Complete Blood Count (CBC) Test: What You Need to Know
A non-invasive eye imaging test that measures the thickness of the retinal nerve fiber layer at a specific location to assess optic nerve health.
{"Uses optical coherence tomography (OCT) to capture detailed images of the retina","Measures retinal nerve fiber layer thickness at the 6 o'clock position (lower portion)","Helps detect and monitor glaucoma and other optic nerve diseases","The test is quick, painless, and does not require blood samples","Results are compared with age-matched normal values"}
Key facts at a glance
- LOINC
- 86310-0
- Short code
- RNFL-6-OD
- Synonyms
- R ret RNFL 6 o'clock thickness OCT
- Category
- cardiac
- Sample
- serum
- Container
- Not applicable - Imaging test
- Units
- N/A
- Ref low
- 95
- Ref high
- 145
- Critical range
- — – 10000 N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- ≥ 18 years
- Method
- Optical Coherence Tomography (OCT)
- CPT
- 92134
- SNOMED
- 371631009
- Specimen volume
- Not applicable - Non-invasive imaging procedure
- Stability
- Not applicable - Real-time imaging
- Hemolysis max
- 500
- Lipemia max
- 1000
- Icterus max
- 500
- TAT
- 1–24 hours
- Order
- routine
A complete blood count (CBC) is a common blood test that measures the different cells in your blood, including red blood cells, white blood cells, and platelets. It gives your doctor a broad picture of your overall health and can help detect a wide range of conditions, such as infections, anemia, and other blood disorders.
This test uses Optical Coherence Tomography (OCT), an advanced imaging technology, to measure the thickness of the retinal nerve fiber layer (RNFL) in your right eye. Specifically, it measures the thickness at the 6 o'clock position, which corresponds to the lower portion of the optic nerve head.
The test is performed by drawing a small sample of blood, usually from a vein in your arm. A healthcare professional will clean the site, insert a thin needle, and collect the blood into a tube. The process typically takes only a few minutes, and you may feel a quick pinch or slight discomfort when the needle goes in. A small bandage is placed over the spot afterward, and you can return to your normal activities right away.
During the test, you will sit in front of an OCT machine. Your chin will rest on a support while the device scans your eye using harmless light waves. The process takes only a few minutes and is completely painless. No blood draw is required for this examination.
This test is important because it helps your doctor evaluate your overall health and screen for many conditions before symptoms appear. It can identify infections, anemia, clotting problems, and certain blood disorders, and it is often used to monitor ongoing treatments or chronic conditions. Early detection through a CBC allows for timely treatment and better health outcomes.
The retinal nerve fiber layer contains nerve fibers that carry visual information from the retina to the brain. Measuring its thickness helps doctors detect early changes that may indicate glaucoma or other optic nerve disorders. The 6 o'clock position is particularly important because it corresponds to the inferior quadrant of the optic nerve, which is often affected first in glaucoma.
Reference range
Retinal nerve fiber layer thickness at clock hour 6 (inferior quadrant): Normal range 95-145 microns. Values below 85 microns may indicate early glaucomatous damage; below 70 microns suggests significant nerve fiber loss.N/A
Patient should remove contact lenses if applicable. Pupil dilation may be performed for better image acquisition. Inform technician of any eye surgery history, laser treatments, or retinal diseases. Avoid eye makeup on day of examination.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Thinning of the retinal nerve fiber layer may indicate damage to the optic nerve, commonly associated with glaucoma. Early detection of RNFL thinning allows for timely treatment to prevent vision loss. This test is also useful for monitoring disease progression and evaluating treatment effectiveness. The results are compared with a database of normal values matched for age to determine if the thickness is within normal range.
Frequently asked questions
Do I need to prepare for this test?
Is the test painful?
How long does the test take?
When will I get my results?
What happens if my results are abnormal?
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