Detection of herpes simplex virus and varicella-zoster virus in clinical swabs: Frequent inhibition of PCR as determined by internal controls

Background: PCR-based detection of microorganisms is widely used for diagno stic purposes. Most routine PCR applications do not control for inhibition of PCR, thus leading to false-negative results. Methods and Results: One hundred eighteen swab samples obtained from skin a nd mucosa were investigated for the presence of herpes simplex virus (HSV), varicella-zoster virus (VZV), and the control gene beta -globin by interna lly controlled PCR with purified and unpurified DNA in parallel. With unpur ified DNA, inhibition of PCR was detected in 23% of beta -globin PCRs, 25% of VZV PCRs, and 16% of HSV PCRs versus 3% each for purified DNA. Approxima tely 20% of the samples with positive results for HSV or VZV had negative o r inhibited results using unpurified DNA. Conclusion: These results indicate that PCR from clinical swab specimens sh ould be performed exclusively with internal controls because the positive c ontrol alone cannot exclude PCR inhibition in individual samples. Purificat ion of DNA will decrease, but not exclude, PCR inhibition.