Analyzing trends in HSV Western Blot results at a Reference Laboratory

Objective: To better understand herpes simplex virus (HSV) Western Blot result trends over the past 20 years and to determine the proportion of Western Blot positivity for HSV-1 and HSV-2 and the proportion of results that are indeterminate. To examine the relationship between indeterminate results and age.

Methods: We used de-identified HSV Western Blot data performed at the University of Washington Virology Laboratory between 1999-2020. We included all results collected between 1999-2020 from persons aged 18 to 99 with a result of negative, positive, or indeterminate. We excluded results from the oncology service with the label Seattle Cancer Care Alliance (“SCCA”) but included all other referral locations. We assessed overall total proportion of negative, positive, and indeterminate for both HSV-1 and HSV-2 in the sample. Linear regression was used assess the trends in positive and indeterminate results over time. We then analyzed the proportion of Western Blots done in the 5 year time period before and after the 2015 CDC guidelines. Linear regression was utilized to assess the association between indeterminate results and age.

Results: Results from 188,342 samples collected from 1999 to 2020 were included in the analysis. 107,468 (57%) were HSV-1 positive while 35,294 (19%) were HSV-2 positive. There were 20,138 (11%) samples that were positive for both HSV-1 and HSV-2 and 61,533 (33%) that were negative for both types. Overall, 4,419 samples were indeterminate for HSV-2 (2%) and 2,152 (1%) for HSV-1. The proportion of HSV-1 positives decreased by 0.45% each year (p<0.001) while the proportion of indeterminates increased by 0.08% each additional year (p<0.001). For HSV-2, the proportion of positives decreased by 0.6% annually (p<0.001) and indeterminates increased by 0.15% each year (p<0.001). When looking at HSV-2 indeterminates by age category, the highest proportion was in the 56- 65 age group with a total of 3.3% compared to the lowest at 1.7% in those 18-25 (p<0.003).

Conclusion: HSV-1 and HSV-2 seropositivity has declined over the last twenty years, in parallel to national trends. The frequency of indeterminate results for both HSV-1 and HSV-2 may be rising possibly due to increased testing in the setting of declining seroprevalence leading to a decrease in the positive predictive value of testing and increase in false positives. Our analysis also showed that there was a statistically significant correlation between proportion of indeterminate results and older age.