Until the company is more forthcoming about the instruments’ algorithm, we are forced to rely on the information it makes available to the general public. According to the TonoPen XL instructions manual “each single valid IOP reading is digitally displayed on the LCD. When four valid readings are obtained, the mean IOP and the standard deviation for those readings are shown on the LCD.” This text is at odds with Mr Taylor’s statement that “one should not expect that the individual readings which appear on the screen … are all used in the final displayed IOP.” Our interpretation of the instructions manual text is that, contradictory to Mr Taylor’s letter, “each valid reading is displayed” and that after displaying four such readings, “the mean IOP and standard deviation for those readings are shown.” This brief description of the instrument’s mode of action formed the basis for our investigation.1
Mr Taylor is concerned that our results and conclusions also indirectly condemn the AVIA model (which we did not evaluate), even though it “uses significantly different methodology in its calculation of IOP values.” This revelation about the AVIA technology (which, to the best of our knowledge, is not made public on the Reichert Technologies website) begs an obvious question—if TonoPen’s reliability has been established “through dozens of publications over the past 30+ years,” why did the company feel compeled to use significantly different methodology in its new model?
We are well aware of the publications cited by Mr Taylor regarding TonoPen’s accuracy, but draw his attention to one of his own references. According to Frenkel et al,2 the TonoPen “lacks good correspondence [with the Goldmann applanation tonometer] in the 31 to 45 mm Hg interval,” which are the highest values evaluated by these researchers. Similarly, Iester et al3 found significant differences between TonoPen and Goldmann values for individuals with intraocular pressure (IOP)>20 mm Hg, and, when compared with manometry, TonoPen underestimated IOP in the higher pressures of rabbits, dogs, cats, and horses.