Type of test : Cerebrospinal fluid (CSF) analysis
 Normal findings
> 450 units/L
Test explanation and related physiology
 This test is used to help diagnose Alzheimer disease (AD) and other forms of neurodegenerative diseases. Amyloid beta protein is formed by cleavage of amyloid precursor protein (APP). There are many isoforms of sBPP depending on how many amino acids are cleaved into the form (30-51). The 42-amino-acid peptide (AB42 ) and AB40 are most significant to the diagnosis of AD. These beta amyloid proteins have been shown to be neurotrophic and neuroprotective. Beta amyloid is deposited on the brain in the form of plaques in patients with AD. As a result of this deposition, levels of beta amyloid are decreased in the cerebrospinal fluid of patients with AD and other forms of dementia. The AB42 / AB40 ratio in CSF may be more accurate in the diagnosis of AD. This test is often performed as a diagnostic panel that includes Apolipoprotein E  testing.
Ongoing research has also focused on using CSF levels of tau protein as another biochemical marker for AD. Serum tau protein levels can also be quantified but are not as accurate of a predictor of dementia as CSF tau levels. However, plasma tau protein is more easily obtained and can be used to determine risk of dementia.
 Recently, PET scanning using amyloid and tau specific markers has shown promise for the differential diagnosis of AD . Pittsburgh Agent B (PIB) appears to reliably detect brain amyloid caused by the accumulation of A beta 42 within plaques. Studies so far have revealed high levels of amyloid retention in the brain at prodromal stages of AD and the possibility of discriminating AD from other dementia disorders by scanning with PIB. Because amyloid accumulation is one of the earliest signs of AD, early diagnosis may be facilitated by identifying amyloid early in the disease progression, perhaps before symptoms emerge.
Procedure and patient care
Before
* Explain the procedure to the patient.
• Refer to the instructions for a lumbar puncture and CSF examination .
During
• Collect a CSF specimen as per the lumbar puncture discussion.
After
 • Follow the postprocedure guidelines after a lumbar puncture.
Abnormal findings
 Decreased levels
 Alzheimer disease