sample size was calculated to measure a difference of greater 
 than 2 mm of hg between the two instruments with an 
 estimated standard deviation of 4 mm , for 80% power and 
 a type 1 error of 5% .
we needed 34 patients to measure this 
 difference and recruited 40 consecutive adult subjects attending 
 a glaucoma prevalence study for the trial .
all subjects who could undergo visual acuity measurement , 
 refraction , slit - lamp examination and applanation tension 
 measurement were eligible .
corneal pathology including 
 astigmatism of 2 diopter ( d ) or greater , inability to measure 
 iop or history of allergy to proparacaine or fluorescein were 
 reasons for exclusion .
intraocular pressure was measured by one of two examiners , 
 using the zeiss at 030 ( gatz ) ( carl zeiss , jena , germany ) 
 and the inamil-5110 ( gati ) ( inami and co. , tokyo , japan ) 
 goldmann type applanation tonometers . at the start of the day
the instruments were calibrated as per 
 the manufacturers instructions and were used for examination 
 only if they were accurately calibrated . to minimize the influence of any iop - lowering effect 
 induced by applanation tonometry on the results , the 
 sequence of measurements was randomized .
applanation 
 tonometry was performed first on one randomly selected 
 instrument followed immediately after by measurement 
 on the second instrument .
the iop was measured after 
 anesthetizing the cornea with sterile 0.5% proparacain eye 
 drops ( paracain , sunways , mumbai , india ) and staining the 
 tear film with fluorescein strips .
the tonometer was set to the 
 zero mark prior to the start of the examination .
measurements 
 on both instruments for a subject were performed by a single 
 observer who was blinded to the actual readings , which 
 were read and recorded by the second examiner who then 
 reset the applanation tonometer to the zero mark .
the iop 
 was measured two consecutive times for each eye on each 
 instrument , and the mean of the two readings was used for 
 analysis ; if there was a difference of greater than 2 mm of 
 hg between the readings , a third measurement was taken 
 and the median of three readings was taken . by convention , 
 the right eye was examined first for every patient .
the iop 
 was measured on the second instrument , placed in the same 
 examination room , almost immediately after completing 
 recording on the first instrument .
intraocular pressure was compared using the paired 
 t test and agreement was assessed by the altman and bland 
 plot.6
forty eyes ( 40 subjects , mean age : 53.3 sd  7.9 years , 22 males , 
 18 females ) were included .
mean iop ( sd ) on gatz was 15.32 
 ( 6.80 ) mm hg ( range:9 mm hg - 36 mm hg ) and on gati 13.52 
 ( 5.65 ) mm hg ( range:7 mm hg - 30 mm hg ) ( p<0.001 , 95% ci of 
 the difference : -2.48 to -1.11 ) .
the bland and altman 
 plot [ figure 1 ] revealed a tendency for higher iop recordings 
 on the gatz ( 95% limits of agreement:-2.47 to 6.16 mm hg ) .
the absolute level of iop may not 
 be as important to glaucoma diagnosis as was once thought , 
 however , the trend of iop measurements is relevant in the 
 management of glaucoma patients .
we attempted to minimize their influence on the study results . 
 in order to avoid an observer bias ,
two observers were 
 used for the study - however , the same observer made iop 
 measurements on both instruments for a single patient . to 
 minimize the effect of a possible transient lowering of iop 
 following applanation tonometry , on the results , we randomly 
 allocated the order of iop measurement by either machine .
the 
 iops on the second instrument were measured within a few 
 minutes of the measurement on the first machine to minimize 
 any temporal variation in iop .
lower iop readings were consistently 
 recorded on the inami at . in 70% of subjects
iop differences 
 were within the clinically acceptable range of 2 mm . with both 
 instruments calibrated
accurately as per the manufacturers 
 recommendations better agreement between the instruments 
 would be expected .
since manometric measurements were not 
 made in any of our subjects it is difficult to comment on which 
 instrument was giving erroneous results .
the clinical implications of this variation on disease 
 diagnosis are considerable since both instruments are widely 
 used in asia .
it could result in potential misdiagnosis of normal 
 tension glaucoma ( ntg ) or primary open angle glaucoma 
 ( poag ) .
follow - up may not be affected as significantly as 
 long as all measurements are made on a single instrument by 
 a single observer keeping other factors such as calibration and 
 the time of measurement constant . however , with increasing 
 cross - referrals and the use of multiple instruments in various 
 clinics the possibility of lack of agreement between devices has 
 to be kept in mind .
with the large number of similar applanation 
 tonometers available , agreement between other commercially 
 available devices needs to be assessed . in this study
if this was indeed the case there should not have 
 been a significant difference between devices and certainly 
 not a difference of the magnitude that was found .
perhaps , 
 the manufacturers need to reassess the calibration procedures 
 for these devices . in conclusion , we demonstrate yet another variable in iop 
 measurements , the effect of which could be minimized by 
 performing baseline and follow - up iop measurements on a 
 single instrument .
additionally , therapeutic decisions should 
 be made keeping in mind the possibility of an inter - instrument 
 variability .
 the aim of the study was to assess agreement between two 
 commercially available applanation tonometers for the 
 measurement of intraocular pressure ( iop ) . 
 forty subjects 
 underwent iop measurement on two accurately calibrated 
 goldmann type applanation tonometers ( zeiss at 030 ( gatz ) 
 and inami l-5110(gati ) ) . 
 the order of examination was 
 randomized and observers were masked to the iop recorded . 
 
 the mean of two consecutive readings , from a randomly selected 
 eye for each subject , was used for analysis . 
 agreement was 
 assessed using the altman and bland plot . 
 the mean ( sd ) iop 
 readings on gatz was 15.32 ( 6.80 ) mm hg and on gati was 
 13.52 ( 5.65 ) mm hg ( p<0.001 , 95% ci of the difference : -2.48 to 
 -1.11 ) . 
 the 95% limits of agreement on the altman and bland plot 
 were:-2.47 to 6.16 mm hg ) . 
 there was significant inter - instrument 
 variability between the two accurately calibrated goldmann type 
 applanation tonometers studied . 
Materials and Methods
Results
Discussion