Although a fasting specimen is recommended, it is not required. There are no special instructions with regard to diet if a fasting sample cannot be obtained. Vitamin D does not follow diurnal variation so timing is not important. Specimens for vitamin D analysis can be fresh or frozen serum or plasma. Specimens may be collected by using regular red-top or serum-separator tubes as well as green top tubes that contain sodium heparin. Specimens can be stored at < -20ºC, and because vitamin D is very stable, serum/plasma samples can be frozen for a long period of time if analysis will be greatly delayed. Several freeze-thaw cycles do not seem to adversely affect the assay, although repeated freeze-thaw cycles should be avoided if possible. Specimens may be stored in glass or plastic vials, as long as the vials are tightly sealed to prevent desiccation of the sample.
Moderately hemolyzed specimens may be used in certain testing methods because red blood cells do not contain Vitamin D. If using an assay that involves spectrophotometric testing methods, hemolysis may interfere with test results.
