GDx test, i.e. scanning laser polarimetry, is a test that allows you to measure the thickness of the layer of retinal nerve fibers. The GDx scanning laser polarimetry test is considered to be one of the most useful in the diagnosis of glaucoma. Check what this test is about and how to interpret the results.
GDx test , or scanning laser polarimetry, is a test that allows you to measure the thickness of the layer of nerve fibers in the retina. The measurement is taken around the optic disc, where the nerve fibers converge across the entire surface of the retina to form the optic nerve that leaves the eyeball and runs to the brain. Even using the best slit lamp, a doctor examining a patient is not able to accurately count the number of nerve fibers on the surface of the retina, much less assess their disappearance. In a he althy person, the number of nerve fibers is about one million, therefore appropriate equipment is needed to enable their evaluation and analysis. The GDx device measures the thickness of the fiber layer, their distribution, determines the places of cavities and compares the result with a standard database, which is not possible for a doctor who only has a "slide and an eye".
GDx test - indications for the test
GDx scanning laser polarimetry is considered to be one of the most useful in the diagnosis of glaucoma. This test is often difficult to perform, and it also happens that a completely unreliable result can be obtained. It is therefore inappropriate to base a glaucoma diagnosis solely on the GDx test result alone. First of all, the assessment of the optic nerve, which is enabled by the HRT laser technology, as well as the assessment of the GCL ganglion cell layer are needed. An indispensable supplementary examination is the performance of the field of vision examination on standardized new generation cameras. The case of each patient must be individually analyzed, and all obtained results must be properly interpreted.
GDx study - results
As a test result, a special report is obtained showing the thickness of the layer of nerve fibers in the form of numbers and graphs. The result shows a laser image of the area of the optic nerve disc where the measurement is made, below the laser image of the thickness of the layer of nerve fibers itself is presentedvividly in blue, yellow and red. In the lower part of the report there is a statistical analysis of the examination result superimposed on the fundus image, determining the probability of glaucoma damage in the form of colored squares, where red indicates a very high probability of this damage, and yellow and blue - less. In the middle part of the report there is a table showing the absolute values of the thickness of the nerve fiber layer and their statistical analysis in the following colors: white - normal values, red - values below the norm with a high probability of glaucomatous damage, yellow and blue - suspected glaucomatous values. At the bottom of the report, the thickness of the retinal nerve fiber layer is shown as a bactrian curve overlapping the bactrian range of the norm, so that the location of the lesions can be assessed.
A separate issue is the assessment of the GDx test result in patients with myopia, whose results often fall outside the statistical norm and cannot be interpreted in the same way as the results of tests obtained in people without visual impairment. In such cases, you should never rely solely on the GDx test and perform other additional tests.
Three generations of GDx cameras
There are three generations of GDx units:
- GDx FCC, oldest version with fixed corneal compensation
- GDx VCC, slightly newer version with variable corneal compensation
- GDx ECC, which is Pro, with improved compensation. This is the latest version.
The next generations were created along with the modernization of the method, successively eliminating the factors disturbing the measurement. FCC version cameras, burdened with the highest measurement error, are hardly ever encountered, as are the second generation VCC cameras.
According to the latest guidelines of the European Glaucoma Society, only the GDx Pro is the device recommended for the diagnosis of glaucoma. This device, in addition to the ability to perform tests using the ECC technique, can also perform VCC measurements and automatically compare subsequent tests. The name "Pro" indicates the possibility of analyzing the progression of changes, which is extremely important in monitoring the progress of the disease.
About the authorBarbara Polaczek-Krupa, MD, PhD, specialist in eye diseases, Centrum Opulystowa Targowa 2, WarsawDr. Barbara Polaczek-Krupa, MD, initiator and founder of the T2 Center. She specializes in modern diagnostics and treatment of glaucoma - this was also the subject of her PhD thesis defended with honors in 2010.
Dr. med. Polaczek-Krupa has been gaining experience for 22 years since she started workingat the Ophthalmology Clinic of the CMKP in Warsaw, with which she was associated in the years 1994-2014. During this period, she obtained two degrees of specialization in ophthalmology and the title of doctor of medical sciences.
In the years 2002-2016 she worked at the Institute of Glaucoma and Eye Diseases in Warsaw, where she gained knowledge and medical experience by consulting patients from all over Poland and abroad.
For years, as part of cooperation with the Medical Center of Postgraduate Education, he has been a lecturer at courses and trainings for doctors specializing in ophthalmology and primary he alth care.
He is the author or co-author of numerous publications in scientific journals. Member of the Polish Ophthalmology Society (PTO) and the European Glaucoma Society (EGS).