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Conversion formulas between automated-perimetry indexes as measured by two different types of instrument
11st Department of Ophthalmology, Semmelweis Medical School, Budapest, Hungary. pappand@szem1.sote.hu
Purpose:
Comparison between the results of measurements made using different types of automated-perimetry equipment is difficult, because of the lack of standardisation. The purpose of the study was to compare the two main indexes, mean deviation or mean defect (MD) and pattern standard deviation (PSD), as measured by two widely used instruments (Interzeag Octopus 500EZ and Humphrey Field Analyser 740) in the same group of patients.
Patients And Methods:
Thirty eyes of 17 patients with different stages of primary open-angle glaucoma were tested, and for 25 eyes the indexes measured using program G1 of the Octopus perimetry instrument were compared with those from program C30-2 of the Humphrey Field Analyser.
Results:
Using the one-tailed Student t test, a significant difference was found between the MD and the PSD values of the Humphrey and of the Octopus perimetry equipments. However, subsequent use of the conversion formulas to adjust the measurements then indicated a good correlation between the two instruments (MD: r = 0.91; PSD: r = 0.79).
Conclusion:
MD and PSD values measured by the Octopus and Humphrey perimetry instruments are based on measurements made under significantly different conditions, so the direct comparison of the indexes is not reliable. Nonetheless, empirically valid conversion formulas exist and can be used to compare the perimetric results for a patient tested with the two types of instrument.