Real time summarization in microblog aims at providing new relevant and non redundant information about an event as soon as it occurs. In this paper, we introduce a new tweet summarization approach where the decision of selecting an incoming tweet is made immediately when a tweet is vailable. Unlike existing approaches where thresholds are predefined, the proposed method estimates thresholds for decision making in real time as soon as the new tweet arrives. Tweet selection is based upon three criterion namely informativeness, novelty and relevance with regards of the user's interest which are combined as conjunctive condition. Only tweets having an informativeness and novelty scores above a parametric-free threshold are added to the summary. The evaluation of our approach was carried out on the TREC MB RTF 2015 data set and it was compared with well known baselines. The results have revealed that our approach produces the most precise summaries in comparison to all baselines and official runs of the TREC MB RTF 2015 task.