Xanthochromia, from the Greek xanthos (ξανθός) “yellow” and chroma (χρώμα) “colour”, is the yellowish appearance of cerebrospinal fluid that occurs several hours after bleeding into the subarachnoid space caused by certain medical conditions, most commonly subarachnoid haemorrhage. Its presence can be determined spectrophotometry as described in 

Revised National Guidelines for analysis of Cerebrospinal Fluid in suspected subarachnoid haemorrhage. Cruikshank,A., Auld,P., Beetham,R., et al. Ann Clin Biochem 2008;45:238-244  

The Torax xanthochromia precision control is a two level control  with indicative values as below

Elevated  NOA(414nm) 0.150            NBA(476nm) 0.050

Low         NOA(414nm)  0.100           NBA(476nm) 0.015