Three blood tests play an important role in diagnosis of giant cell arteritis: erythrocyte sedimentation rate, c-reactive protein and platelet count, usually obtained as part of a complete blood count. Here’s how these tests can help guide your diagnostic decisions.
Why it matters: These tests can 1) inform your decision on whether to prescribe empiric treatment and perform additional workup, such as temporal artery biopsy or vascular imaging tests, in undiagnosed cases and 2) help you monitor treatment in a patient with known GCA.
Here are three tips:
1. Don’t order ESR/CRP/Plt to rule out GCA in a patient with a very low pretest probability of GCA
For the initial evaluation of possible GCA in a patient with vision loss, the utility of diagnostic tests depends on the pretest probability of disease. For example, a patient under the age of 50 with vision loss due to retinal detachment is highly unlikely to have GCA.
Why is GCA unlikely? First, because GCA affects older individuals and second, because it does not cause retinal detachment.
In such a case, the pre-test probability of GCA is low. Therefore, ESR/CRP/Plt elevation has low positive predictive value and high negative predictive value. The test results won’t affect the post-test probability of whether or not that patient has GCA. Therefore the results will not influence your decision regarding additional workup and the need for treatment for unlikely GCA.
2. Do order ESR/CRP/Plt to rule out GCA in a patient with a moderate pretest probability of GCA
Now consider an 80-year-old patient with anterior ischemic optic neuropathy and no systemic symptoms. In such a case, the pre-test probability of GCA is moderate.
Why is GCA possible? First, because prevalence of GCA increases with age and second, because vasculitities such as GCA can cause anterior ischemic optic neuropathy, but so can non-arteritic processes.
In this case, elevated ESR/CRP/Plt has high positive predictive value and reasonable negative predictive value. You should order these tests for this patient. The results will affect the post-test probability of whether or not the patient has GCA and can help guide your management regarding disease treatment and workup for GCA.
3. Do order ESR/CRP/Plt in a patient with a high pretest probability of giant cell arteritis — but for purposes of management rather than diagnosis
On the far end of the spectrum, consider an 80-year-old patient with posterior ischemic optic neuropathy and jaw claudication. Here you have high pretest probability of GCA.
Why is GCA likely? First, because prevalence of GCA increases with age, second, because vasculitities such as GCA commonly cause posterior ischemic optic neuropathy and third, because the patient has systemic symptoms consistent with GCA.
In this patient, elevated ESR/CRP/Plt also has high positive predictive value and low negative predictive value. As with the first case, the test results in this patient will not affect post-test probability or influence your decision regarding the need for treatment or additional workup for likely GCA.
However, knowing the results of ESR/CRP is still important because it will help you monitor likely GCA by establishing a baseline against which to compare the patient’s test results in the future.
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About the author: Heather E. Moss, MD, PhD, is a neuro-ophthalmologist at Byers Eye Institute at Stanford and an assistant professor in the Departments of Ophthalmology and Neurology & Neurosciences in the Stanford University School of Medicine.