26600 Alzheimer Disease Evaluation, Spinal Fluid (ADEVL)

​​​Amyloid Beta 42, Beta-Amyloid 42, ABeta, Total Tau, Phosphorylated Tau, pTau, tTau

​Assessment of adults with cognitive impairment being evaluated for Alzheimer disease and other causes of cognitive impairment.

These assays should not be used to predict the development of dementia or other neurologic conditions or to monitor response to therapies.​

​Patient Preparation: For 12 hours before specimen collection do not take multivitamins or dietary supplements containing biotin (vitamin B7), which is commonly found in hair, skin, and nail supplements and multivitamins.

Supplies: Alzheimer's Disease Evaluation (ADEVL) Collection Kit (T836)​

Collection Instructions:

1. Perform lumbar puncture and discard the first 1 to 2 mL of cerebrospinal fluid (CSF).

2. Collect CSF directly into one of the listed collection tubes until the tube is at least 50% full.*

3. Send CSF specimen in original collection tube. Do not aliquot.

Note: Polystyrene collection tubes are not acceptable. Exposure of CSF to polystyrene tubes may result in falsely low Abeta42 concentrations.

*The Alzheimer's Association consensus protocol for handling of CSF for clinical measurements of Abeta42 and tau recommends using the drip method for CSF collection and directly collecting into a low bind polypropylene tube. although some clinicians prefer the syringe pull method due to speed of collection, the drip method reduces the risk of Abeta42 binding to the plastic of any syringe used. 

Gross hemolysis

Gross icterus

​A positive cerebrospinal fluid (CSF) beta-amyloid 42 (Abeta42), total Tau (t-Tau), or phosphorylated Tau (p-Tau181) result, or p-Tau181/Abeta42 ratio does not establish a diagnosis of Alzheimer disease (AD) or other cognitive disorder. These results should be interpreted in combination with other clinical diagnostic and radiologic evaluations.

An abnormal p-Tau181/Abeta42 ratio in the context of a normal Abeta42 may be observed in some individuals. In some situations, interindividual differences in overall concentration of Abeta peptide production and/or in p-Tau elevation stemming from other neurodegenerative disease may result in an abnormal p-Tau181/Abeta42 ratio.

To achieve the best clinical performance (ie, keep patient misclassification rate at a minimum), it is important that the recommended pre-analytical protocol for sample collection is followed. Based on the Alzheimer's Association international guidelines for consensus handling of CSF for clinical measurements of Abeta42 and Tau, CSF should be collected using the drip method and directly collected into a low bind polypropylene tube. The low bind polypropylene tube should be filled to at least 80% of the tube volume capacity. Failure to adhere to this sample collection recommendation may impact the measured Abeta42 concentration and may influence the interpretation relation to the laboratory used cut-offs.

Additionally, it is recognized that using the recommended Alzheimer's Association drip method may not be feasible for every patient collection scenario. If a different method for CSF collection is used, it is critical that the CSF is collected directly into a low bind polypropylene tube and sent to Mayo Clinic Laboratories per test recommendations.

Improper specimen handling or interindividual differences in overall concentration of Abeta peptide production may yield an abnormally low Abeta42 in the context of a normal p-Tau181/Abeta42 ratio. Results should be interpreted in concordance with other clinical information.

Exposure of CSF to polystyrene tubes can reduce concentrations of the amyloid Abeta42 by as much as 20% to 50% due to adherence of the sticky amyloid protein to polystyrene tube surface material, potentially altering clinical interpretation, including the p-Tau181/Abeta 42 ratio. p-Tau181 and t-Tau protein do not substantially adhere to polystyrene collection tubes.

Failure to adhere to the specimen collection instructions provided may result in falsely low Abeta42 concentrations and potential misdiagnosis of AD.

The performance of the test for African American, Asian, and other races has high uncertainty due to the limited number of patients studied.

There is no high-dose hook effect at Abeta42 concentrations up to 6000 pg/mL, p-Tau181 concentrations up to 300 pg/mL, and t-Tau concentrations up to 4267 pg/mL.​

There is no interference from biotin up to concentrations of 1200 ng/mL for the Abeta42 and p-Tau assays. There is no interference from biotin up to concentrations of 50 ng/mL for the t-Tau assay.

In rare cases, some individuals can develop antibodies to mouse or other animal antibodies (often referred to as human anti-mouse antibodies [HAMA] or heterophile antibodies), which may cause interference in some immunoassays. The presence of antibodies to streptavidin or ruthenium can also rarely occur and may interfere in this assay. Caution should be used in interpretation of results, and the laboratory should be alerted if the result does not correlate with the clinical presentation.

