enhance its automated rt-pcr. in the fret reaction, a donor and acceptor probe exchange excitation energy when within 1-5 base pairs (bp) on the target sequences. the energy is emitted in the form of heat or fluorescence. the probes are designed to bind 1-5 bp from each other. the energy emission is proportional to the concentration of viral particles. the linear range for this assay is 48 copies/ml of blood. quantitation of hiv-15/11/2007roche molecular systems, inc pleasanton, ca non-nucleic acid-based tests exavir™ load version 3 from cavidi ab is a largely manual test, which has the european regulatory approval (ce-mark) for clinical use and is also used for viral load monitoring. virus-associated reverse transcriptase (rt) activity is measured and can therefore detect all types and subtypes of hiv. the technology does not require sophisticated laboratories.