<prompt> CSF should be collected within4-6 hours after death12-16 hours after death16-20 hours after death20-24 hours after death  <response>: Cistern fluid is examined. The amount of potassium increases at a constant rate in relation to the temperature of the body during the first 20 hours. The concentrations of sodium,calcium and magnesium have no obvious relationship to time since death. Ref: K.S.Narayan Reddys Synopsis of Forensic Medicine and Toxicology, 29th edition, Chapter 7, page-98. 