During Friday morning’s Uveitis Subspecialty Day session, Lyndell Lim, MD, PhD, shared the importance of being intentional in selecting laboratory tests for uveitis. She explained that many of her residents choose to run the entire uveitis panel—something she advises against. The results can be confusing and misleading.
“If you order the test, you own that test. It’s up to you to interpret that for your patient because you decided to order it,” she recalled telling her residents when they look to her for answers.
The solution: Bayes theorem. “What you want to know is the likelihood of your patient having that disease for which you’ve tested: the positive predictive value,” she explained.
Bayes theorem is a formula that helps calculate the likelihood of a patient having a particular disease. Simply put, the formula determines the positive predictive value based on the sensitivity and specificity of the test, and the prevalence of disease in the patient’s population.
Using examples, she showed how the wrong or unnecessary tests can lead to high levels of false positives. According to Dr. Lim, a physician should not order a test if the pretest probability of that disease is less than 20%.
Target your investigations. “So if we go back to that uveitis panel on the EMR, it’s not a list that you actually order indiscriminately,” she added.
She emphasized the importance of collecting a detailed history and examination. Use the patient’s presentation to deselect things on the panel that you don’t think your patient has.
Syphilis serology for all. Prior to the COVID-19 pandemic, we were in a syphilis epidemic, and ocular syphilis is the “great masquerader,” Dr. Lim said. Showing numbers from the United States and Australia, she explained that cases have been on the rise. Although there was a dip in case numbers during the pandemic in Australia, the numbers are higher than they were five years ago.
The overall prevalence of ocular syphilis is rare among all syphilis infections, but at her own institution in Australia, there’s been an increase in cases to at least one or two per week.
“Reverse testing” (a treponemal-specific test followed by a nontreponemal test) has nearly perfect sensitivity and specificity. With a low prevalence of ocular syphilis in your population (for example, 0.1%), 15% of the positive tests will be false positives. But as the syphilis prevalence increases from 0.1% to 1.0%, she explained, the positive predictive value could increase from 85% to more than 98%.
If you don’t think about syphilis before you inject an unopposed steroid into an eye, you might get a nasty surprise “and the patients will not thank you,” she warned. —Kanaga Rajan
Financial disclosures: Lyndell Lim, MD, PhD: Allergan, Inc.: L; Bayer Healthcare Pharmaceuticals: L,S; Implicit Bioscience: C; Novartis, Alcon Pharmaceuticals: L; Novotech: C.
Disclosure key: C = Consultant/Advisor; E = Employee; EE = Employee, executive role; EO = Owner of company; I = Independent contractor; L = Lecture fees/Speakers bureau; P = Patents/Royalty; PS = Equity/Stock holder, private corporation; S = Grant support; SO = Stock options, public or private corporation; US = Equity/Stock holder, public corporation For definitions of each category, see aao.org/eyenet/disclosures.
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