The use of in-vitro sensitivity testing to predict clinical response of recurrent herpes simplex to suppressive oral acyclovir.

Twenty-six pre-treatment isolates of herpes simplex virus from 20 immunocompetent patients with frequently recurring mucocutaneous herpes were tested for in-vitro sensitivity by a plaque reduction assay. When correlated with the occurrence of breakthrough attacks during subsequent courses of suppressive treatment with oral acyclovir the mean ID50, ID90 and ID99 were all found to be significantly higher for isolates associated with breakthroughs. However, cut-off values could only be set for the ID90 and ID99 values. An ID99 less than 0.75 mg/l was found in 17/21 (81%) of isolates not associated with breakthrough attacks, but none of five isolates associated with breakthroughs. The corresponding findings for an ID90 less than 0.5 mg/l were 17/21 (81%) without breakthroughs and 1/5 with breakthroughs. It is suggested that the ID90 and ID99 are better predictors of the clinical response to suppressive oral acyclovir than is the ID50. Possibly this is because they better reflect the presence of viral strains with reduced sensitivity which may be responsible for breakthrough attacks.