2. A quick, painless imaging scan that measures the thickness of the retinal nerve fiber layer in your left eye, helping your eye doctor evaluate the overall health of your optic nerve.
A quick, painless imaging scan that measures the thickness of the retinal nerve fiber layer in your left eye, helping your eye doctor evaluate the overall health of your optic nerve.
{"Uses light waves to create detailed images of your retina","Helps detect early signs of glaucoma and optic nerve damage","The test is quick, painless, and requires no blood sample","Results help your doctor monitor eye health over time","No special preparation is needed before the test"}
Key facts at a glance
- LOINC
- 86296-1
- Short code
- RNFL-CH4-OCT
- Synonyms
- L ret RNFL 4 o'clock thickness OCT
- Category
- cardiac
- Sample
- serum
- Container
- Not applicable - this is an imaging test, not a blood test
- Units
- N/A
- Ref low
- 95
- Ref high
- 145
- Critical range
- 60 N/A – 10000 N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- ≥ 18 years
- Method
- Optical Coherence Tomography (OCT)
- CPT
- 92134
- SNOMED
- 371632003
- Specimen volume
- Not applicable - imaging test
- Stability
- Not applicable - imaging test
- Hemolysis max
- 500
- Lipemia max
- 1000
- Icterus max
- 500
- TAT
- 1–24 hours
- Order
- routine
What is this test?
This test uses Optical Coherence Tomography (OCT) to measure the thickness of the retinal nerve fiber layer (RNFL) at a specific location (clock hour 4) in your left eye. The retinal nerve fiber layer contains nerve fibers that carry visual information from your retina to your brain. Measuring its thickness helps doctors assess the health of your optic nerve.
How is the test performed?
During the test, you will sit in front of an OCT machine. You will rest your chin on a support and look at a fixed target. The machine will scan your eye using light waves, creating detailed cross-sectional images of your retina. The process takes only a few minutes and is completely painless. No eye drops or blood draw are required for this imaging test.
Why is it important?
Measuring RNFL thickness helps doctors detect and monitor conditions that affect the optic nerve, particularly glaucoma. Glaucoma damages the optic nerve slowly over time, often without noticeable symptoms in early stages. This test can detect thinning of the RNFL even before vision loss occurs, allowing for early treatment.
Reference range
Retinal nerve fiber layer thickness at clock hour 4 typically ranges from 95-145 microns in healthy adults. Reference ranges may vary by instrument manufacturer and patient age. Values below 80 microns may indicate early glaucomatous damage.N/A
Pupil dilation may be required for optimal imaging. Patients should remove contact lenses before the procedure. No special dietary restrictions. Inform technician of any eye surgeries, laser treatments, or eye conditions.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Abnormal RNFL thickness at clock hour 4 in the left eye may indicate early glaucomatous damage, optic neuritis, or other optic nerve disorders. Thinning of the RNFL suggests nerve fiber loss, while thickening may occur with conditions like optic neuritis or retinal edema. Your doctor will compare your results with normal reference values and track changes over time to determine if treatment is needed.
Frequently asked questions
Does the test hurt?
How long does the test take?
Do I need to fast before this test?
When will I get my results?
How often do I need this test?
Sources
- 1
- 2
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.
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 17, 2026. · Read our editorial policy