, according to a report featured in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report.
Only about half of clinically diagnosed varicella cases — cases diagnosed by examining rashes without laboratory testing — were positive for the varicella-zoster virus (VZV), suggesting lab testing is important to avoid consequences such as children being kept out of school longer than necessary.
Clinical diagnosis continues to be the primary method for diagnosing varicella, said authors of the report, led by Alison Ruprecht, MPH, a state epidemiologist with the MDH. But the signs and symptoms of those who have received the varicella vaccine (including fewer skin lesions, mostly maculopapular) make it difficult to diagnose.
Minnesota Offers Free Tests
In December 2016, the MDH expanded polymerase chain reaction (PCR) laboratory testing for varicella in the state. The program reached out to clinicians through newsletters, webinars, advisories, and conferences describing the importance of lab testing when clinicians suspect a patient’s rash is varicella. The department also offered free testing at MDH Public Health Laboratory (PHL) through an agreement with the CDC and follow-up, if needed, with clinicians on testing practices.
MDH also provided specimen collection kits (containing a collection swab for vesicular fluid and slides for collection of scabs or scraping of maculopapular lesions) to clinics. Free testing was available for people with suspected varicella, including those who had been clinically diagnosed, or people who self-reported suspected varicella or whose school or child care reported the suspected cases. In addition to testing for varicella, MDH-PHL performed PCR testing for herpes simplex virus 1 (HSV-1), herpes simplex virus 2 (HSV-2), and enterovirus on all samples.
The state then saw lab-confirmed varicella cases double from 17% (235 of 1,426) during January 2013–November 2016 to 36% (619 of 1,717) during December 2016–March 2023 (P < .001).
During December 2016–March 2023, MDH-PHL tested specimens for 420 patients with suspected varicella; the median patient age was 5 years (range = 0-68 years). Of those, 23% provided specimens collected at home.
Clinical Diagnosis Versus Lab Test Confirmation
The researchers found that among 208 patients receiving a clinical diagnosis of varicella after only examination at a medical facility, fewer than half (45%) had positive varicella-zoster virus (VZV) lab test results. VZV detection was 66% lower in those who received varicella vaccine compared with those who did not.
The researchers acknowledged that outreach, at-home specimen collection, and free testing likely increased lab testing numbers.
They added that, “This increase in varicella testing likely also contributed to an increase in appropriate clinical management and school exclusion recommendations for suspect varicella cases.
“Clinicians should incorporate routine laboratory testing whenever varicella is suspected,” the researchers wrote. “Public health and school health professionals should emphasize the importance of laboratory confirmation in their recommendations to clinicians and parents.”
Presentation May Also Be Different in Immunocompromised
Sam Dominguez, MD, infectious disease specialist at Children’s Colorado in Aurora, who was not part of the research, said in addition to presentation being harder to recognize in those who are vaccinated, varicella is harder to diagnose in the immunocompromised population, where the rash may not be as prominent or more localized or appear in any number of atypical presentations.
In addition, he said, clinicians don’t see many cases these days. “Providers aren’t as familiar with what varicella looks like, especially younger providers who weren’t trained in the prevaccination era,” he said.
Cost is often an issue with lab testing as well as turn-around time and access, he said, and those factors can be barriers.
Dr. Dominguez said some classic presentations are easily diagnosed as varicella. “If you have a normal, healthy kid, who you’re seeing in the outpatient world who presents with a very classic rash for chickenpox, I don’t think laboratory testing is necessarily warranted in that scenario.”
But when clinicians aren’t confident in their diagnosis, “I think in those scenarios, testing can be very helpful in terms of management from a treatment standpoint as well as a potential infection control standpoint,” he said.
The authors reported no relevant financial relationships. Dr. Dominguez is a consultant for diagnostic companies Karius and BioFire. He has grant support from Pfizer and BioFire.