Prognostic studies include clinical studies of variables predictive of future events as well as epidemiological studies of aetiological risk factors. As multiple similar studies accumulate it becomes increasingly important to identify and evaluate all of the relevant studies to develop a more reliable overall assessment. For prognostic studies this is not straightforward.
Box BoxB1B1 summarises the clinical importance of information on prognostic factors. Many of the issues discussed are also relevant to aetiological studies, especially cohort ones. Some features of prognostic studies lead to particular difficulties for the systematic reviewer. Firstly, in most clinical prognostic studies the outcome of primary interest is the time to an event, often death. Meta-analysis of such studies is rather more difficult than that for binary data or continuous measurements. Secondly, in many contexts the prognostic variable of interest is often one of several prognostic variables. When examining a variable of interest researchers should consider other prognostic variables with which it might be correlated. Thirdly, many prognostic factors are continuous variables, for which researchers use a wide variety of methods of analysis.
- Systematic reviews are applicable to all types of research design, and studies of prognostic variables are an important additional area where appropriate methodology should be applied
- Prognostic variables should be evaluated in a representative sample of patients assembled at a common point in the course of their disease—ideally they should all have received the same medical treatment or been in a randomised trial
- When examined critically, a high proportion of prognostic studies are found to be methodologically poor
- Meta-analysis of published data is hampered by difficulties in extraction of data and variation in the characteristics of the study and patients
- The poor quality of the published literature is a strong argument in favour of systematic reviews but also an argument against formal meta-analysis
- Meta-analysis of prognostic studies using individual data from patients may overcome many of these difficulties
The emphasis in this paper is on clinical studies to examine the variation in prognosis in relation to a single putative prognostic variable of interest (also called a prognostic marker or factor). A more detailed discussion can be found elsewhere.2