From Wikipedia and very helpful reading for all:

"A differential diagnosis (sometimes abbreviated DDx, ddx, DD, D/Dx, or ΔΔ) is a systematic diagnostic method used to identify the presence of an entity where multiple alternatives are possible (and the process may be termed differential diagnostic procedure), and may also refer to any of the included candidate alternatives (which may also be termed candidate condition). This method is essentially a process of elimination or at least of obtaining information that shrinks the "probabilities" of candidate conditions to negligible levels. The "probabilities" at issue are epistemic rather than ontological in that they are imaginative parameters in the mind of the diagnostician (or, for computerized or computer-assisted diagnosis, the software of the system), while in reality the target (such as a patient) either has a condition or not with an actual probability of either 0 or 100%.
Differential diagnostic procedures are used by physicians, psychiatrists, and other trained medical professionals to diagnose the specific disease in a patient, or, at least, to eliminate any imminently life-threatening conditions.
Differential diagnosis can be regarded as implementing aspects of the hypothetico-deductive method in the sense that the potential presence of candidate diseases or conditions can be viewed as hypotheses which are further processed as being true or false."

en.wikipedia.org/wiki/Differential_diagnosis

OP, it would be especially useful for you to read this wikipedia article because it has direct relevance to your situation.