Studies have shown that cyanide has variable instability in biological specimens and is particularly unstable in some postmortem specimens. The loss of cyanide can be minimized by shipping the sample to the laboratory for analysis as soon as possible, preferably using refrigerated or frozen transportation and preservation using sodium fluoride / potassium oxalate (grey-top tube). Samples should not be refrozen if previously thawed. The potential for increases in cyanide concentrations, although rare, have also been demonstrated and may be due to microbial action. Preservation with sodium fluoride may reduce this possibility.