To reuse or not reuse? That is a rebound question!

In this era of environmental and cost-awareness, it is generally recommended to reduce wastage, reuse multiple times, and recycle waste to reduce environmental impact. In the medical field, this has to be balanced with clinical safety and accuracy. Rebound tonometers (RTs) are portable, do not need topical anesthesia, and are comfortable for the patient and technician. The most commonly used type of RT uses a disposable probe consisting of a 1.8 mm diameter plastic spheroid on a magnetized steel wire.[1] Reuse of these disposable RT probes has become a topic of debate in recent years. However, the ease of use of these devices has made them popular among ophthalmic fraternity; the cost of disposable probes has led to their reuse. A question arises: Is the reuse of these devices safe or does it carry a risk of infection transmission?

One of the primary concerns is cross-contamination. When a tonometer is reused on multiple patients, there is a risk of transferring microorganisms from one patient to another, leading to the spread of infectious diseases such as conjunctivitis or even more serious conditions like meningitis. To reduce cross-contamination, it is essential to thoroughly disinfect the tonometer after each use.[2]

The accompanying article[3] highlights the potential risk of infection transmission associated with the reuse of disposable RT probes. The authors conducted two distinct experiments – one in vitro and one in vivo.

Five-Second Rule

The first experiment involved dipping the RT probe and a Goldman applanation tonometer (GAT) prism in a bacterial suspension containing Pseudomonas fluorescens and Escherichia coli for 5 s. The probes were then swabbed with a cotton swab, which was tested for bacteria. Although that is not a plausible clinical scenario, the experiment showed that even with a smaller surface area, the RT probe carried more bacteria per unit surface area.

The five-second rule is a popular belief that food dropped on the floor is safe to eat if picked up within 5 s as long as it is not visibly dirty. However, there is no scientific evidence to support this claim, and it is generally recommended to discard food that has come into contact with a dirty surface as it can quickly become contaminated with bacteria. Television shows such as “Mythbusters” and scientific studies have shown that bacteria can get transferred almost instantaneously (<1s) on contact, although longer contact times increase the cross-contamination. Also, the nature of the food and surface are important factors.[4]

The RT probe may harbor more bacteria per unit surface area because of the nature of its tip surface compared to the smooth flat surface of a GAT prism. That would also make it more difficult to physically clean as well.

Worst Case Clinical Scenario

The second experiment was done using the RT probe on 11 patients of corneal ulcer and then inoculated on agar culture media. In this worst case plausible scenario, 36% of the tests showed bacterial growth when incubated at 37°C. This shows that there is risk of cross-infection on reuse without sterilization, especially if done on patients of corneal ulcer or conjunctivitis. What about after disinfection?

Standard Clinical Scenario

Briesen et al. in 2020 had published their study[5] where used RT probes from 100 healthy eyes were inoculated on blood agar and on a brain heart infusion broth without any disinfection and did not find any bacterial or fungal growth. Five additional RT probes were wipe disinfected with isopropanol 70% and five were wipe disinfected with sekusept 4% (disinfective agent containing 10% natriumperborat and 10% tetracetylglycoluril) and analyzed by environmental scanning electron microscopy. There probes did not have any microorganisms, but some had signs of remaining cell elements. They concluded that RT probes carry only a minimal cross-infection risk if disinfected and reused.

But Is It Accurate?

However, cross-infection is not the only problem. Another concern is accuracy of the measurement. Rebound tonometry depends on accurate measurements of the deceleration of a precisely weighed RT probe on the cornea. Over multiple uses, the RT probe may become worn or damaged, which may affect the accuracy of the measurement, leading to a misdiagnosis of glaucoma. This can have serious consequences for the patient.

Boonyaleephan and Watanasupt published their study[6] in 2022, where they measured IOP of 58 participants by rebound tonometry, four times for each person. Each participant was measured with a new RT probe, then repeated three times after wiping with 70% isopropyl alcohol swab each time. They concluded that there was good agreement of IOP measurements between the new, first reuse, and second reuse measurement values. They noted a statistically significant difference on the third reuse.

Another laboratory study by Lee et al. published in 2017[7] used a pressure calibration apparatus to take hundreds of measurements at different pressure settings with and without wiping with an alcohol pad. Although there was some statistical significance in measurements at some settings, these were not clinically significant. IOP was statistically significantly higher in the group where the tips were wiped, and they suggest that it may be because of deformation of the probe while wiping.

Wipe Or Wash Or Gas?

With the conclusion that there is a risk of cross-infection from a highly infective patient, we conclude that if reused, the RT probes have to be sterilized. Accuracy is not compromised at least for three readings. Deformation of the probe while handling and wiping may be the cause for measurement errors.

One option for cleaning would be wiping with an alcohol swab, but that would be more likely to deform and damage the RT probe. Another option would be to dip or spray in isopropanol 70% or other agents[8] – this may prevent inadvertent deformation of the probe. Another option would be ethylene oxide (EtO) sterilization.[9] Because EtO can sterilize at relatively lower temperatures, the main advantage of this gas for hospital sterilization is that it will not damage heat-sensitive materials, such as plastics.[10] There is some discussion that even this temperature may be enough to alter the magnetic property of the probe, although personally no issues were seen with the EtO-sterilized probes. One thing is certain – risk of infection is present, however minimal it may be. A safe method of sterilization that avoids damage to the probe's structure and magnetic property would allow cost-effective usage with fairly reliable measurements.