A method comprising subjecting one or more sample portion(s) to a single amplification step, thereby amplifying a single molecule in the sample portion to a detectable level, and, in some embodiments, then determining whether the sample portion contains at least one molecule of the target nucleic acid. In some embodiments, the sample portion is in a porous sample structure, or in a sample chamber which comprises means for minimizing diffusion of the sample portion, or in a sample chamber which is inside a microcapillary device, or in a sample retaining means.