell tumors. The demonstrated antigen-specific T-cell responses support a CD8+ T-cell-mediated cytotoxic paraneoplastic and antitumor potential. ANN NEUROL 2021;89:1001-1010.

Alzheimer's disease cerebrospinal fluid biomarkers differentiate patients with Creutzfeldt-Jakob disease and autoimmune encephalitis.

BACKGROUND AND PURPOSE: Autoimmune encephalitis (AE) is a potentially treatable cause of rapidly progressive dementia that may mimic Creutzfeldt-Jakob disease (CJD). Alzheimer disease (AD) cerebrospinal fluid (CSF) biomarkers may discriminate CJD from AD, but utility in discriminating CJD and AE is unclear. This study compared AD CSF biomarkers in CJD and AE. METHODS: Patients with probable or definite CJD and probable or definite AE who underwent Roche Elecsys AD CSF biomarker testing at Mayo Clinic from March 2020 through April 2021 were included. Total-tau, phosphorylated181 tau and amyloid-ß42 levels were compared. RESULTS: Of 11 CJD cases, four were autopsy proven; the rest had positive real-time quaking-induced conversion testing. Disease-associated autoantibodies were detected in 8/15 cases of AE: leucine-rich glioma-inactivated 1 and neuronal intermediate filaments (two cases each), and N-methyl-d-aspartate receptor, contactin-associated protein-like 2, dipeptidyl-peptidase-like protein 6 and immunoglobulin-like cell adhesion molecule IgLON family member 5. Total-tau provided excellent discrimination between CJD and AE in a univariate model (odds ratio 1.46 per 100 pg/ml, 95% confidence interval 1.17-2.11, p < 0.05, c = 0.93). Total-tau was elevated in 91% of CJD cases (median > 1300, range 236->1300 pg/ml), of which 55% were above the limit of assay measurement (>1300 pg/ml). Total-tau was elevated in 20% of AE cases (median 158, range 80->1300 pg/ml). CONCLUSION: Total-tau was greater in CJD than AE. Given that amyloid-ß42 and phosphorylated181 tau were comparable, the ratio differences were probably driven by elevated total-tau in CJD. This study supports the role for AD biomarker testing in patients with rapidly progressive dementia.

Associations of amyloid and neurodegeneration plasma biomarkers with comorbidities.

INTRODUCTION: Blood-based biomarkers of amyloid pathology and neurodegeneration are entering clinical use. It is critical to understand what factors affect the levels of these markers. METHODS: Plasma markers (Aß42, Aß40, NfL, T-tau, Aß42/40 ratio) were measured on the Quanterix Simoa HD-1 analyzer for 996 Mayo Clinic Study of Aging (MCSA) participants, aged 51 to 95 years. All other data were collected during in-person MCSA visits or abstracted from the medical record. RESULTS: Among cognitively unimpaired (CU) participants, all plasma markers correlated with age. Linear regression models revealed multiple relationships. For example, higher Charlson Comorbidity Index and chronic kidney disease were associated with higher levels of all biomarkers. Some relationships differed between mild cognitive impairment and dementia participants. DISCUSSION: Multiple variables affect plasma biomarkers of amyloid pathology and neurodegeneration among CU in the general population. Incorporating this information is critical for accurate interpretation of the biomarker levels and for the development of reference ranges.

Detection of Alzheimer's disease amyloid beta 1-42, p-tau, and t-tau assays.

INTRODUCTION: We aimed to provide cut points for the automated Elecsys Alzheimer's disease (AD) cerebrospinal fluid (CSF) biomarkers. METHODS: Cut points for Elecsys amyloid beta 42 (Aß42), total tau (t-tau), hyperphosphorylated tau (p-tau), and t-tau/Aß42 and p-tau/Aß42 ratios were evaluated in Mayo Clinic Study of Aging (n = 804) and Mayo Clinic Alzheimer's Disease Research Center (n = 70) participants. RESULTS: The t-tau/Aß42 and p-tau/Aß42 ratios had a higher percent agreement with normal/abnormal amyloid positron emission tomography (PET) than the individual CSF markers. Reciever Operating Characteristic (ROC)-based cut points were 0.26 (0.24-0.27) for t-tau/Aß42 and 0.023 (0.020-0.025) for p-tau/Aß42. Ratio cut points derived from other cohorts performed as well in our cohort as our own did. Individual biomarkers had worse diagnostic properties and more variable results in terms of positive and negative percent agreement (PPA and NPA). CONCLUSION: CSF t-tau/Aß42 and p-tau/Aß42 ratios are very robust indicators of AD. For individual biomarkers, the intended use should determine which cut point is chosen.

Comparison of CSF phosphorylated tau 181 and 217 for cognitive decline.

