Whether CTA can be used for the primary diagnosis of sSAH, eliminating the need for LP, is a topic of current debate. A common practice is to obtain a LP for all patients with suspected SAH after a negative noncontrast CT. However, only 25% to 50% of patients with CT-negative, worst-of-life headache actually underwent LP in the ED,64,127,128 likely because this test is burdensome for the patient and the provider. If CTA could improve the diagnostic capability of noncontrast CT, perhaps it could reduce the need for LP. Only 2 small studies comparing CTA with noncontrast CT and LP for primary SAH diagnosis exist.142,143 The best evidence comes from a prospective study of 106 patients in whom CTA detected 6 cases of aneurysm that were not seen on noncontrast CT.142 All of these patients had positive LP results and were picked up by routine workup. However, there were 3 cases of negative CT and spinal fluid that had a positive CTA, likely owing to detection of unruptured aneurysms.
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