Suspected subarachnoid haemorrhage with a negative CT head scan: What next?

The current recommended ‘next investigation’ for patients with a suspected subarachnoid haemorrhage and a negative non‐contrast computed tomography head scan, is a lumbar puncture. The cerebrospinal fluid obtained at lumbar puncture may be examined for the presence of blood and xanthochromia. Xanthochromia may be assessed by macroscopic examination or by spectrophotometry. Total red cell count and the three‐tube test have poor sensitivity and specificity as investigations for diagnosing subarachnoid haemorrhage. Macroscopic examination for xanthochromia is unreliable, with a risk of false‐negative results. Spectrophotometric examination for xanthochromia is associated with false positive results. The use of cerebrospinal fluid derivative spectroscopy and D‐Dimer testing using an enzyme‐linked immunosorbent assay need further evaluation. In the future, computed tomography angiography or magnetic resonance angiography may change current practice. Until then, all patients should be advised to have a lumbar puncture. A cerebrospinal fluid specimen obtained at least 8 h after the onset of headache and analysed for xanthochromia via spectrophotometry has the best chance of placing the patient in the lowest possible risk category. The emergency physician must be aware of the methods of cerebrospinal fluid analysis used locally.