2. This imaging test uses optical coherence tomography (OCT) to measure the thickness of the retinal nerve fiber layer at the clock hour 3 position on your left retina, helping your eye doctor detect early signs of glaucoma and optic nerve damage.
This imaging test uses optical coherence tomography (OCT) to measure the thickness of the retinal nerve fiber layer at the clock hour 3 position on your left retina, helping your eye doctor detect early signs of glaucoma and optic nerve damage.
{"OCT uses light waves to create detailed cross-sectional images of the retina","The test measures the thickness of the retinal nerve fiber layer (RNFL)","Thinner RNFL may indicate glaucoma or other optic nerve diseases","The test is quick, non-invasive, and requires no special preparation","Results help your doctor monitor eye health and guide treatment decisions"}
Key facts at a glance
- LOINC
- 86295-3
- Short code
- RNFL-L3-OCT
- Synonyms
- L ret RNFL 3 o'clock thickness OCT
- Category
- cardiac
- Sample
- serum
- Container
- Not applicable - this is an imaging procedure, not a blood collection
- Units
- N/A
- Ref low
- 71
- Ref high
- 130
- Critical range
- 50 N/A – 10000 N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- ≥ 18 years
- Method
- Optical Coherence Tomography (OCT)
- CPT
- 92134
- SNOMED
- 397540003
- 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?
The Left Retina Retinal Nerve Fiber Layer (RNFL) Clock Hour 3 Thickness test by OCT is an imaging test that uses Optical Coherence Tomography to measure the thickness of the retinal nerve fiber layer at a specific location on your left retina. The 'clock hour 3' refers to the 3 o'clock position when viewing the retina, which corresponds to the nasal side of the eye.
How is the test performed?
During the test, you will sit in front of an OCT machine and place your chin on a support. The machine will scan your eye using harmless light waves to create detailed images of your retina and measure the RNFL thickness. The test takes only a few minutes and is completely painless. No eye drops or injections are required.
Why is this test important?
The retinal nerve fiber layer contains nerve fibers that carry visual information from your retina to your brain. Measuring its thickness helps doctors detect early changes that may indicate glaucoma, optic neuritis, or other conditions that can cause vision loss. Early detection allows for timely treatment to prevent further damage.
Reference range
Normal RNFL thickness at clock hour 3 ranges from 71-130 micrometers; values below 71 μm suggest localized nerve fiber layer thinningN/A
No special preparation required. Patient should remove contact lenses if worn. Pupillary dilation may be performed for better imaging quality. Inform technician of any eye surgery history or current eye conditions.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Abnormal RNFL thickness at clock hour 3 may indicate localized nerve fiber loss, which can be an early sign of glaucoma or other optic neuropathies. Comparing test results over time helps doctors track disease progression and evaluate treatment effectiveness. Thinner-than-normal RNFL may require closer monitoring or adjusted treatment plans.
Frequently asked questions
Do I need to fast before this test?
How long does the test take?
Will the test hurt?
Can I wear my glasses or contact lenses?
How often should I have 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