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 diagnostic 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 and mucosa were investigated for the presence of herpes simplex virus (HSV), varicella-zoster virus (VZV), and the control gene beta -globin by internally controlled PCR with purified and unpurified DNA in parallel. With unpurified 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. Approximately 20% of the samples with positive results for HSV or VZV had negative or inhibited results using unpurified DNA. Conclusion: These results indicate that PCR from clinical swab specimens should be performed exclusively with internal controls because the positive control alone cannot exclude PCR inhibition in individual samples. Purification of DNA will decrease, but not exclude, PCR inhibition.