Polymerase chain reaction testing of aqueous and vitreous fluid
Polymerase chain reaction (PCR) testing for diagnosis of infectious uveitis is highly sensitive and specific. It can directly amplify the DNA of a suspected pathogen from a small sample volume of ophthalmic fluid, making it ideal for application in ocular tissue.
Testing of aqueous fluid is an important adjunct in diagnosis of infectious posterior uveitis. Aqueous humor PCR testing that is specific for herpes simplex virus types 1 or 2, varicella-zoster virus, and cytomegalovirus has high diagnostic sensitivity and specificity at levels similar to vitreous biopsy. Anterior chamber paracentesis is generally safer and easier to perform than vitreous sampling. In suspected cases of retinal toxoplasmosis, one can perform anterior chamber paracentesis for PCR, though the diagnostic yield may be lower than vitreous sampling. In toxoplasmosis, a positive anterior chamber PCR result can be more likely in patients with large lesions or immunocompromised patients. Diagnostic utility of isolated aqueous PCR testing for suspected viral anterior uveitis is much lower. In cases for which viral speciation is critical to management, repeat sampling can increase the yield.
Until recently, PCR was not practical for diagnosis of bacterial and fungal uveitis due to the need to specify the selected pathogen of interest for amplification. Now, based on conserved genetic subunits within bacteria (16S) and fungi (5.8S/18S/28S), panbacterial and panfungal PCR tests perform equally or superiorly to culture. The PCR-based techniques for genomic testing of vitreous samples are used in diagnosis of primary intraocular lymphoma. Disadvantages of PCR are cost, limitations in testing for multiple entities due to small fluid sample size, risk of improper amplification of a contaminant, and risk of identification failure when there is a paucity of cellular material.
Doan T, Acharya N, Pinsky BA, et al. Metagenomic DNA sequencing for the diagnosis of intraocular infections. Ophthalmology. 2017;124(8):1247–1248.
Harper TW, Miller D, Schiffman JC, Davis JL. Polymerase chain reaction analysis of aqueous and vitreous specimens in the diagnosis of posterior segment infectious uveitis. Am J Ophthalmol. 2009;147(1):140–147.
Rothova A, de Boer JH, Ten Dam-van Loon NH, et al Usefulness of aqueous humor analysis for the diagnosis of posterior uveitis. Ophthalmology. 2008;115(2):306–311.
Sowmya P, Madhavan HN. Diagnostic utility of polymerase chain reaction on intraocular specimens to establish the etiology of infectious endophthalmitis. Eur J Ophthalmol. 2009;19(5):812–817.
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Ocular serology/local antibody production
European countries consider evaluation of aqueous antibody production based on the Goldmann-Witmer (GW) coefficient to be the gold standard for the diagnosis of toxoplasmosis; however, this is not commercially available in the United States. (Chapter 2 explained calculation of the GW.) Diagnostic yield may increase when PCR and the GW coefficient are used together, especially in viral infections. Aqueous antibody production may persist even in the absence of cellular material, giving it certain advantages over PCR in viral anterior uveitis.
Culture and vital staining
Cell culture and bacterial and fungal staining are useful in cases of suspected bacterial or fungal endophthalmitis. Isolation is time consuming and can lack sensitivity due to the low pathogen load in ocular samples. However, the technique is the traditional first line of testing, widely available, and inexpensive to perform.
Excerpted from BCSC 2020-2021 series: Section 9 - Uveitis and Ocular Inflammation. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.