A method for performing a nucleic acid amplification assay employs a thermally-conductive receptacle holder with one or more receptacle wells, each conforming to an outer surface of a lower portion of a vial. A through-hole extends from an inner surface of each well to an outer surface of the holder. A thermal element is positioned proximal to the holder for altering a temperature of the holder. A signal detection module is configured to generate an excitation signal directed through the through-hole of the well and detect an optical emission from a fluid contained in the vial supported by the well. At least one well supports a capped vial containing a reagent for a nucleic acid amplification assay and including an opaque cap sealing an open end of the vial. The lower portion of the vial is contained within a well, and the cap is situated above a top surface of the holder.