How is the diagnosis of subarachnoid hemorrhage made?
If the patient presents within 6 hours of headache onset, the sensitivity of a noncontrast computed tomography (CT) is 100%. The sensitivity decreases to 93% within 24 hours of onset and 80% at 3 days. A lumbar puncture may be required to demonstrate the presence of blood, particularly if the headache was remote in time. Xanthochromia in the fourth tube of cerebrospinal fluid is most accurate. Magnetic resonance imaging (MRI) sequences are also very sensitive for detecting acute subarachnoid hemorrhage.