A method for detecting and/or quantifying a target DNA or RNA present in a sample by a isometric primer extension method is disclosed. The method includes carrying out a primer extension reaction in the absence of a free nucleotide so that the primer extension reaction is stopped where the absent nucleotide would have been inserted. Thus, as the amount of incorporation of a labeled nucleotide on the primer extended product is detected, the amount of the target RNA or DNA in the sample is measured.