Never has been externally and prospectively validated, authors caution implementation into routine use
100% sensitive to rule out<unk>  (97.1%-100%)
Can exclude<unk>  if all of the following are true
Age < 40
No neck pain or stiffness
No witnessed LOC
No onset during exertion
No thunderclap symptomatology (max intensity at onset)
No limited neck flexion on physical exam
Age < 40
No neck pain or stiffness
No witnessed LOC
No onset during exertion
No thunderclap symptomatology (max intensity at onset)
No limited neck flexion on physical exam
If concerned for<unk>  and CT normal strongly consider LP, especially if CT obtained >6 hrs after symptom onset<unk>  due to aneurysm - look in cisterns (esp. suprasellar cistern)<unk>  due to trauma - look at convexities of frontal and temporal cortices
Elevated RBC count that does not decrease from tube one to four
Note: decreasing RBCs in later tubes can occur in<unk> ; only reliable if RBC count in final tube is nl
Opening pressure >20 (60% of patients)
Can help differentiate from a traumatic tap (opening pressure expected to be normal)
Elevated opening pressure also seen in cerebral venous thrombosis, IIH
Xanthochromia
May help differentiate between<unk>  and a traumatic tap
Takes at least 2hr after bleed to develop (beware of false negative if measure early)
Sn (93%) / Sp (95%) highest after 12hr
If unable to obtain CSF consider CTA
CTA also highly sensitive for predicting delayed cerebral ischemia
If traumatic tap is suspected
Tube 4 RBC count <500 has negative predictive value of 100% for<unk> . Tube 4 RBC decrease of 70% compared to tube 1 excludes a radiographically detectable<unk> .
One study found that >2000 RBCs had a sensitivity of 93% and specificity of 93% for<unk> , sensitivity increased to 100% when xanthochromia added.
Note: decreasing RBCs in later tubes can occur in<unk> ; only reliable if RBC count in final tube is nl
Can help differentiate from a traumatic tap (opening pressure expected to be normal)
Elevated opening pressure also seen in cerebral venous thrombosis, IIH
May help differentiate between<unk>  and a traumatic tap
Takes at least 2hr after bleed to develop (beware of false negative if measure early)
Sn (93%) / Sp (95%) highest after 12hr
CTA also highly sensitive for predicting delayed cerebral ischemia
Tube 4 RBC count <500 has negative predictive value of 100% for<unk> . Tube 4 RBC decrease of 70% compared to tube 1 excludes a radiographically detectable<unk> .
One study found that >2000 RBCs had a sensitivity of 93% and specificity of 93% for<unk> , sensitivity increased to 100% when xanthochromia added.
A CT followed by CTA is an acceptable alternative to CT and LP
CTA has a 98% sensitivity for aneurysms >3mm
Subarachnoid hemorrhage
Intra-axial
Subdural hemorrhage
Hemorrhagic stroke 
Ischemic stroke
Traumatic intracerebral hemorrhage
Hemorrhagic stroke
Extra-axial