Objective: To point out the sources of error in scanning laser polarimetry (SLP or GDx examination), a relatively recent objective test that is supposed to detect early retinal nerve fiber damage before any perimetric defects appear.

Methods: Review of clinical examples of GDx tests that demonstrate the possible sources of error related to the eye itself and to operator performance.

Results: Examples will be shown using actual patient records to demonstrate errors caused by poor compensation of corneal birefringence, improper macular alignment, abnormalities of the optic nerve and of the peripapillary area, pupil size, refractive error, and pseudophakia. Conclusion: SLP is an important addition to the clinical armamentarium of glaucoma diagnosis and follow up, but only if performed correctly and if sources of error are kept in mind and avoided.