Complete Blood Count (CBC) Test: What It Is, How It Works, and Why It Matters
A non-invasive eye imaging test that measures the thickness of the retinal nerve fiber layer to assess eye health, particularly for glaucoma detection.
{"This is an eye imaging test, NOT a blood test","Uses optical coherence tomography (OCT) to measure retinal layers","Helps detect and monitor glaucoma and other optic nerve diseases","The procedure is quick, painless, and requires no special preparation","Important note: The test category (cardiac) and sample type (serum) listed appear to be errors - this is an ophthalmologic imaging test, not a blood test"}
Key facts at a glance
- LOINC
- 86274-8
- Short code
- RNFL TS OD
- Synonyms
- R ret RNFL temp sup thickness OCT
- Category
- cardiac
- Sample
- serum
- Container
- Not applicable - this is an imaging test, not a blood test
- Units
- N/A
- Ref low
- 110
- Ref high
- 150
- Critical range
- 70 N/A – 5 N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–100 years
- Method
- Optical Coherence Tomography (OCT)
- CPT
- 92134
- SNOMED
- 415153001
- 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
The complete blood count, commonly called a CBC, is a routine blood test that measures the different cells circulating in your blood, including red blood cells, white blood cells, and platelets. It provides your healthcare provider with a broad picture of your overall health and can help detect a wide range of conditions, such as infections, anemia, and certain blood disorders, often before symptoms become noticeable.
This test uses Optical Coherence Tomography (OCT), a advanced imaging technology, to measure the thickness of the retinal nerve fiber layer (RNFL) in the temporal superior region of your right eye. The RNFL is a layer of nerve fibers that carries visual information from the retina to the brain.
The test is performed by drawing a small sample of blood, usually from a vein on the inside of your elbow or the back of your hand. A healthcare professional will clean the area, wrap a snug band around your upper arm, and insert a thin needle to collect the sample into a tube. The procedure takes only a few minutes, and you may feel a brief pinch. Afterward, a small bandage is applied, and you can return to your normal activities right away.
During the test, you will sit in front of the OCT machine. You will be asked to place your chin on a support and look at a specific target. The machine will scan your eye using light waves (not radiation) to create detailed cross-sectional images of your retina. The scan takes only a few seconds and is completely painless. No eye drops or blood draw is required.
This test is important because it helps your doctor screen for many conditions during routine checkups, monitor ongoing health problems, and investigate symptoms such as fatigue, weakness, fever, or unusual bruising or bleeding. Because your blood cells change when the body is fighting illness or not functioning properly, the CBC is often one of the first tests ordered to guide diagnosis, track treatment progress, and support informed decisions about your care.
Measuring RNFL thickness helps doctors detect early changes in the optic nerve that may indicate glaucoma or other neurological conditions. Thinning of the RNFL can be a sign of nerve damage, often before vision loss occurs.
Reference range
The normal temporal superior retinal nerve fiber layer (RNFL) thickness range is typically 110-150 microns in healthy adults. Reference ranges may vary by instrument and population.N/A
No special preparation required. Patient should remove eye makeup and contact lenses before the scan. Pupillary dilation may be performed for optimal imaging.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
RNFL thickness measurement by OCT is primarily used for: (1) Glaucoma diagnosis and staging - thinning RNFL is a key indicator of glaucomatous damage; (2) Monitoring disease progression - comparing results over time helps track if glaucoma is getting worse; (3) Detecting other optic nerve disorders - conditions like optic neuritis or ischemic optic neuropathy can also cause RNFL changes; (4) Baseline documentation - establishing a baseline for future comparisons.
Note: The test information indicates 'cardiac' category and 'serum' sample type, which appear to be data entry errors. OCT retinal nerve fiber layer analysis is an ophthalmologic imaging test, not a cardiac test, and does not require blood samples.
Frequently asked questions
Do I need to fast before this test?
Is the test painful?
How long does the test take?
Will I need dilating eye drops?
When will I get the results?
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 16, 2026. · Read our editorial policy