Is it possible to have a retrospective survival analysis? I would like to ask whether it is possible to have a retrospective survival analysis.
We have a population of patients who had an examination twice (one now and one in past with varying time intervals). We measure at the time of the latter examination a specific gene, which of course is a constant factor through time and we are interested in a finding of the examination. 
Is it correct to run a survival analysis for that finding between the gene groups? The concept is that the population basically existed in the first examination although formed by our measurement in the second.
 A: So you have a number of patients of which some have developed the disease during the time between the assessments. You have measured the specific gene of interest, and as you say, this is of course a static factor and may be used as a baseline variable.
Now, there are basically two possible types of information about the disease. The first one is that the patient is re-examined as planned and you discover the disease. You then know only that the disease has developed during the time interval, not the approximate time. The second type of information about the disease is that the patient develops symptoms of the disease and seek medical care for this, and then get their diagnosis. In this case, if you have access to the medical records, you have a better approximation of when the diseased developed.
If there are none or few of this second type (called interval-detected recurrences), you can apply standard methods for survival analysis according to Collett, 2014. If not, you need to apply other methods that I'm not currently familiar with. The answers to this question give some ideas, provided you use R.
