In this journal in the late 1980s, two papers presented contrasting advice about the appropriate investigation of cerebrospinal fluid (CSF) in suspected subarachnoid haemorrhage when computed tomography (CT) of the head revealed no evidence of blood. The first concluded that it was the detection of red blood cells that was important in supporting a decision to proceed to cerebral angiography and not that of the red cell breakdown products, oxyhaemoglobin and bilirubin.¹ This was a conclusion based on the use of visual inspection to detect the colour (xanthochromia) imparted by oxyhaemoglobin and bilirubin. The second, based on a series of 111 patients in whom blood …