An epidemiology-based and a likelihood ratio-based method of differential diagnosis

There are several methods for differential diagnostic procedures, and several variants among those. Furthermore, a differential diagnostic procedure can be used concomitantly or alternately with protocols, guidelines or other diagnostic procedures (such as pattern-recognition or using medical algorithms). For example, in case of medical emergency, there may not be enough time to do any detailed calculations or estimations of different probabilities, in which case the ABC protocol may be more appropriate. At a later, less acute, situation, there may be a switch to a more comprehensive differential diagnostic procedure. The differential diagnostic procedure may be easier in the finding of a 'pathognomonic' sign or symptom, in which it is almost certain that the target condition is present, and in the absence of finding a sine qua non sign or symptom, in which case it is almost certain that the target condition is absent. In reality, however, the subjective prob-ability of the presence of a condition is never exactly 100% or 0%, so in reality the procedure is usually aimed at specifying these various probabilities in order to form indications for further actions. The "probabilities" in this article refer to the 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%. Following are two methods of differential diagnosis, being based on epidemiology and likelihood ratios, respectively.