Cardiovascular risk markers are first hypothesized and then tested. Once a potential marker is identified, concentrations of the serum marker are correlated with patient outcomes. Cardiovascular risk marker studies are typically either retrospective or prospective epidemiology studies.
A retrospective study looks backwards at a patient population. For example, we identify (through a hospital database perhaps) patients who have had myocardial infarcts or some other adverse outcome as well as similar subjects without that outcome to use as controls. We then go back and find archived patient serum samples and relate the concentrations of our new risk marker with patient outcomes. Retrospective studies can only be performed if you have archived samples from the patient.
Prospective studies look forward in time. For example, we first select a group of subjects and measure our new risk marker in these patients. We continue to measure it over time. After a few years, we see how the serum concentrations relate to patient outcomes. Obviously, prospective studies take much longer to perform than retrospective studies.
Whatever study model is used, when assessing the value of a cardiovascular risk marker, we must correlate serum concentrations with a specific outcome. The outcome is determined by the study authors. Outcomes could be events like myocardial infarction, stroke, a diagnosis of coronary artery disease, death, or any other event the study authors wish to include.
Concentrations of risk markers are usually divided into tertiles, quatriles or quintiles. This simply means that the top 33%, top 25% or top 20% of the serum concentration values are compared to the bottom 33%, 25% or 20%. For example, risk marker studies will often compare the outcomes of patients with serum concentrations in the upper tertile (those in the top third) with those in the bottom tertile (those in the bottom third) to see if the top 33% had significantly worse outcomes; if so, the risk marker likely has clinical value.