An Aspergillus-specific lateral-flow device already FDA-approved for pulmonary aspergillosis testing was able to detected MK with high sensitivity for both corneal scrape (0.89, 95% CI 0.74-0.95) and swab samples (0.94, 95% CI, 0.73-1.00).
Overall accuracy was 0.94 (95% CI 0.90-0.97) and 0.88 (95% CI 0.73-0.96), respectively, reported Bethany Mills, PhD, of the University of Edinburgh, Scotland, and colleagues in JAMA Ophthalmology.
The study was "a useful proof-of-concept" for rapid detection of pathogens causing the infection, which could allow a more targeted treatment approach, Mills told MedPage Today.
Microbes causing the condition are bacteria in half of cases and fungi like Aspergillus in the other half, but clinicians usually can't distinguish without testing, Mills said. That makes microbiological diagnosis especially important, she said, but it requires highly skilled lab workers at tertiary care centers. These facilities can be hundreds of kilometers from sites in Africa and Asia, she said, and testing can take a week.
The disease is more severe and more common in tropical and subtropical regions, where it often causes blindness and eye loss, particularly in poorer nations.
The disease "is an acute ophthalmic emergency," Mills said. "Across Asia and Africa, it is the second most common cause of single-eye vision loss following cataracts. In these regions, 60% of patients are left with moderate or worse visual outcomes, and up to 15% require surgery that's often unsuccessful. Current treatment strategies include topical antimicrobials, antibiotics, and/or antifungals, and then surgery."
For the study, researchers used a lateral-flow device known as AspLFD that is FDA-approved to test for pulmonary aspergillosis and costs about $12 in Western countries. "It has never been looked at in the context of keratitis," Mills said. "To our knowledge, no lateral flow or point-of-care device has been looked at for keratitis."
As Mills explained, "think of this as being like a COVID lateral flow test, which everyone is familiar with. Instead of swabbing your nose, your cornea is scraped or swabbed to retrieve the sample, with a numbing drop added first. The sample is then placed into a buffer and then put onto the lateral flow test."
The researchers suggested "swabs could be used to collect corneal samples from patients with suspected MK in settings where the routine method of specimen collection by scraping is not possible, such as primary care."
Their group tested the diagnostic on corneal swab and scrape samples taken from 198 individuals ages 15 or older with MK (63.6% male, mean age 51) from 2022-2023 at a single eye hospital in Madurai, India. Some had already been treated with antibiotics, antifungals, or both at the time of sampling.
The samples were placed into the AspLFD devices, and a laboratory microbiologist inspected them after 20 minutes.
Among corneal scrape samples, 39 were positive for Aspergillus according to the AspLFD test. A reference culture test revealed that 31 were actually positive and eight were false positives. Of the 159 negative scrape samples by the AspLFD test, the reference culture test revealed that 155 were negative and four were false negatives.
Twenty corneal swab samples were positive per the AspLFD test, of which reference culture test revealed that 16 were actually positive and four were false positives. Another 20 samples were deemed negative by the test, with one being a false negative by the reference culture test.
The researchers noted that "while not included within the formal AspLFD analysis, 5 of the 76 culture-negative, fungal smear-positive scrape samples also had positive AspLFD results, suggesting that these patients likely had a missed Aspergillus species infection."
The study authors noted limitations. For one, the subjects often had "relatively advanced" stages of MK, making it unclear how the test would work in those with less severe cases. Researchers also had to eyeball the test results looking for the band to determine whether the results were positive or negative. Some of the bands were faint. To improve resolution, the researchers looked at photos of the devices that were taken using open-source software and a smartphone.
What's next? While testing kits may not be readily available, "we believe that ophthalmologists may start using them now," said study co-author Venkatesh Prajna, MD, of Aravind Eye Hospital in India, in an interview.
Reference:
Rapid Point-of-Care Identification of Aspergillus Species in Microbial Keratitis. Rameshkumar Gunasekaran, MSc; Abinaya Chandrasekaran, MS; Karpagam Rajarathinam, CMLT; et al. JAMA Ophthalmol. Published online September 28, 2023. doi:10.1001/jamaophthalmol.2023.4214
Abstract
Importance: Microbial keratitis (MK) is a common cause of unilateral visual impairment, blindness, and eye loss in low-income and middle-income countries. There is an urgent need to develop and implement rapid and simple point-of-care diagnostics for MK to increase the likelihood of good outcomes.
Objective: To evaluate the diagnostic performance of the Aspergillus-specific lateral-flow device (AspLFD) to identify Aspergillus species causing MK in corneal scrape and corneal swab samples of patients presenting with microbial keratitis.
Design, Setting, and Participants: This diagnostic study was conducted between May 2022 and January 2023 at the corneal clinic of Aravind Eye Hospital in Madurai, Tamil Nadu, India. All study participants were recruited during their first presentation to the clinic. Patients aged 15 years or older met the eligibility criteria if they were attending their first appointment, had a corneal ulcer that was suggestive of a bacterial or fungal infection, and were about to undergo diagnostic scrape and culture.
Main Outcomes and Measures: Sensitivity and specificity of the AspLFD with corneal samples collected from patients with MK. During routine diagnostic scraping, a minimally invasive corneal swab and an additional corneal scrape were collected and transferred to aliquots of sample buffer and analyzed by lateral-flow device (LFD) if the patient met the inclusion criteria. Photographs of devices were taken with a smartphone and analyzed using a ratiometric approach, which was developed for this study. The AspLFD results were compared with culture reports.
Results: The 198 participants who met the inclusion criteria had a mean (range) age of 51 (15-85) years and included 126 males (63.6%). Overall, 35 of 198 participants with corneal scrape (17.7%) and 17 of 40 participants with swab samples (42.5%) had positive culture results for Aspergillus species. Ratiometric analysis results for the scrape samples found that the AspLFD achieved high sensitivity (0.89; 95% CI, 0.74-0.95), high negative predictive value (0.97; 95% CI, 0.94-0.99), low negative likelihood ratio (0.12; 95% CI, 0.05-0.30), and an accuracy of 0.94 (95% CI, 0.90-0.97). Ratiometric analysis results for the swab samples showed that the AspLFD had high sensitivity (0.94; 95% CI, 0.73-1.00), high negative predictive value (0.95; 95% CI, 0.76-1.00), low negative likelihood ratio (0.07; 95% CI, 0.01-0.48), and an accuracy of 0.88 (95% CI, 0.73-0.96).
Conclusions and Relevance: Results of this diagnostic study suggest that AspLFD along with the ratiometric analysis of LFDs developed for this study has high diagnostic accuracy in identifying Aspergillus species from corneal scrapes and swabs. This technology is an important step toward the provision of point-of-care diagnostics for MK and could inform the clinical management strategy.