A rapid point-of-care test of 14 pathogens found in encephalitis and meningitis was heralded as a breakthrough, but because the test is not able to determine ciHHV-6 status or viral load, it now has physicians frustrated over how to interpret a positive result.
The FilmArray panel is an FDA approved test of 14 bacteria, fungi and viruses commonly found in encephalitis. A retrospective review led by Kiran Thakur of Columbia University reviewed the cases of 705 patients who had cerebrospinal fluid samples tested with the FilmArray panel. They found that HHV-6 was the most common pathogen, with 13 positive cases (1.8%). The problem is that since the prevalence of ciHHV6 is at least 1% and may be as high as 2% in patients (Pellett 2012), one would expect between 7 and 14 cases to be ciHHV-6. In other words, one must expect a very high percentage of ciHHV6 among HHV-6 positive cases, especially in patients older than the age of 3, or past the typical age of primary infections. Only 1/3 of the cases were pediatric.
A simple qPCR test on whole blood can easily determine ciHHV-6 status in almost all cases (Pellet 2012), so a policy of routinely testing all patients for ciHHV-6 using a qPCR or ddPCR assay would eliminate this confusion. In a whole blood qPCR test, individuals with ciHHV-6 have millions of copies per ml so are very easy to spot on a whole blood test, and impossible to identify with a plasma/serum test.
Absent this type of routine ciHHV6 status determination, physicians are left to guess whether the HHV-6+ patients should be treated. An expert panel did their best to determine if the 13 HHV-6+ cases were clinically significant. Unfortunately, some of the criteria developed to make this critical decision are not well suited for HHV-6. For example, pleocytosis > 5 cells/uL was one criteria but pleocytosis is known to be low or absent in HHV-6 encephalitis, particularly during primary infection (Yoshikawa 2009).
Also, imaging abnormalities are often delayed for HHV-6 encephalitis (Fida 2019, Takaya 2007,Yoshinari 2007), so imaging done only at the outset would be negative.
Finally, another complication is the fact that integrated HHV-6 in these ciHHV-6+ can activate in response to immunosuppression (Endo 2014,Hill 2015), pregnancy (Gravel 2013) or HDAC inhibitors such as vorinostat (Politikos 2016). Therefore, positive ciHHV-6 status does not rule out an active infection. This issue was not addressed by the authors of the review but is an important point since ciHHV6 status does not rule out an active infection of HHV-6, and in fact may make it more likely.
The panel concluded that 10/13 HHV6+ cases were either ciHHV6 or ‘clinically insignificant’.
In another review of pediatric cases tested using the BIOFIRE panel on CSF, HHV-6 was the second most common pathogen identified in cerebrospinal fluid, after enterovirus. Bacteria was found in 2.9% and viruses were found in 8.8% of 787 samples tested. This data was presented to the American Society of Microbiology in June 2019. This group also found that CSF pleocytosis was absent in many viral infections, most commonly with HHV-6, enterovirus and HSV1.
The Columbia group concludes that further study is needed to gain insight into the role of the meningitis/encephalitis panel in the diagnostic evaluation and antimicrobial treatment of CNS infections.
Read the full retrospective analysis: Radmard 2019.