INTRODUCTION: The prognostic utility of cerebrospinal fluid (CSF) phosphorylated tau 217 (p-tau217) and p-tau181 is not understood. METHODS: Analyses included 753 Mayo Clinic Study on Aging participants (median age = 71.6; 57% male). CSF amyloid beta (Aß)42 and p-tau181 were measured with Elecsys immunoassays. CSF p-tau181 and p-tau217 were also measured with Meso Scale Discovery (MSD). We used Cox proportional hazards models for risk of mild cognitive impairment (MCI) and linear mixed models for risk of global and domain-specific cognitive decline and cortical thickness. Analyses were stratified by elevated brain amyloid based on CSF Aß42 or amyloid positron emission tomography for those with imaging. RESULTS: CSF p-tau217 was superior to p-tau181 for the diagnosis of Alzheimer's disease (AD) pathology. CSF MSD p-tau181 and p-tau217 were associated with risk of MCI among amyloid-positive individuals. Differences between CSF p-tau measures predicting cortical thickness were subtle. DISCUSSION: There are subtle differences for CSF p-tau217 and p-tau181 as prognostic AD markers.

False Positives in Thyroglobulin Determinations Due to the Presence of Heterophile Antibodies: An Underrecognized and Consequential Clinical Problem.

OBJECTIVE: To report a case series of thyroid cancer patients in whom false positive results in immunometric assays for thyroglobulin (TgIMA) were caused by heterophilic antibody interference, describe the clinical scenario in which this interference should be suspected, and recommend methods to demonstrate the interference. METHODS: Three patients with unexpectedly elevated thyroglobulin results (range, 1.6-75 ng/mL) were studied. In the first patient, thyroglobulin was elevated despite the presence of Tg antibody. In the second patient, suppressed thyroglobulin was higher than a recent stimulated thyroglobulin. In the third patient, thyroglobulin became detectable years after treatment and did not change after thyroid-stimulating hormone stimulation. TgIMA concentration determination was compared to determination by a mass spectrometry method (TgMS). Thyroglobulin was also remeasured after preabsorption with heterophile antibody blocking reagents and after serial dilutions. RESULTS: In all cases, thyroglobulin was undetectable by TgMS. In 2 of 3 patients, dilutions provided nonlinear thyroglobulin results. After blocking agent preabsorption, thyroglobulin dropped by 35%, 45%, and 91% in the 3 samples. CONCLUSION: False positive thyroglobulin concentrations from heterophilic antibody interference have significant impact on the management of thyroid cancer. Here we show that TgMS assays can be used to rule out heterophilic antibody interference. This interference should be suspected when a detectable thyroglobulin by TgIMA does not respond to thyroid-stimulating hormone or is discordant from the clinical assessment.

Hyperphosphatemia with elevated serum PTH and FGF23, reduced 1,25(OH)2D and normal FGF7 concentrations characterize patients with CKD.

BACKGROUND: Hyperphosphatemia confers adverse cardiovascular outcomes, and commonly occurs in late-stage CKD. Fibroblast growth factor 7 (FGF7) is a phosphaturic peptide which decreases renal phosphate transport in vitro and in vivo. Serum FGF7 concentrations are reduced in hyperphosphatemic patients with hypophosphatasia and are elevated in some hypophosphatemic patients with tumor-induced osteomalacia. No data, however, are available on whether circulating FGF7 concentrations increase to compensate for phosphate retention in CKD patients. METHODS: This was a cross-sectional study performed among 85 adult patients with varying estimated glomerular filtration rates (eGFR). We measured serum intact FGF7 (iFGF7) concentration using an iFGF7 immunoassay and determined its associated factors. Relationships between eGFR and mineral metabolism biomarkers [phosphate, iFGF7, iFGF23, parathyroid hormone (PTH), and 1,25-dihydroxyvitamin D (1,25(OH)2D)] were explored. RESULTS: For eGFRs of ≥ 60 (n = 31), 45-59 (n = 16), 30-44 (n = 11), 15-29 (n = 15), and < 15 mL/min/1.73 m2 (n = 12), median (IQ25-75) iFGF7 concentrations were 46.1 (39.2-56.9), 43.1 (39.0-51.5), 47.3 (38.3-66.5), 47.7 (37.7-55.8), and 49.6 (42.5-65.6) pg/mL, respectively (P = 0.62). Significant increases in serum iFGF23, PTH, and phosphate were observed at eGFRs of < 33 (95 % CI, 26.40-40.05), < 29 (95 % CI, 22.51-35.36), and < 22 mL/min/1.73 m2 (95 % CI, 19.25-25.51), respectively, while significant decreases in serum 1,25(OH)2D were observed at an eGFR of < 52 mL/min/1.73 m2 (95 % CI, 42.57-61.43). No significant correlation was found between serum iFGF7 and phosphate, iFGF23, PTH or 1,25(OH)2D. In multivariable analyses, body mass index (per 5 kg/m2 increase) was independently associated with the highest quartile of serum iFGF7 concentration (OR, 1.20; 95 % CI, 1.12-1.55). CONCLUSIONS: Compensatory decreases in circulating 1,25(OH)2D and increases in circulating iFGF23 and PTH, but not iFGF7, facilitate normalization of serum phosphate concentration in early stages of CKD. Whether other circulating phosphaturic peptides change in response to phosphate retention in CKD patients deserves further study.