Laboratory Test: What It Is, Why It Is Done, and How to Get Ready

This test is a laboratory analysis, usually performed on a small sample of your blood, urine, or other body fluid. It helps your doctor check how your body is working, screen for certain health conditions, and monitor your overall health over time.

Optical Coherence Tomography (OCT) is an advanced imaging technique that uses light waves to capture detailed cross-sectional images of your retina. This specific test measures the thickness of the retinal nerve fiber layer (RNFL) in the upper nasal area of your right eye. The RNFL contains nerve fibers that carry visual information from your retina to your brain.

Your doctor may order this test to help diagnose a medical condition, find the cause of symptoms you are having, check how well organs such as the liver, kidneys, or thyroid are working, or see how well a treatment is going. It is also often part of a routine checkup even when you feel well.

This test is primarily used to detect and monitor glaucoma, a condition that damages the optic nerve. It can also help identify other optic nerve and retinal disorders. By measuring the RNFL thickness, doctors can detect early signs of damage before vision loss occurs, allowing for earlier treatment and better outcomes.

The test is most often done by drawing a small amount of blood from a vein in your arm. A healthcare professional will clean the skin, wrap a band around your upper arm, and use a thin needle to collect the sample into a tube. You may feel a brief pinch or sting. The sample is then sent to a laboratory, where it is analyzed and the results are reported to your doctor.

During the test, you will sit in front of the OCT machine and place your chin on a support. You will be asked to look at a fixed target while the scanner takes images of your eye. The procedure is completely painless and takes only a few minutes. No eye drops or radiation are involved.

Follow any specific instructions your doctor gives you before the test. You may need to fast, meaning no food or drinks except water, for 8 to 12 hours beforehand. Tell your doctor about all medications, vitamins, and supplements you take, since some can affect the results. Drink plenty of water unless told otherwise, wear a sleeve that rolls up easily, and try to stay relaxed before the blood draw.

No special preparation is needed. You can eat and drink normally before the test. If you wear glasses or contact lenses, bring them with you. Avoid using eye makeup on the day of the test as it may interfere with the imaging.