[2] Cerebrospinal fluid, which fills the subarachnoid space between the arachnoid membrane and the pia mater surrounding the brain, is normally clear and colorless.

Within several hours, the red blood cells in the cerebrospinal fluid are destroyed, releasing their oxygen-carrying molecule heme, which is then metabolized by enzymes to bilirubin, a yellow pigment.

[3] The most frequently employed initial test for subarachnoid hemorrhage is a computed tomography scan of the head, but it detects only 98% of cases in the first 12 hours after the onset of symptoms, and becomes less useful afterwards.

[4] Therefore, a lumbar puncture ("spinal tap") is recommended to obtain cerebrospinal fluid if someone has symptoms of a subarachnoid hemorrhage (e.g., a thunderclap headache, vomiting, dizziness, new-onset seizures, confusion, a decreased level of consciousness or coma, neck stiffness or other signs of meningismus, and signs of sudden elevated intracranial pressure), but no blood is visible on the CT scan.

[1][9] There is still disagreement about whether or not to routinely use spectrophotometry or whether visual inspection is adequate, and one group of authors has even advocated measuring bilirubin levels.