[Maintenance of measuring quality of transfer standards for the metrological control of air impulse tonometers].
BACKGROUND
According to the German Medical Devices Act ("Medizinproduktegesetz") from 2.8.1994 and the German Directive for Operators of Medical Devices ("Medizinprodukte-Betreiberverordnung") from 29.6.1998, human eye tonometers in Germany have to be checked by metrological controls every two years. Every ophthalmologist using such instruments in his medical practice has to ensure they are metrologically checked by independent testing agencies. One class of tonometers frequently used by ophthalmologists is the air impulse tonometer. PTB has designed a test device for this particular type of tonometer. A small series of these devices has been built and are in use by verification offices and private test agencies since 1984. PROCEDURE AND METHOD: At a seminar hosted by the "Deutsche Akademie für Metrologie" in 2005 the accuracy of 14 test devices for air impulse tonometers was checked in a comparative set of measurements. All test devices are in use for metrological control by verification offices or private test agencies. In the comparison a clinically calibrated air impulse tonometer "XPERT NCT PLUS" (manufactured by Leica in Buffalo, USA) was used as a reference instrument Three simulated intraocular pressure values, covering the full range of the tonometer (0 mmHg to 60 mmHg), had to be set on each test device and measured with the tonometer. For every device and every pre-set pressure value, a minimum of ten usable measurements were performed and statistically analysed.


RESULTS
In this comparative measurement the averaged readings given by the reference tonometer were generally in good agreement with the pre-set reference values of 15.0 mmHg, 30.0 mmHg, and 45.0 mmHg on the testing devices. Only in the 15 mmHg case did two out of 13 test devices give a result slightly out of the permissible limits. One device had to be eliminated from the test due to a defect bearing.


CONCLUSIONS
With only two exceptions, the results for all test devices agreed with the nominal values within permissible tolerances. These two exceptions may have been caused by slight misalignments between the respective test devices and the tonometer. In summary, it can be concluded that even 20-year-old test devices, if maintained and re-calibrated by the Physikalisch-Technische Bundesanstalt at least tri-annually, are still well suited for metrological control measurements.
Schwenteck
T
T
Physikalisch-Technische Bundesanstalt, Berlin. thomas.schwenteck@ptb.de
Thiemich
H-J
HJ
D000388:Air; D002138:Calibration / Q000592:standards; D019544:Equipment Failure Analysis / Q000592:standards*; D005858:Germany; D019983:Guideline Adherence; D007429:Intraocular Pressure; D008283:Maintenance / Q000592:standards*; D017410:Practice Guidelines as Topic; D012015:Reference Standards; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014065:Tonometry, Ocular / Q000295:instrumentation* / Q000379:methods