International Journal of Epidemiology 2003;32:125–130

Observational epidemiological studies can often be greatly enhanced by the inclusion of biochemical analyses in stored blood samples collected from the population being studied. Biochemical analyses can be used to assess risk factor exposure, to control for confounding, or to measure the effects of bias. In randomized trials, biochemical analyses can be used to monitor the safety and biochemical efficacy of treatment. For epidemiological studies to be informative, they often need to be large (perhaps involving tens or hundreds of thousands of indi-viduals), which in turn requires methods for blood collection that are practical and cost-effective. Standard guidelines for blood sample handling state that plasma or serum should be separated from cells as soon as possible and certainly within 2 hours.1 Whilst this is necessary for particular analytes, it might be assumed that many blood analytes deteriorate within a matter of hours in unseparated samples kept at ambient temperature. This perceived need for either immediate local separation of blood samples or their rapid chilled transfer t