Endogenous endophthalmitis is essentially a clinical diagnosis, based on the presence of characteristic ocular signs and often systemic risk factors. The diagnosis may be aided by B-scan ultrasonography that, besides showing vitreous exudates, may also reveal choroidal abscesses that appear as dome-shaped choroidal lesions, underneath the vitreous exudates (Fig. 10.2, left). Rarely, optical coherence tomography may help delineate intra-retinal, subretinal or subretinal pigment epithelium location of infective foci.

The key test for confirmation of diagnosis is diagnostic vitrectomy. Since the infection originates at the back of the eye, sampling the vitreous near the infective focus greatly increases the diagnostic yield of the procedure. Rarely, subretinal biopsy of the subretinal exudates can help in identifying the causative organism (Fig. 10.2, right). Vitreous samples are subjected to standard microbiological examination (microscopy and culture) protocols for bacteria and fungi. Recently, quantitative real-time polymerase chain reaction (rt-PCR) has shown excellent sensitivity in diagnosis of bacterial and fungal infections, as compared to culture. Sugita et al. described a procedure that can be performed as quickly as 90 min [10]. Despite these advantages, current molecular techniques lack the ability to test for the entire range of antibiotic susceptibility and are prone to false-positive results, especially if care is not taken to prevent contamination of test samples.

Additional tests are required to rule out presence of systemic infection that may have been missed during clinical evaluation. The most effective method is blood culture using blood drawn on three consecutive days under sterile precautions. It is also useful to sample other extraocular sites of infection such as urine or pus from skin or other abscesses. A thorough internist evaluation can help in identifying occult infections (Table 10.1).