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Total Tau Protein with CSF-to-Plasma Ratio (CSF/Plasma Ratio)

Published: August 9, 2026Updated: August 16, 2026

This test measures the ratio of tau protein in cerebrospinal fluid (CSF) compared to its level in blood plasma. Tau protein is a marker often studied in conditions affecting the brain’s nerve cells.

Quick summary

["The CSF-to-plasma tau ratio helps assess potential brain damage or diseases like Alzheimer’s disease."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
97102-8
Short code
p-tau181/AL beta42 CSF-Rto
Synonyms
p-tau181/AL beta42 CSF-Rto
Category
coagulation
Sample
csf
Container
Sterile tube
Units
N/A
Ref low
0.8
Ref high
1.2
Critical range
— – 5 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
CPT
85610
SNOMED
396252006
Specimen volume
1-2 mL
Stability
Immediate analysis required, Refrigerated: 1h
Hemolysis max
200
TAT
4–24 hours
Order
routine

Why This Test Is Done

Your doctor may order this test if you have symptoms like memory loss, confusion, or other neurological concerns. It can also be part of investigations for conditions like traumatic brain injury, brain tumors, or neurodegenerative diseases.

How the Test Works

A small amount of CSF is collected via lumbar puncture (spinal tap), while blood is drawn from a vein. The tau protein levels in both samples are measured and compared to determine the ratio.

What to Expect Before the Test

You may need to fast for a few hours before the spinal tap, depending on your doctor’s instructions. Avoid heavy lifting or strenuous activity for 24 hours before the procedure.

What to Expect During the Test

The spinal tap involves inserting a needle into your lower back to collect CSF. You may feel pressure or discomfort, but it’s usually well-tolerated. Blood tests are quick and done in a clinic or lab.

Reference range

INR: 0.8-1.2 (warfarin: 2.0-3.0)N/A

Specimen: csf

Inform your healthcare provider if you are taking blood thinners. No special preparation required.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

CSF Tau Ratio Test

Frequently asked questions

What does a high CSF tau ratio mean?
A high CSF tau ratio may suggest conditions like Alzheimer’s disease, traumatic brain injury, or other brain-related illnesses that damage nerve cells. Your doctor will discuss findings with you in detail.
How long does it take to get results?
Results from both CSF and plasma tests are typically ready within 1–3 business days, depending on lab processing times.
Can I eat or drink before the blood draw?
You can usually eat and drink normally before the blood draw, but avoid heavy meals or alcohol for 24 hours before the spinal tap (if applicable). Always follow your doctor’s specific instructions.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy