One overnight sample will give you some mitotic cells, but mitotic index will be lower than that of fresh sample. More the 2 over night sample will give you no mitotic cells.
we had over the more than 50 years thousands of blood samples sent at room temperature by surface mail, never did any study of the number of viable cells but
virtually always had a succesfull lymphocytre culture.
Vacutainers with heparin is better for cytogenetic studies than ones with EDTA. If you don't need to use a dividing cells (how Eisuke Gotoh has wrote) in your work (e.g. comet assay) you can freeze samples. But if you need it (e.g. micronucleus test, analysis of chromosomal aberrations) - you can't.
First thing: after you took the blood-sample make sure you mixed the blood with the heparin in the siringe proper (sounds logical but my experience is that this is not always done in the way it should be)! Second thing: you should work on it as soon as possible. If you cannot do this you should store the blood-sample at 4 degrees in a fridge. If u want to perform chromosome analysis don't store the blood-sample for longer than 10-14 days! After this time you will not be able to stimulate the lymphocytes any more! "Conservation" of a blood-sample for chromosome diagnostics for longer time is not possible except u have the time and resources to establish a immortalized cell line like LCL (Lymphoblastoid Cell Line).
For Cytogenetic analysis must store Heparinised blood at normal fridge not more than 24 hrs to get good result. For molecular biology can store EDTA blood in -20 for months. Better to inoculate the buffy coat after 24 hrs in the culture media and then the cytogenetic result will come good.
my experience is limited in storing blood samples. prefer to work with fresh sample. but my question is blood banks store blood bags using citrate phosphate adenine dextrose solition. will it work for ytogenetic analysis. how about citrate interference.