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Miami-In uveitis cases, laboratory testing often fails because of the low prevalence of the types of uveitis diagnosable by this means.
Clinical skill can make a difference in testing, said Janet L. Davis, MD, professor of ophthalmology, Bascom Palmer Eye Institute, University of Miami School of Medicine, Miami.
"Try to figure out the diagnosis before you test, and assess the clinical importance of the knowledge so that you know how far to push your testing," Dr. Davis said. "It's a common finding among most clinicians that they can do all the testing they want. Yet, they may end up with no positive results whatsoever.
Dr. Davis noted, for example, that it is common to have sarcoid eye disease despite an array of negative test results. However, unless the patient has pulmonary symptoms, he or she is not likely to be treated for systemic disease, so the negative results may not matter.
In the case of an infectious disorder, a cure might not be found if testing fails to reveal the correct illness. Clinicians understandably worry that they may fail to order the one test that could determine the correct diagnosis.
Most of the important false negatives are going to be infectious, a category that accounts for only about 16% of all uveitis cases, Dr. Davis said. Regardless of negative test results, clinical suspicions can be a very good guide that the eye is infected.
Classification of uveitis
Uveitis is classified into two main groups: conditions that are associated with systemic disease, in which laboratory tests stand a chance of finding useful diagnostic clues; and those conditions that are not.
Elaborating on this point, Dr. Davis referred to a study in which the authors found a definite association between uveitis and systemic disease in 26% of cases and a presumed association in 23%. While some of the remaining disorders involved ophthalmologically defined, named entities, the remainder were cases that could not be further defined other than by saying that some part of the eye was inflamed.
"If you have only a 25% chance of finding a systemic disease, it's because only 25% have a systemic disease that can be found," she said. "Most people still feel obligated to look for it, but you have to look at this from a realistic perspective in terms of what you can actually expect to accomplish."
Poor test selection can also be a factor in the failure of lab tests, she added, reminding clinicians of their responsibility for the validity of the workups they select.
Dr. Davis also stated that certain signal clinical features could be very useful in evaluating patients with uveitis and deciding whether to order a test. If symptoms such as abrupt onset, unilaterality, redness, pain, and photophobia are present, HLA-B 27 is an excellent test to order and will be positive in about 50% of patients, Dr. Davis said. If focal chorioretinitis is observed, a toxoplasmosis IgG test is advisable. However, a positive IgG result does not necessarily confirm the disease.
Hypopyon is also an important sign and could indicate acute anterior uveitis, Behçet's disease, endophthalmitis, Churg-Strauss syndrome, or occasionally syphilis. Large keratic precipitates are typical in sarcoid as well as toxoplasmosis and cases of acute anterior uveitis that has been present for some time. Retinal vasculitis may occur in conjunction with connective tissue diseases and a variety of infections; a fluorescein angiogram is sometimes necessary to confirm the diagnosis.
Retinal neovascularization is seen in a limited spectrum of uveitic diseases, including sarcoidosis, Behçet's disease, juvenile rheumatoid arthritis, and pars planitis.
In addition, some angiographic patterns are characteristic of a disease, such as the fern pattern of pars planitis, in which small- vessel leakage follows the pattern of the major vascular structure.
"This is a very helpful sign when present," Dr. Davis said.