It is a very important, but also a very difficult task, to establish how long a certain species has to remain unregistered before we can declare it extinct. The wrong assumption regarding species extinction could result in a type I or type II statistical error, leading to inappropriate management actions or even species extinction. Recent development of the methods for inferring the threat of extinction, when the only available information is a record of sightings, has enabled a quantitative approach to the problem. In our study we present an index that infers extinction probability based on trends in sighting intervals. Our study comprises a description of the sighting trend index, a sensitivity analysis and an application of the index to the sighting record of the black-footed ferret Mustela nigripes. The main advantage of this method could be its sensitivity to changes in sighting frequency within the sighting record. However, further testing of the method on different data sets could be important for gaining additional knowledge regarding its adequate application in the field of conservation biology.