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Have you calibrated your tonometer?

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By Helen Carter
Journalist

 

How often do you verify the calibration of your tonometers? Monthly, every few months, annually?

Experts suggest that spending a few minutes every month checking for calibration errors and keeping your instruments calibrated is essential. It has been shown that tonometers can lose their accuracy within weeks of purchase or recalibration. Monthly checks will help ensure accurate readings in the treatment and care of your glaucoma patients.

Senior fellow and lead optometrist of the University of Melbourne Glaucoma Clinic, Dr Graham Lakkis, explains why calibration is so important.

‘IOP readings are vital in detecting elevated pressure in routine patient examinations, monitoring glaucoma suspects for any increase in IOP over time, in the monitoring and treatment of glaucoma, and assessing efficacy of intraocular pressure-lowering drugs in patients currently on glaucoma treatment,’ he told Equipment.

‘Lowering IOP from baseline is the only treatment that we have, and an accurately-calibrated tonometer will let us determine the effectiveness of our treatment drugs or lasers, as well as monitoring for changes in pressure over time. However, accurate calibration is vital to ensure any pressure change is real, rather than due to an error in the instrument,’ he said.

False positives

Lakkis points out that inaccurate calibration tends to give an excessively high reading leading to false positives in glaucoma suspects, that is, normal patients being misclassified as having ocular hypertension. ‘In patients already undergoing glaucoma treatment, a high reading would falsely indicate that therapy was no longer as effective, and patients may have unnecessary changes or additions to their glaucoma medications,’ he said.

In practices with multiple IOP measuring devices, Lakkis advises practitioners to make sure the same device is used each time on the same patient, as the various instruments use different measurement principles and may not be interchangeable.

What does the manual say?

Ideally, the manufacturer’s recommendations on calibration should be followed. These include how to calibrate each instrument and how often it should be done.

‘Calibration instructions are provided with each instrument. Copies of manuals are available online in case the original has been misplaced and online video tutorials on calibration are also available,’ Lakkis said.

‘The handheld (Perkins and Kowa) and slitlamp-mounted (Goldmann) tonometers can be easily checked for calibration. They can also be adjusted to rectify errors by the practitioner if they have a modicum of technical ability. Otherwise, the faulty instrument needs to be serviced by a technician.’

Clinical guidelines

If there are no manufacturer’s recommendations on calibration, clinical guidelines should be consulted.

Optometry Australia’s Clinical Guideline Tonometry (2005) and Clinical Guideline Tonometer Probes (2011) advise that tonometers should be calibrated in accordance with the manufacturer’s recommendations. These guidelines are available for members, on the Optometry Australia website.

The tonometry guideline clearly states: ‘a number of instruments are used for the measurement of IOP and it is important that these instruments are calibrated regularly as described in the manufacturer’s instructions and that appropriate disinfection procedures are followed.’

The guideline goes on to say that measurement of IOP is subject to variability due to factors including ‘instrument errors and calibration of tonometers (this assumes a constant value for corneal thickness and rigidity; a thin cornea may be underestimated by up to 5.9 mmHg and a thick cornea overestimated by up to 6.8 mmHg).’

As Lakkis points out, Optometry Australia’s tonometry guidelines do not suggest a frequency for calibration and the Optometry Board of Australia’s Guidelines for the use of scheduled medicines also do not discuss calibration frequency. Similarly, NHMRC Guidelines for the Screening, Prognosis, Diagnosis, Management and Prevention of Glaucoma state ‘regular calibration of the tonometer is required’ without stating a frequency.

Lakkis suggests a 30-day recurrent schedule. ‘If a monthly calibration check is to be done, it would be easiest to remember on the first day of the new month,’ he said.

The Royal Victorian Eye and Ear Hospital’s three tonometer types (Haag Streit Applanation, iCare and Tono-Pen) all require a different type of calibration or calibration check due to the different types of technology they use to measure IOP.

Haag Streit tonometers use a mechanical weighting system and clinicians use a small weighted bar supplied with the unit to check the calibration monthly.

The iCare tonometers require a calibration or calibration check by the original equipment manufacturer on a 24-month schedule as recommended by that manufacturer. The Tono-Pen tonometer has a small self-check done before each pressure is taken or when indicated by the tonometer to calibrate the strain gauge inside the unit.

Chief staff optometrist at the Centre for Eye Health, UNSW, Michael Yapp has initiated monthly calibration checks of its tonometers according to the recommendation in the Goldmann manual.

Non-contact (puff) tonometers are computer controlled rather than mechanical devices. They go through an automated self-calibration when they are switched on. If they fail the calibration check, an error message appears on the computer screen and they have to be serviced by a technician as there are no user-adjustable parts.

False readings

As optometrist Caroline Pate writes in the US magazine Review of Optometry, time and thousands of repeated motions and procedures can take their toll on equipment. Constant use and abuse will set it out of calibration and even a few millimetres off might affect treatment decisions.

‘Would lowering IOP by a few millimetres make a difference for a patient with progressing advanced glaucoma?’ Pate asked. ‘Then it is important to verify the calibration on your tonometer. It could easily be off, giving you false readings, IOPs that are too low or too high.

‘We want to be sure that every time the patient comes in, he gets the exact same tonometry test from visit to visit and from exam room to exam room. That’s why it’s so important to keep your tonometer calibrated and to verify it regularly.’

Logbook

Australian College of Optometry head of primary and specialist eye-care services, Roman Serebrianik, who manages the college’s equipment, says many manufacturers recommend that their tonometers are calibrated monthly.

‘It would be admirable and a good habit to get into, perhaps on the first or last day of every month to make it easier to remember. It’s easy and takes only 30 seconds. The ideal is to record when calibration occurs in a logbook,’ he said.

Lakkis agrees, saying calibration is not a time-consuming process and is best performed by the practice’s principal optometrist.

‘Tonometers don’t stay calibrated because applanation tonometers are mechanical devices with dials, levers, springs and counterweights,’ he said. ‘There is potential for miscalibration due to dirt build-up, bent components from being dropped or misused, loss of spring tension from heavy use and so on.’

Serebrianik says applanation tonometers including Goldmann and Perkins have a mechanical spring to guide the probe and with wear and regular use, the spring can loosen or stretch.

‘All tonometers need regular calibration because you want accuracy and reproducibility,’ he said. ‘When a patient comes for a repeat IOP reading, you want to be assured that the tonometer is accurate so it doesn’t falsely indicate their IOP has risen or dropped from the first visit.

‘A tonometer which is off affects the care you provide patients because if it under- or over-estimates pressure it could alter management and impede ability to monitor glaucoma progression. If calibration is out, return it for servicing to be recalibrated.

‘Calibration can affect patient care and the quality of treatment decisions. When you maintain your equipment properly, the information won’t be skewed but if equipment is not well maintained, it can damage people’s eyes.’ 



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