Therefore, differentiation of septic arthritis from other causes is not always possible based on cell counts alone. Synovial fluid PCR analysis is useful in finding less common organisms such as Borrelia species. However, measuring protein and glucose levels in joint fluid is not useful for diagnosis.The Gram stain can rule in the diagnosis of septic arthritis, however, cannot exclude it.Synovial fluid cultures are positive in over 90% of nongonoccocal arthritis; however, it is possible for the culture to be negative if the person received antibiotics prior to the joint aspiration. Cultures are usually negative in gonoccocal arthritis or if fastidious organisms are involved.If the culture is negative or if a gonococcal cause is suspected, NAAT testing of the synovial fluid should be done.Positive crystal studies do not rule out septic arthritis. Crystal-induced arthritis such as gout can occur at the same time as septic arthritis.A lactate level in the synovial fluid of greater than 10 mmol/L makes the diagnosis very likely. Blood tests
Laboratory testing includes white blood cell count, ESR and CRP. These values are usually elevated in those with septic arthritis; however, these can be elevated by other infections or inflammatory conditions and are, therefore, nonspecific. Procalcitonin may be more useful than CRP.Blood cultures can be positive in up to half of people with septic arthritis. Imaging
Imaging such as x-ray, CT, MRI or ultrasound are nonspecific. They can help determine areas of inflammation but cannot confirm septic arthritis.When septic arthritis is suspected, x-rays should generally be taken.
Ischemic optic neuropathy (ION) is the loss of structure and function of a portion of the optic nerve due to obstruction of blood flow to the nerve (i.e. ischemia). Ischemic forms of optic neuropathy are typically classified as either anterior ischemic optic neuropathy or posterior ischemic optic neuropathy according to the part of the optic nerve that is affected. People affected will often complain of a loss of visual acuity and a visual field, the latter of which is usually in the superior or inferior field.When ION occurs in patients below the age of 50 years old, other causes should be considered, such as juvenile diabetes mellitus, antiphospholipid antibody-associated clotting disorders, collagen-vascular disease, and migraines. Rarely, complications of intraocular surgery or acute blood loss may cause an ischemic event in the optic nerve. Presentation
Anterior ION presents with sudden, painless visual loss, developing over hours to days. Diagnosis
Examination findings usually include decreased visual acuity, a visual field defect, color vision loss, a relative afferent pupillary defect, and a swollen optic nerve head. Posterior ION occurs arteritic, nonarteritic, and surgical settings. It is characterized by acute vision loss without initial disc edema, but with subsequent optic disc atrophy. Management
Although there is no recognized treatment that can reverse the visual loss, upon recent reports, optic nerve health decompression may be beneficial for a select group of patients with a gradual decline in vision due to ION. See also
Ocular ischemic syndrome

References

Dictionary of Eye Terminology,  Triad Publishing Company, 1990.