If there is a sudden "worst headache of your life" that reaches its peak within an hour, go to the emergency room for a head CT right away

High value
Evidence A
Buys life
Costs a little
Takes some hours
No willpower

In a real emergency, call 911 first, then read this.

In plain words

What to look at is how fast the pain reaches its worst. Of severe headaches that peak within an hour, about 6% are a ruptured blood vessel on the surface of the brain. If you are 40 or older, have neck pain or stiffness, were seen to faint, the headache began during exertion, it came on like an instant explosion, or you can't bend your neck forward, then any one of these means going to the emergency room for a head CT. Most people who go are fine, but missing one means a second bleed, which is death.

Cost: The emergency visit itself costs little. A head CT in the emergency room costs several hundred CNY.
What you get: 10 emergency departments in Canada ran a prospective cohort study of 2,131 cases of "severe headache reaching its worst within one hour," with criteria set in advance and the patients followed and recorded throughout. Of these, 132 cases (6.2%) were subarachnoid hemorrhage (a ruptured vessel on the surface of the brain). The Ottawa SAH Rule has six items: aged 40 or older, neck pain or stiffness, and someone seeing the person faint. The other three are onset during exertion, onset like an explosion, and limited neck flexion on the doctor's exam. Meeting any one means testing. This rule picked out every case of bleeding, with a sensitivity of 100%, 95% CI 97.2%–100.0% (confidence interval, the range the true value likely falls in). The cost is that of the people without bleeding, only 15.3% were ruled out by it (specificity 15.3%)
Notes: What matters is not "how much it hurts" but "how fast it reaches the worst." A headache that explodes within seconds to a minute is the danger sign, and a migraine that builds slowly is not. Vomiting, a stiff neck, sensitivity to light or a brief faint make it even more serious. A specificity of only 15.3% means most people tested by this rule turn out fine. But missing a case means a second bleed, which is death, so it is worth testing. Don't take a painkiller to push it down and go back to sleep
Evidence grade: A : Specific numbers from meta-analyses, large cohorts or randomized trials; for law and policy, the statute or official text.
Sources: Perry JJ, Stiell IG, Sivilotti MLA, et al. (2013). Clinical decision rules to rule out subarachnoid hemorrhage for acute headache. JAMA, 310(12), 1248-1255. https://doi.org/10.1001/jama.2013.278018

Translated from the Chinese original. Original book, section 13, tip 9. Content may be out of date and rules differ by place. When you actually deal with an agency, a doctor or a court, follow what local officials, doctors and lawyers tell you. Disclaimer