For PCR testing, collect swab specimens up to 10 days after rash onset. Use dacron or rayon swabs with plastic shafts. Do not use cotton swabs with wooden shafts. The material in cotton and wood will interfere with the PCR process. Real-time RT-PCR testing is the most sensitive test for identifying whether a person is infected with the measles virus.
For IgM serologic testing, collect serum specimen three or more days after rash onset.