​Two neuropathologic features observed in the brain of patients with Alzheimer disease (AD) dementia are the presence of plaques composed of beta-amyloid (Abeta) peptides and intracellular neurofibrillary tangles containing hyperphosphorylated Tau (tubulin-associated unit) proteins. These 2 groups of molecules are the most established biomarkers of the disease used in clinical and research practice. Positron emission tomography (PET) imaging using US Food and Drug Administration approved amyloid radiotracer (amyloid-PET) to visualize the presence of amyloid lesions in the cerebral cortex is available in some specialized centers. Measuring Abeta42 peptides and certain phosphorylated Tau (such as p-Tau181) proteins in cerebrospinal fluid (CSF) may be used as a means to assess the presence of amyloid pathology. In particular, the use of the p-Tau181/Abeta42 ratio has been shown to be an excellent surrogate marker of amyloid plaque burden, caused by increased deposition of beta-amyloid 1-42 in the brain. The use of these biomarkers has been included in the new consensus research diagnostic criteria for AD, mild cognitive impairment (MCI), and preclinical AD proposed by the National Institute on Aging and Alzheimer's Association Research Framework.

The CSF assays included in this evaluation are beta-amyloid (1-42; Abeta42), total-Tau (t-Tau), and phosphorylated-Tau (p-Tau181).

Abeta42 is approximately 4-kDa protein of 42 amino acids that is formed following proteolytic cleavage of a transmembrane protein known as amyloid precursor protein. Due to its hydrophobic nature, Abeta42 has the propensity to form aggregates and oligomers. Oligomers form fibrils that accumulate into amyloid plaques. These pathological changes in Abeta42 are reflected by the decrease in the CSF concentrations of Abeta42 and/or by the increase in the brain uptake of specific tracers during beta-amyloid PET.

Tau is present as six isoforms in human brain tissue. These isoforms are generated by alternative splicing of the pre-mRNA. The t-Tau assay measures all these isoforms. The most common post-translational modification of Tau proteins is phosphorylation. During neurodegeneration, abnormal phosphorylation leads to the formation of intracellular neurofibrillary tangles composed of the Tau protein that has undergone hyper-phosphorylation and developed aggregates of hyper-phosphorylated Tau proteins called p-Tau. The p-Tau assay detects phosphorylated Tau at threonine 181 (p-Tau181).

Pathological changes associated with AD are reflected by an increase in the CSF concentrations of t-Tau and p-Tau. Increases in CSF t-Tau reflect the intensity of the neuronal and axonal damage and degeneration and are associated with a faster progression from MCI to AD. Increases in CSF p-Tau concentrations are also associated with a faster progression from MCI to AD with more rapid cognitive decline in AD patients and in mild AD dementia cases.

The Alzheimer's Association has developed appropriate use criteria to guide safe and optimal use of CSF testing for AD pathology detection in the diagnostic process. The use of CSF biomarker testing may be indicated for the following patient groups:
1) Patients with subjective cognitive decline who are considered at increased risk for AD
2) Patients with MCI that is persistent, progressing, and unexplained
3) Patients with symptoms that suggest possible AD
4) MCI or dementia with an onset at an early age (less than 65)
5) Patients meeting core clinical criteria for probable AD with typical age of onset
6) Patients whose dominant symptom is a change in behavior (eg, Capgras syndrome, paranoid delusions, unexplained delirium, combative symptoms, and depression) and where AD diagnosis is being considered.

​A beta-amyloid (1-42; Abeta42) result greater than 834 pg/mL is consistent with a negative amyloid positron emission tomography (PET) scan. A negative amyloid PET scan indicates the presence of no or sparse neuritic plaques and is inconsistent with a neuropathological diagnosis of Alzheimer disease (AD). An Abeta42 result greater than 834 pg/mL is associated with a reduced likelihood that a patient's cognitive impairment is due to AD.

Total Tau (t-Tau) and phosphorylated Tau (p-Tau181) cerebrospinal fluid (CSF) concentrations increase approximately 2 to 3-times as much in patients with mild-moderate AD as compared to age-matched controls. A t-Tau and/or p-Tau181 concentration of less than or equal to 238 pg/mL and less than or equal to 21.6 pg/mL, respectively, reduces the likelihood that a patient's cognitive impairment is due to AD.

The use of p-Tau181/Abeta42 ratio provides better concordance with amyloid PET scan when compared to Abeta42, p-Tau181, and t-Tau individually. The p-Tau/Abeta42 ratio provides better concordance with amyloid PET imaging when compared to Abeta42, phospho-Tau and total-Tau individually. A cut-off of 0.028 provides optimal balance between negative percent agreement (NPA) and positive percent agreement (PPA) when compared to amyloid PET results. A p-Tau/Abeta42 ratio of 0.028 or less has a 92% NPA with normal amyloid PET. A ratio above 0.028 has a 92% PPA with abnormal amyloid PET.

High CSF t-Tau protein concentrations are found in other neurodegenerative diseases such as prion disease or Creutzfeldt-Jakob disease (CJD). In this situation, an elevated t-Tau concentration and an increased t-Tau to p-Tau ratio has a very high specificity for differential diagnoses of CJD.

 
This table and interpretations are based on the National Institute on Aging and Alzheimer's Association research framework diagnostic recommendations.