| Patient factors |
- The time of onset of the headache is clearly defined.
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- The CT is performed within six hours of headache onset.
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- The presentation is an isolated severe rapid-onset headache (no primary neck pain, seizure, or syncope at onset, or other atypical presentations).
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- There is no meningismus and the neurologic examination result is normal.
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| Radiologic factors |
- The CT scanner is a modern, third-generation or newer machine with thin cuts through the brain.
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- The CT is technically adequate, without significant motion artifact.
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- The hematocrit level is >30%.
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- The physician interpreting the scan is an attending-level radiologist (or has equivalent experience in reading brain CT scans).
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- Radiologists should specifically examine the brain CTs for subtle hydrocephalus, small amounts of blood in the dependent portions of the ventricles, and small amounts of isodense or hyperdense material in the basal cisterns.
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| Communication factors |
- The clinician should communicate the specific concern to the radiologist (eg, "severe acute headache; rule out SAH").
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- After a negative CT result, the clinician should communicate the posttest risk of SAH that persists (1 to 2 per 1000).
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