Virus isolation is less suitable as a routine diagnostic procedure than either of the aforementioned tests. Infectivity is slowly but progressively lost after fetal death; as a result, isolation of virus from mummified fetuses that have died as a result of infection is sometimes unsuccessful. Moreover, the procedure is time-consuming, and contamination is a constant threat because of the stability of PPV in the laboratory and because cell cultures are sometimes unknowingly prepared from infected tissues. IF microscopy is often used to determine whether PPV has been isolated in cell culture.